Ketosis

Q- What are ketone bodies ? Discuss their biological significance ?

Answer- Ketone bodies can be regarded as  water-soluble, transportable form of acetyl units. Fatty acids are released by adipose tissue and converted into acetyl units by the liver, which then exports them as ketone bodies.

Acetoacetate, D(-3) -hydroxybutyrate (Beta hydroxy butyrate), and acetone are often referred to as ketone bodies (Figure-1).

Figure-1- showing the structure of ketone bodies.

The term “ketones”  is a misnomer because 3-hydroxybutyrate is not a ketone and there are ketones in blood that are not ketone bodies, eg, pyruvate, fructose.

Biological Significance

Ketone bodies serve as a fuel for extra hepatic tissues

The  brain  is an  important  organ.  It  is  metabolically  active  and  metabolically privileged. The brain generally uses 60-70% of total body glucose requirements, and always  requires  some  glucose for  normal  functioning.  Under  most  conditions, glucose is essentially the sole energy source of the brain. The brain cannot use fatty acids, which  cannot cross  the  blood-brain  barrier. Because animals  cannot synthesize significant amounts of glucose from fatty acids, as glucose availability decreases, the brain is forced to use either amino acids or ketone bodies for fuel.

Individuals eating diets extremely high in fat and low in carbohydrates,or starving, or suffering  from  a severe lack of insulin  (Type I diabetes  mellitus) therefore increase the synthesis and utilization of ketone bodies

During high rates of fatty acid oxidation, primarily in the liver, large amounts of acetyl-Co A are generated. These exceed the capacity of the TCA cycle, and one result is the synthesis of ketone bodies. The synthesis of the ketone bodies (ketogenesis) occurs in the liver mitochondria allowing this process to be intimately coupled to rate of hepatic fatty acid oxidation. Conversely, the utilization of the ketones (ketolysis) occurs in the peripheral cells, in the cytosol.

The acetyl CoA formed in fatty acid oxidation enters the citric acid cycle only if fat and carbohydrate degradation are appropriately balanced. The reason is that the entry of acetyl CoA into the citric acid cycle depends on the availability of oxaloacetate for the formation of citrate, but the concentration of Oxaloacetate is lowered if carbohydrate is unavailable or improperly utilized. Oxaloacetate is normally formed from pyruvate, the product of glycolysis, by pyruvate carboxylase (Figure-2).This is the molecular basis of the adage that fats burn in the flame of carbohydrates.

Figure-2-showing the pathway of ketogenesis in conditions of non availability of Oxaloacetate

In fasting or diabetes, oxaloacetate is consumed to form glucose by the gluconeogenic pathway (figure-2) and hence is unavailable for condensation with acetyl CoA. Under these conditions, acetyl CoA is diverted to the formation of acetoacetate and β-hydroxybutyrate.

These substances diffuse from the liver mitochondria into the blood and are transported to peripheral tissues. These ketone bodies were initially regarded as  degradation products of little physiological value. However, the results of studies revealed that these derivatives of acetyl CoA are important molecules in energy metabolism. Acetoacetate and β-hydroxybutyrateare normal fuels of respiration and are quantitatively important as sources of energy. Indeed, heart muscle and the renal cortex use acetoacetate in preference to glucose. In contrast, the brain adapts to the utilization of acetoacetate during starvation and diabetes. In prolonged starvation,75% of the fuel needs of the brain are met by ketone bodies.

Q.- Describe the pathway for the synthesis of ketone bodies by naming substrates, the first ketone body made in the pathway, the next two ketone bodies made in the pathway, the intermediates in the pathway that can be used either for ketone body synthesis or cholesterol synthesis, and the enzyme that actually produces the first ketone body as a product.

Answer-Ketogenesis takes place in liver using Acetyl co A as a substrate or a precursor molecule. Enzymes responsible for ketone body formation are associated mainly with the mitochondria.

Steps of synthesis-Acetoacetate (First ketone body) is formed from acetyl CoA in three steps (Figure-3 ).

1)Two molecules of acetyl CoA condense to form acetoacetyl CoA. This reaction, which is catalyzed by thiolase, is the reverse of the thiolysis step in the oxidation of fatty acids.

2) Acetoacetyl CoA then reacts with acetyl CoA and water to give 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) and CoA. The reaction is catalyzed by HMG co A synthase. This enzyme is exclusively present in liver mitochondria. There are two isoforms of this enzyme-cytosolic and mitochondrial. The mitochondrial enzyme is needed for ketogenesis while the cytosolic form is associated with cholesterol biosynthesis.

This condensation resembles the one catalyzed by citrate synthase. This reaction, which has a  favorable equilibrium owing to the hydrolysis of a thioester linkage, compensates for the unfavorable equilibrium in the formation of acetoacetyl CoA.

3) 3-Hydroxy-3-methylglutaryl CoA is then cleaved to acetyl CoA and acetoacetate in the presence of HMG Co A  lyase (Figure-3)

The carbon atoms split off in the acetyl-CoA molecule are derived from the original acetoacetyl-CoA molecule. Both enzymes must be present in mitochondria for ketogenesis to take place. This occurs solely in liver and rumen epithelium,

The sum of these reactions is

The other two ketone bodies-Acetone and D(-)- 3-Hydroxybutyrate are formed from Acetoacetate, the primary ketone body.

4) Acetone is formed by decarboxylation in the presence of decarboxylase enzyme and, because it is a beta-keto acid, acetoacetate also undergoes a slow, spontaneous decarboxylation to acetone. The odor of acetone may be detected in the breath of a person who has a high level of acetoacetate in the blood.  “Acetone-breath” has been used as  a  crude  method  of  diagnosing  individuals  with  untreated Type I diabetes mellitus.

 5) D (-)-3-Hydroxybutyrate is formed by the reduction of acetoacetate in the mitochondrial matrix by D(-)3-hydroxybutyrate dehydrogenase. D(-)-3-Hydroxybutyrate is quantitatively the predominant ketone body present in the blood and urine in ketosis.

The β-hydroxybutyrate dehydrogenase reaction has two functions: 1) it stores energy equivalent to an NADH in the ketone body for export to the tissues, and 

2) it  produces  a  more  stable molecule.

Acetoacetate and β-hydroxybutyrate, in particular, also serve as major substrates for the biosynthesis of neonatal cerebral lipids.

The ratio of β hydroxybutyrate to acetoacetate depends on the NADH/NAD+ ratio inside mitochondria. if NADH concentration is high, the liver releases a higher proportion of β-hydroxybutyrate.

In vivo, the liver appears to be the only organ in nonruminants to add significant quantities of ketone bodies to the blood. Extrahepatic tissues utilize them as respiratory substrates. The net flow of ketone bodies from the liver to the extrahepatic tissues results from active hepatic synthesis coupled with very low utilization. The reverse situation occurs in extra hepatic tissues.

While an active enzymatic mechanism produces acetoacetate from acetoacetyl-CoA in the liver, acetoacetate once formed cannot be reactivated directly except in the cytosol, where it is used in a much less active pathway as a precursor in cholesterol synthesis. This accounts for the net production of ketone bodies by the liver.

Why are three enzymes required to synthesize acetoacetate?

An enzyme that cleaves the thioester  bond of the thiolase  product  acetoacetyl-CoA  would  also produce acetoacetate, but such a thioesterase does not seem to exist. The reason for the multienzyme pathway is not really understood. However, the pathway that does exist is not especially wasteful; the third acetyl-CoA used merely acts catalytically.

Because the cell needs to have HMG-CoA synthase for other purposes, the choice is in having HMG-CoA lyase. It is possible that having two mitochondrial enzymes (HMG-CoA synthase and HMG-CoA lyase) reuired for ketone body synthesis assists in controlling the pathway.

Figure-3- Showing the steps of ketogenesis

Q.- Name the tissues that oxidize ketone bodies. Why not the liver? What happens to blood ketone bodies? Name the intermediates in the pathway from β-Hydroxybutyrate to acetyl CoA.

Answer- The ketone bodies are water soluble and are transported across the inner mitochondrial membrane as well as across the blood-brain barrier and cell membranes. Thus they can be used as a fuel source by a variety of tissues including the CNS. They are preferred substrates for aerobic muscle and heart, thus sparing glucose when they are available.

Tissues that can use fatty acids can generally use ketone bodies in addition to other energy sources. The exceptions are the liver and the brain. The liver synthesizes ketone bodies, but has little β-ketoacyl-CoA transferase, and therefore little ability to convert acetoacetate into acetyl-CoA. The brain does not normally use fatty acids, which do not cross the blood-brain barrier; under ordinary circumstances, the brain uses glucose as its sole energy source.

The metabolic rate of the brain is essentially constant. While other tissues reduce their metabolic requirements during starvation, the brain is unable to do so. After a few  days  of  fasting,  the  brain  undergoes  metabolic  changes  to  adapt  to  the decreased availability of glucose. One major change is increased amounts  of the enzymes necessary to metabolize ketone bodies.

Ketone bodies are utilized by extrahepatic tissues via a series of cytosolic reactions (Figure-4) that are essentially a reversal of ketone body synthesis, the ketones must be reconverted to acetyl CoA in the mitochondria:

Figure-4- Showing the steps of utilization of ketone bodies.

Steps- (Figure-4)

1) Beta-hydroxybutyrate, is first oxidized to acetoacetate with the production of one NADH (1). It is important to appreciate that under conditions where tissues are utilizing ketones for energy production their NAD⁺/NADH ratios are going to be relatively high, thus driving the β-hydroxybutyrate dehydrogeanse catalyzed reaction in the direction of acetoacetate synthesis. 

2) Coenzyme A must be added to the acetoacetate. The thioester bond is a high energy bond, so ATP equivalents must be used. In this case the energy comes from a trans esterification of the CoAS from succinyl CoA to acetoacetate by Coenzyme A transferase (2), also called Succinyl co A : Acetoacetate co A transferase, also known as Thiophorase.

The succinyl CoA comes from the TCA cycle. This reaction bypasses the succinyl-CoA synthetase step of the TCA cycle, hence there is no GTP formation at this steps although it does not alter the amount of carbon in the cycle.

The liver has acetoacetate available to supply to other organs because it lacks this particular CoA transferase and that is the reason that “Ketone bodies are synthesized in the liver but utilized in the peripheral tissues”. The latter enzyme is present at high levels in most tissues except the liver. Importantly, very low level of enzyme expression in the liver allows the liver to produce ketone bodies but not to utilize them. This ensures that extrahepatic tissues have access to ketone bodies as a fuel source during prolonged fasting and starvation, and also,lack of this enzyme in the liver prevents the futile cycle of synthesis and breakdown of acetoacetate.

3) The acetoacetyl CoA is now cleaved to two acetyl CoA’s with Thiolase (3).

This implies that the TCA cycle must be running to allow ketone body utilization; a fact  which is necessarily true,  because  the TCA cycle is necessary to allow generation of energy from acetyl-CoA.

D(-)-3-Hydroxybutyrate is oxidized to produce acetoacetate as well as NADH for use in oxidative phosphorylation.

If the blood level is raised, oxidation of ketone bodies increases until, at a concentration of approximately 12 mmol/L, they saturate the oxidative machinery. When this occurs, a large proportion of the oxygen consumption may be accounted for by the oxidation of ketone bodies.

In most cases, ketonemia is due to increased production of ketone bodies by the liver rather than to a deficiency in their utilization by extrahepatic tissues. While acetoacetate and D(-)-3-hydroxybutyrate are readily oxidized by extrahepatic tissues, acetone is difficult to oxidize in vivo and to a large extent is volatilized in the lungs.

Both β-hydroxybutyrate and acetoacetate are organic acids. These compounds are released in the protonated form, which means that their release tends to lower the pH of the blood. In  normal  individuals, other mechanisms compensate  for  the increased proton release. Individuals with untreated Type I diabetes mellitus often release  ketone  bodies  in  such  large  quantities  that the normal  pH-buffering mechanisms are overloaded; the reduced pH, in combination with a number of other metabolic  abnormalities  associated  with lack  of  insulin results  in  diabetic ketoacidosis, a life-threatening acute disorder of Type I diabetes. In most cases, the increase in ketone body concentration in blood is due to increased synthesis in liver; in severe ketoacidosis, cells begin to lose ability to use ketone bodies also.

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Ketosis

Q. Give the details of the energy yield from the complete oxidation of ketone bodies.

Answer-The energy provided to the peripheral tissues from acetoacetate and for beta-hydroxybutyrate are shown below:

Reaction	Energy product	Multiplier	ATP equivalents
Co A transferase	- GTP (No GTP formation  from Succinyl coA to Succinate in TCA cycle	1	-1
Acetyl co A OxidationIn TCA cycle	12 ATP	2	24
Acetoacetate oxidationTotal	-	-	23
β-OH ButyrateDehydrogenase	1NADH-3ATP	1	3
Total β-OH Butyrate oxidation	-	-	26

This may be appreciated when it is realized that complete oxidation of 1 mol of palmitate involves a net production of 129 mol of ATP via beta oxidation and CO₂ production in the citric acid cycle, whereas only 23 mol of ATP are produced when acetoacetate is the end product and only 26 mol when 3-hydroxybutyrate is the end product. Thus, ketogenesis may be regarded as a mechanism that allows the liver to oxidize increasing quantities of fatty acids within the constraints of a tightly coupled system of oxidative phosphorylation.

Q.- Discuss in brief about  the regulation of ketosis.

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What is the effect of insulin, glucagon, or epinephrine upon lipolysis in adipose tissue? How can a decrease in the insulin/glucagon ratio explain the increased production of ketone bodies during a fast?

Answer- Ketogenesis is regulated at three steps-

(1) Lipolysis in Adipose tissues- Ketosis does not occur unless there is an increase in the level of circulating free fatty acids that arise from lipolysis of triacylglycerol in adipose tissue. Fatty acid release from adipose tissue is controlled via the activity of hormone-sensitive lipase (HSL). When glucose levels fall, pancreatic glucagon secretion increases resulting in phosphorylation of adipose tissue HS (Figure-1), thus resulting in increased hepatic ketogenesis due to increased substrate (free fatty acids) delivery from adipose tissue. Conversely, insulin, released in the well-fed state, inhibits ketogenesis via the triggering dephosphorylation and inactivation of adipose tissue HSL.

Figure-1- Showing the degradation of Triglycerides in adipose cell by hormone sensitive lipase. Hormone sensitive lipase exists in two forms inactive- dephosphorylated (brought by Insulin) and active phosphorylated form (brought by glucagon, ACTH and catecholamines). Insulin promotes lipogenesis while the other hormones  promote lipolysis.

Free fatty acids are the precursors of ketone bodies in the liver. The liver, both in fed and in fasting conditions, extracts about 30% of the free fatty acids passing through it, so that at high concentrations (After high fat diet or in conditions of excessive lipolysis) the flux passing into the liver is substantial. Therefore, the factors regulating mobilization of free fatty acids from adipose tissue are important in controlling ketogenesis.

2) Fate of  Fatty acid- After uptake by the liver, free fatty acids are either oxidized to CO₂ or ketone bodies or esterified to triacylglycerol and phospholipid.  If the liver has sufficient supplies of glycerol-3-phosphate, most of the fats will be turned to the production of triacylglycerols.

There is regulation of entry of fatty acids into the oxidative pathway by carnitine Acyl transferase-I (CAT-I), and the remainder of the fatty acid taken up is esterified. CAT-I activity is low in the fed state, leading to depression of fatty acid oxidation. Malonyl-CoA, the initial intermediate in fatty acid biosynthesis (Figure-2), formed by acetyl-CoA carboxylase in the fed state, is a potent inhibitor of CAT-I . Under these conditions, free fatty acids enter the liver cell in low concentrations and are nearly all esterified to acylglycerols and transported out of the liver in very low density lipoproteins (VLDL).

Figure-2- Showing the inhibition of CAT-1 by Malonyl Co A

However, CAT-1 activity is higher in starvation, allowing fatty acid oxidation to increase. Since the concentration of free fatty acids increases with the onset of starvation, acetyl-CoA carboxylase is inhibited directly by acyl-CoA, and malonyl-CoA level decreases, releasing the inhibition of CAT-I and allowing more acyl-CoA to be oxidized. These events are reinforced in starvation by decrease in the[insulin]/[glucagon] ratio.  

Figure-3-Showing the regulation of Acetyl co A carboxylase by covalent modification. During starvation glucagon causes inhibition of Acetyl co A carboxylase by c AMP dependent phosphorylation. Reverse occurs in the presence of Insulin.

Glucagon In addition, results in phosphorylation and inhibition of acetyl-CoA carboxylase (ACC), the rate limiting enzyme of de novo fatty acid synthesis (Since the enzymes gets inactivated upon phosphorylation). Conversely, under conditions of insulin release, in fed state, hepatic ACC is activated, by dephosphorylation and the excess acetyl-CoA  is converted into malonyl-CoA and then to fatty acids, CAT-1 is inhibited and fatty acid oxidation is also inhibited. Thus, -oxidation from free fatty acids is controlled by the CAT-I gateway into the mitochondria.

(3) Fate of Acetyl co A- In turn, the acetyl-CoA formed in  beta-oxidation is oxidized in the citric acid cycle, or it enters the pathway of ketogenesis to form ketone bodies. If the hepatic demand for ATP is high the fate of acetyl-CoA is likely to be further oxidation to CO₂. This is especially true under conditions of hepatic stimulation by glucagon which results in increased gluconeogenesis and the energy for this process is derived primarily from the oxidation of fatty acids supplied from adipose tissue.

As the level of serum free fatty acids is raised, proportionately more free fatty acids are  converted to ketone bodies and less are oxidized via the citric acid cycle to CO₂. The partition of acetyl-CoA between the ketogenic pathway and the pathway of oxidation to CO₂ is so regulated that the total free energy captured in ATP which results from the oxidation of free fatty acids remains constant as their concentration in the serum changes.

Q.- Enlist the conditions causing ketosis,  Discuss the underlying defect in each of them responsible for causing ketosis.

Answer- The production of ketone bodies occurs at a relatively low rate during normal feeding and under conditions of normal physiological status. Ketosis is basically observed in conditions of glucose deprivation and excess lipolysis.

A) Conditions causing glucose deprivation- Normal physiological responses to carbohydrate shortages cause the liver to increase the production of ketone bodies from the acetyl-CoA generated from fatty acid oxidation. This allows the heart and skeletal muscles primarily to use ketone bodies for energy, thereby preserving the limited glucose for use by the brain. The common causes of glucose deprivation are as follows-

a) Starvation

b) Chronic alcoholism

c) Von- Gierke’s disease

d) Heavy exercise

e) Low carbohydrate diet- For weight loss

f) Glycogen storage disease type 6(Due to phosphorylase kinase deficiency)

g) Pyruvate carboxylase deficiency

B) Conditions causing excessive Lipolysis- All conditions causing hypoglycemia cause lipolysis to compensate for the energy needs, but in uncontrolled diabetes mellitus (Type 1 especially) glucose is available yet cannot be utilized due to insulin deficiency. There is an imbalance between Insulin to Glucagon ratio. Excess glucagon in such conditions induces a state of catabolism , causing lipolysis and thus enhanced ketogenesis. Similarly extreme stress and glucagon producing tumors can cause ketosis.

C) Prolonged ether anesthesia, toxaemia of pregnancy and certain conditions of alkalosis associated with excessive vomiting can also cause ketosis.

D) Nonpathologic forms of ketosis are found under conditions of high-fat feeding and after severe exercise in the post absorptive state.

Q. What is the biochemical basis of ketosis in prolonged fasting or starvation

Answer-Prolonged fasting may result from an inability to obtain food, from the desire to lose weight rapidly, or in clinical situations in which an individual cannot eat because of trauma, surgery, neoplasms, burns etc. In the absence of food the plasma levels of glucose, amino acids and triacylglycerols fall, triggering a decline in insulin secretion and an increase in glucagon release. The decreased insulin to glucagon ratio, and the decreased availability of circulating substrates, make this period of nutritional deprivation a catabolic state, characterized by degradation of glycogen, triacylglycerol and protein. This sets in to motion an exchange of substrates between liver, adipose tissue, muscle and brain that is guided by two priorities (i) the need to maintain glucose level to sustain the energy metabolism of brain ,red blood cells and other glucose requiring cells and (ii) to supply energy to other tissues by mobilizing fatty acids from adipose tissues and converting them to ketone bodies to supply energy to other cells of the body (Figure-4)

Figure-4 Showing the distribution of fuels in different tissues during starvation

After about 3 days of starvation, the liver forms large amounts of acetoacetate and d3- hydroxybutyrate. Their synthesis from acetyl CoA increases markedly because the citric acid cycle is unable to oxidize all the acetyl units generated by the degradation of fatty acids. Gluconeogenesis depletes the supply of oxaloacetate, which is essential for the entry of acetyl CoA into the citric acid cycle. Consequently, the liver produces large quantities of ketone bodies, which are released into the blood. At this time, the brain begins to consume appreciable amounts of acetoacetate in place of glucose. After 3 days of starvation, about a third of the energy needs of the brain are met by ketone bodies. The heart also uses ketone bodies as fuel. After several weeks of starvation, ketone bodies become the major fuel of the brain. Acetoacetate is activated by the transfer of CoA from succinyl CoA to give acetoacetyl CoA .Cleavage by Thiolase then yields two molecules of acetyl CoA, which enter the citric acid cycle.

In essence, ketone bodies are equivalents of fatty acids that can pass through the blood-brain barrier. Only 40 g of glucose is then needed per day for the brain, compared with about 120 g in the first day of starvation. The effective conversion of fatty acids into ketone bodies by the liver and their use by the brain markedly diminishes the need for glucose. Hence, less muscle is degraded than in the first days of starvation. The breakdown of 20 g of muscle daily compared with 75 g early in starvation is most important for survival.

A person’s survival time is mainly determined by the size of the triacylglycerol depot.

Q. Discuss in detail about the causes, clinical manifestations, laboratory diagnosis and treatment of diabetic ketoacidosis.

Answer- Explained in Complications of diabetes Mellitus.

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Multiple Choice Questions- Lipid Metabolism

1-  All except one are true about gastric lipase-
a)   Main preduodenal lipase
b)   Hydrolyzes triglycerides containing short and medium chain fatty acids
c)   Primary site of hydrolysis is sn-3ester linkage
d)   Optimum pH is 6.0-7.0                                                  
                                                                                                         (d)
2-  Pancreatic juice does not contain which of the followings-
a)   Bile salts
b)   Phospholipase A2
c)   Lipase and colipase
d)   Lipoprotein lipase                                                            
                                                                                                        (d)
3-  The dietary fats are transported as –
a)   Micelles
b)   Chylomicons
c)   Fatty acid – Albumin complex
d)   Liposomes
                                                                                                       (b)
4-  One and the only energy requiring step in fatty acid oxidation is catalyzed by which of the following enzymes-
a)   Thiolase
b)   Acyl co A dehydrogenase
c)   Thiokinase
d)   Beta-OH Acyl co A dehydrogenase                                              
                                                                                                       (c)
5-  A fatty acid with 14 carbon atoms will undergo how many cycles of beta oxidation
a)   7
b)   4
c)   6
d)   5                                                                                                  
                                                                                                        (c)
6-  Which statement best describes the oxidation of odd chain fatty acids?
a)   Additional specific enzymes are needed for the oxidative process
b)   One carbon is removed in one cycle
c)   End product is propionyl co A
d)   Hydroxy fatty acids are produced
                                                                                                               
                                                                                                        (c)
7-  Which statement out of the followings is incorrect about Zellwegar syndrome?
a)   Results from the absence of functional peroxisomes
b)   Characterized  by hypoglycemia and ketosis
c)   Caused by a defect in the import of enzymes into the peroxisomes
d)   Death occurs within 6 years of life.
                                                                                                             
                                                                                                        (b)
8-   What is the molecular basis of the  statement “fats burn in the flame of carbohydrates”-
a)   Fats are hydrolyzed in the presence of carbohydrates
b)   Fatty acids and glucose are simultaneously oxidized
c)   Acetyl co A is the common product of fattyacid  and glucose oxidation
d)   Acetyl co A is oxidized completely in the presence of oxaloacetate in TCA cycle
                                                                                                           
                                                                                                        (d)      
9-  Which of the following statements correctly describes the enzyme Thiolase?
a)   It yields Acetoacetyl co A as a product
b)   It yields Malonyl co A as a product
c)   Forms Co A ester as a product
d)   Requires beta keto acyl co A as a substrate
                                                                                                         (d)
10-NADPH required for the fatty acid synthesis can be generated from-
a)   HMP pathway
b)   Glycolysis
c)   TCA cycle
d)   All of the above                                                                      
                                                                                                          (a)
11-Which of the followings is not used for fatty acid synthesis?
a)   Cobalamine
b)   NADPH
c)   Biotin
d)   Bicarbonate
                                                                                                        (a)
12-The key regulatory enzyme of fatty acid synthesis is-
a)   Acyl co A synthetase
b)   Acetyl co A carboxylase
c)   Keto acyl synthase
d)   Thioesterase
                                                                                                       (b)
 13-  Malonyl co A is a direct inhibitor of which enzyme of fatty acid oxidation?
a)   Carnitine Acyl Transferase –I
b)   Carnitine Acyl Transferase –II
c)   Thiokinase
d)   None of the above
                                                                                                        (a)
14-When the liver is  actively synthesizing fatty acids, a concomitant decrease in beta oxidation of fatty acids is due to-
a)   Inhibition by end product
b)   Decrease in adipolysis
c)    Inactivation of  specific enzymes of  fatty acid oxidation
d)   Inhibition of translocation between cellular compartments
                                                                                                        (d)
15-  Which out of the followings is the primary ketone body?
a)   Acetone
b)   Acetoacetate
c)   Beta-hydroxy butyrate
d)   Hydroxy Methyl glutarate
                                                                                                          (b)
16-All are conditions of ketosis except one -
a)   Starvation
b)   Uncontrolled diabetes mellitus
c)   High carbohydrate  diet
d)   Von Gierke’s disease
                                                                                                         (c)
17-The key enzyme for the utilization of ketone bodies is-
a)   Thiolase
b)   Thiophorase
c)   Thiokinase
d)   Thioesterase
                                                                                                        (b)
18-The key regulatory enzyme of cholesterol synthesis is-
a)   HMG- Co A synthase
b)   HMG Co A lyase
c)   HMG Co A reductase
d)   Mevalonate kinase
                                                                                                        (c)
19-Choose the incorrect statement about cholesterol synthesis
a)   All the carbon atoms are derived fromAcetyl CoA
b)   Process is cytoplasmic
c)   NADPH is the main coenzyme
d)   Highly expensive energetically
                                                                                                         (b)
20-  The process by which bile acid sequestrants lower serum cholesterol  level  is-
a)   Direct Inhibition of cholesterol synthesis
b)   Promote cholesterol catabolism
c)   Promote cholesterol excretion
d)   Divert cholesterol metabolism towards bile acid formation
                                                                                                          (d)



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Questions Biochemistry

Q.-1- All amino acids except one participate in phase 2 reactions of detoxification
a) Serine
b) Glycine
c) Glutamine
d) Cysteine
(a)
Q.2- Which out of the following pathways helps in reductive biosynthesis
a) Uronic acid pathway
b) HMP pathway
c) Glycolysis
d) All of the above
(b)
Q.3- All of the following clinical manifestations except one are present in hemochromatosis-
a) Bronze discoloration of skin
b) Diabetes mellitus
c) Iron overload
d) Hemolytic anemia
(d)
Q.4- A person on ingestion of Primaquine develops hemolytic anemia, what is the possible defect?
a) Deficiency of Iron
b) Vitamin K deficiency
c) Glucose-6-P dehydrogenase deficiency
d) Vitamin C deficiency
(c)
Q.5- The enzyme responsible for conversion of Biliverdin to Bilirubin is-
a) Bilirubin esterase
b) Bilirubin oxidase
c) Glucuronyl transferase
d) Biliverdin reductase
(d)
Q.6-Biotin is involved in which of the following types of reactions-
a) Deamination
b) Decarboxylation
c) Carboxylation
d) Transamination
(c)
Q.7-The Xanthurenic acid test (Xanthurenic index) can be used to measure pyridoxine deficiency, it involves the metabolism of-
a) Glycine
b) Histidine
c) Tryptophan
d) Tyrosine
(c)
Q.8-McArdle’s disease is characterized by the deficiency of-
a) Muscle phosphorylase
b) Liver phosphorylase
c) Glucose-6-phosphatse
d) Phosphofructokinase
(a)
 Q.9-Out of 24 mols of ATP formed in citric acid cycle, 2 mols of ATP can be formed at substrate level by which of the following reaction?
a) Citrate——> Isocitrate
b) Isocitrate——–>Oxalo succinate
c) Succinate——->  Fumarate
d)Succinyl co A–> Succinate
(d)
 Q.10- All of the following metabolic abnormalities are observed in Diabetes mellitus, except-
a) Increase plasma free fatty acids
b) Increased pyruvate carboxylase activity
c) Decreased PDH complex activity
d) Increased lipoprotein lipase activity
(d)
Q.11-Beta oxidation of odd chain fatty acids yields

a) Succinyl co A
b) Propionyl co A
c) Acetoacetyl co A
d) Dicarboxylic acids
(b)
 Q.12-Iron therapy is ineffective in which of the following conditions-
a) Chronic blood loss
b) Inadequate iron intake
c) Thalassemia major
d)Acute blood loss
(c)
Q.13-Which of the following vitamins is not a component of electron transport chain-
a) Nicotinamide
b) Ubiquinone
c) Biotin
d) Riboflavin
(c)
Q.14-Which of the following enzymes does not have an impaired activity in Vitamin B1 deficiency?
a) Succinatedehydrogenase
b) Pyruvate dehydrogenase
c) Transketolase
d) Alpha keto glutarate dehydrogenase
(a)
Q.15- Considering the citric acid cycle steps between alpha keto glutarate and Malate, how many high-energy phosphate bonds or net ATP molecules can be generated?
a) 5
b) 6
c) 8
d) 10
(b)
Q.16- The standard free energy change (in terms of net ATP production) when glucose is converted to 6CO2 and 6H2O is about how many times as great as the free energy change when glucose is converted to  two lactate molecules ?
a) 2
b) 4
c) 19
d) 10
(c)
Q.17- The rate of flow of electrons through the electron transport chain is regulated by
a)ATP:ADP ratio
b) Concentration of Acetyl co A
c) Feed back inhibition by H2O
d) Catalytic rate of cytochrome oxidase
(a)
 Q.18- The major product of fatty acid synthase complex is-
a) Oleate
b) Palmitate
c) Palmityl co A
d) Stearoyl co A
(b)
Q.19- The primary enzyme for utilization of ketone bodies is-
a) Thiokinase
b) Thioesterase
c) Thiophorase
d) Thiolase
(c)
Q.20-All of the following processes except one are mitochondrial-
a) Glycolysis
b) TCA cycle
c) Beta oxidation of fatty acids
d) Ketogenesis
(a)


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Choose the correct Answer- Enzymes

1) The enzyme that catalyzes the change in location of the functional group from one position to another within a compound is called Isomerase/Mutase.
                                                                                             
(Mutase)
2) The enzyme having low affinity for the substrate will have low/high km.
                                                                                           
(Highkm)
3) The competitive inhibitors increase the Vmax/km but the Vmax/km remains constant.
                                       
(Increase km but the Vmax remains constant)
4) The non- competitive inhibitors decrease the Vmax/km but the Vmax/km remains constant.
                                       
(DecreaseVmax but the km remains constant)
5) The process of induction of an enzyme is a coarse/fine control for regulation of enzyme activity.
                                                                                           
(Coarse control)
6) Arsenate binds to SH group of the enzyme to inhibit its activity and this mode of inhibition is competitive/Non competitive.
                                                                                           
(Noncompetitive)
7) In normal health LDH-1/LDH-2 is in greater concentration in plasma.
                                                                                           
(LDH-2)
8) Transferases are the enzymes which catalyze the transfer of reducing equivalents/groups other than reducing equivalents.
                                                         
(Groups other than reducing equivalents)
9) The enzyme activity increases 2/4 fold with every ten degree rise of temperature
                                                                                           
(2 fold)
10) The reaction rate is zero/first order when the enzyme activity is directly proportional to substrate concentration
                                                                                           
(first-order)
11) The reaction rate is zero/first order when the enzyme activity is independent of substrate concentration
                                                                                           
(Zero order)
12) When the substrate concentration is equal to km the reaction velocity is equal to Vmax/Half Vmax.
                                                                                           
(Half Vmax)
13) Pepsin/Trypsin has an optimum p H of 2.0.
                                                                                           
(Pepsin)
14) Hexokinase/ Glucokinase is inducible
                                                                                           
(Glucokinase)
15) Hexokinase/ Glucokinase is inhibited by feed back inhibition
                                                                                           
(Hexokinase)
16) Alkaline Phosphatase/Acid phosphatase rises in malignancy of prostate gland.
                                                                                           
(Acidphosphatase)
17) LDH/SGOT rises in hemolytic anemias.
                                                                             
(LDH-Lactate dehydrogenase)
18) Methotrexate inhibits dihydrofolate reductase by competitive/Non competitive inhibition.
                                                                                       
(Competitive inhibition)
19) Streptokinase/tPA causes intravascular bleeding as a side effect.
                                                                                         
(Streptokinase)
20) All the Proteolytic/ Lipolytic enzymes are Zymases in nature.
                                                                                     
(Lipolytic enzymes)


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Multiple choice questions- Enzymes

Q.1- Glycogen phosphorylase, which mobilizes glycogen for energy, requires which of the followings as a cofactor?
a)Pyridoxal phosphate
b)Tetra hydro folate
c)Adenosyl Cobalamine
d)Coenzyme A                                                          
 (a)
 Q.2-Choose the incorrect statement about Active Site of an enzyme-
a) The active site is a three-dimensional cleft
b) The active site takes up a large part of thetotal volume of an enzyme
c) Substrates are bound to enzymes by multiple weakattractions
d) The specificity of binding depends on theprecisely defined arrangement of atoms in an active site.
(b)
Q.3- Any of the following processes except one are involved at the active site of an enzyme to accelerate the rate of reaction-
a) Catalysis by BondStrain
b) Catalysis byProximity and Orientation
c) Non covalentcatalysis
d) Acid basecatalysis                                                  
 (c)
 Q.4-A given substrate may be acted upon by a number of different enzymes, each of which uses the same substrate(s) and produces the same product(s). The individual members of a set of enzymes sharing suchcharacteristics are known as-
a)Group specific enzymes
b)Isoenzymes
c)Substrate specific enzymes
d)Allosteric enzymes
(b)
Q.5-A recently diagnosed hypertensive patient has been prescribed an ACE inhibitor(Angiotensin convertase inhibitor) which is known to act by lowering V max,what is the possible mechanism of inhibition of this drug?
a)Competitive
b)Non Competitive
c)Uncompetitive
d)None of the above.
(a)
Q.6-Which statement out of the followings is incorrect about the effect of increasing temperature on enzyme activity-
a)Raising the temperature increases the kinetic energy of molecules
b) Aten degree Centigrade rise in temperature will increase the activity of mostenzymes by 50 to 100%.
c)Most animal enzymes rapidly become denatured at temperatures above 40oC
d)Storage of enzymes at 5°C or below is generally not suitable.
(d)
Q.7-A54-year –old male was rushed to emergency when he collapsed in the middle of abusiness meeting. Examination revealed excessive sweating and high bloodpressure.ECG chest was conclusive of Acute Myocardial infarction. Which biochemical investigation out of the followings would  be of no help inthe confirmation of diagnosis?
a) Cardiac Troponins
b)Serum myoglobin
c)Lactate dehydrogenase
d)Creatine Phospho kinase-MB(CPK-MB)
 (c)
Q.8-A coal mine worker was brought in an unconscious state to emergency room aftera blast in the mine. His blood Carboxy hemoglobin level was high and he wasdiagnosed with CO poisoning.  CO is aknown inhibitor of electron transport chain. Which complex of electrontransport chain is inhibited by CO?
a)Complex I
b)Complex II
c)Complex III
d)Complex IV
(d)
Q.9-A 42-year-old obese female presented to theemergency center with complaints of worsening nausea, vomiting, and abdominal pain.Her pain was located in the midepigastric area and right upper quadrant. Blood biochemistry revealed high serum amylase level.What is the probable diagnosis for this patient?
a)Viral hepatitis
b)Acute Pancreatitis
c)Renal colic
d)Acute gastritis
(b)
Q.10-A 60 year old chronic alcoholic was brought to thehospital with complaints of protuberant abdomen (ascites) and edema feet. He also had history of hemorrhages.  Blood biochemistry revealed – High serumtransaminases, low Serum total proteins, Albumin and a prolonged prothrombintime. Urine analysis was normal. What could be the possible diagnosis?
a)Renal failure
b)Protein malnutrition
c)Cirrhosis of liver
d)Heart failure
(c)
Q.11-A 2 -week –old child was brought to theemergency. The parents were fearful that the child had been given some poisonas they noted black discoloration on the diaper. Adiagnosis of Alkaptonuria was made and the child was given Vitamin C as asupplement. Alkaptonuria occurs due to reduced activity of Homogentisic acid oxidaseenzyme. What is the role played by vitamin C in this defect?
a)Acts as an oxidant
b)Acts as a coenzyme
c)Acts as an inducer
d)Acts as a positive allosteric modifier
(b)
Q.12-A67- year-old army officer in good health previously presented with sudden painin the great toe.  Serum uric acid levelwas high, and a diagnosis of gouty arthritis was made He was advised bed rest,pain killers and Allopurinol. What is the mechanism of action of Allopurinol inlowering serum uric acid levels?
a)Suicidal inhibition
b)Non competitive inhibition
c)Allosteric inhibition
d)Feed back inhibition
(a)
Q.13-One out of the following enzymes has absolute specificity for its substrate; choose thecorrect option-
a) Urease
b) Carboxy peptidase
c) Pancreatic lipase
d)Lipoproteinlipase
(a)
Q.14-Which out of the followings is a substrate-specific enzyme?
a) Hexokinase
b)Thiokinase
c) Lactase
d)Decarboxylase
(c)
Q.15-Which out of thefollowings is not a substrate-specific enzyme?
 a)Glucokinase
 b)Fructokinase
 c) Hexokinase
 d)Phosphofructokinase
(c)
Q.16-Group I Co enzymes participate in  which of the followingreactions-
a) Oxidation-reduction
b)Transamination
c) Phosphorylation
d) All of the above
(a)
Q.17-Which out of thefollowing co enzymes takes part in hydrogen transfer reactions in the electron transport chain-
a) Tetrahydrofolate
b) Methyl Cobalamine
c)Co enzyme Q
d) Biotin
(c)
Q.18.The conversion of Pyruvate to oxaloacetate involves the participation of—-  as a coenzyme -
a) NAD+
b) NADPH
c) Biotin
d) All of the above                                                        
(c)
Q.19- The drugFluorouracil is recommended for the treatment of cancers. It undergoes a seriesof changes and then binds to Thymidylate synthase enzyme resulting in itsinhibition and blockage of cell division. This mode of inhibition is mostprobably due to-
a) Allosteric inhibition
b) Competitive inhibition
c) NoncompetitiveInhibition
d) Suicidal inhibition                                                      
(d)
Q.20-The activities ofmany enzymes, membrane transporters and other proteins can be quickly activatedor inactivated by phosphorylation of specific amino acid residues. This regulation is called-
a) Allostericmodification
b) Covalent modification
c) Induction
d) Repression
                                                                                                           
(b)


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