Glycolysis

we study carbohydrate digestion and know that the end of digestion is production of Glucose, Fructose and Galactose

Also we know that glucose enters the liver through the hepatic portal vein and glucose should be oxidized through glycolysis

Glycolysis

A)  Definition:

It is the breakdown of glucose in the cell cytosol (= cytoplasm) producing pyrurate in the presence of oxygen or lactate in the absence of oxygen.

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B)   Site:

Glycolysis occurs in cytoplasm but:

• In presence of O₂, oxidation of glucose is complete in mitochondria where pyruvate enters the kerb’s cycle and the electron transport chain to complete the oxidation of glucose resulting in a high amount of energy.

• In absence of O­₂, pyruvate is converted into lactate in the cytoplasm giving a small amount of energy but it is important to some tissue.

Therefore:

Occurrence of glycolysis is of physiological importance in:

1. Tissues with no mitochondria such as RBCs, cornea and lens.

2. Tissues with few mitochondria: Testis, leucocytes, medulla of the kidney, retina, skin and gastrointestinal tract

3. Tissues undergo frequent oxygen lack: skeletal muscles especially during exercise.

Where they depend only on the glycolysis not on kerbs cycle and the electron transport chain

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C)    Stages of glycolysis:

<<<<<<<Stage (I)>>>>>>>

It is the energy requiring stage

in this stage:

• One molecule of glucose is converted into two molecules of glyceraldehyde-3-phosphate.

• These steps consume 2 molecules of ATP .

Step one:

1.      Event: (glucose phosphorylation)­

===> A phosphate group is transferred from ATP molecule to the carbon number 6 in the glucose molecule forming glucose-6-phosphate, thus this step is energy consuming.

===> Glucose-6-phosphate is an intermediate forming an important branch point in the metabolism.

===> This step is fast irreversible step

2.  Enzymes stimulating this step:

===> The enzymes used are either glucokinase or hexokinase enzymes which are responsible for entry of glucose into the cell and phosphorylation of glucose which lead to glucose trapping inside the cell, therefore this step is irreversible because if it is reversible the glucose-6-ph will return to glucose and could exit from the cell again.

===>They are activated after a carbohydrate rich meal for 2 hours to lower the glucose blood level.

NOTE: Kinase enzyme always add phosphate group on the substrate

What is the difference between Hexokinase and glucokinase? (click to open)

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ٍStep 2:

1.   Event: (formation of fructose-6-phosphate from glucose-6-phosphate)­

Isomerization of glucose-6-phosphate to fructose 6-phosphate, I.e. a conversion of an aldose into a ketose by phosphoglucose isomerase.

It doesn’t need energy because it occurs spontaneosly.

2.   Enzymes stimulating this step: phosphoglucose isomerase.

NOTE: Isomerase enzyme always catalyzes the structural rearrangements.

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Step 3:

a.  Event: (Phosphorylation of Fructose-6-phosphate to fructose-1,6-­bisphosphate)­

-  Phosphorylation of Fructose-6-phosphate by ATP to fructose-1,6-­bisphosphate (F-1 ,6-BP)

-   This step is irreversible.

b.  Enzymes stimulating this step: by phosphofructokinase (PFK)

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step 4:

a. Event: (cleave of 6-carbon sugar into two 3-carbon fragments)­

-  Splitting of fructose-1,6-bisphosphate into tow 3-carbon fragments:

• Glyceraldehyde 3-phosphate (GAP)
• Dihydroxyacetone phosphate (DHAP)

-   Reversible under intracellular conditions

b.  Enzymes stimulating this step:

-  Aldolase. (This enzyme derives its name from the nature of the reverse reaction, an aldol condensation).

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Step 5:

a.  Event: (isomerisation of (GAP) to (DHAP))­

-  Isomerisation of Glyceraldehyde 3-phosphate (GAP) to Dihydroxyacetone phosphate (DHAP)

-  It is fast reversible step.

b. Enzymes stimulating this step: triose phosphate isomerase (TPI or TIM).

c.  Importance of this step:

To get full energy from glucose molecules in other words to get energy from the 6 carbons. EXPLAIN?

-   The cell can’t oxidize DHAP to get energy from it , thus it will be lost as a waste product.

-    Thus, the cell will obtain energy from GAP only (i.e. from 3 carbon of the glucose), not from the 6-carbons.

-    But, the GAP will continue the glycolysis and give energy, while DAHP is not

-     Thus, in order to get energy from the 6-carbons of glucose, the DHAP must be converted to GAP

After this step we get 2 molecules of Glyceraldehyde 3-phosphate (GAP):

One form step 4 and one form step 5.

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<<<<<Stage (II)>>>>>

(The energy producing stage)

In this stage:

Tow moleculse of Glyceraldehyde 3-phosphate (GAP) is converted into two molecules of pyruvate producing 4 molecules of ATP.

 

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Step 6

a.  Event: (Oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate)

Conversion of glyceraldehyde-3-phosphate into 1,3-­bisphosphoglycerate (1,3-BPG), a reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase.

b.  Enzymes stimulating this step: glyceraldehyde 3-phosphate dehydrogenase.

Note that, the phosphate used in this step is inorganic phosphate.

The oxidation of the aldehyde to an acid is coupled to the reduction of NAD⁺ to NADH/H⁺

Importance of 1,3-bisphosphoglycerate:

- During the glycolysis inside the RBCs, part of the 1,3-bisphosphoglycerate is converted into 2,3-bisphosphoglycerate by the enzyme bisphosphoglycerate mutase

-   2,3-BPG decreases the affinity of Haemoglobin for Oxygen (i.e. decreases the attachment of O₂ to haemoglobin) thus the oxygen can leave hemoglobin easily and this is good in case of oxygen shortage because cells will be able to get their needs of O₂ easily even if the oxygen supply is low.

Clinical and physiological aspects of 2,3-BPG تطبيقات للكلام اللى قولناه

1. smokers and people who live in high altitude, where the Hb increase in each RBC and the number of RBCs increase and increase the amount of 2,3-bisphosphoglycerate leading to decrease the affinity of Hb for O₂ causing dissociation of O₂ form Hb easily into the blood capillary and thus the cells takes its need easily.

2. Fetus gets oxygen form the mother thus fetal Hb has a high affinity for O₂because it has a plenty of O₂ from his mother, thus competing with the Hb of the mother because the oxygen leave the Mother Hb because of it low affinity and attach to the Fetus Hb which has a high affinity for the O₂.

3. The sotred blood for blood transfusion has diminished levels of 2,3-BPG, thus before transfusion of inosine or the glycolytic substrate dihydroxyphosphate to regenerate 2,3-BPG before blood transfusion because in absence of 2,3-BPG suffocation اختناق occurs.

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step 7:

a.  Event: (Conversion of 1,3-bisphosphoglycerate to 3­phosphoglycerate)

-  Conversion of 1,3-bisphosphoglycerate to 3-­phosphoglycerate

-  This step gproduce one ATP molecule.

b.  Enzymes stimulating this step: phosphoglycerate kinase.

Note that:

Formation of ATP from transferring a Phosphate group to ADP from organic substrate is called Substrate level phosphorylation.

Formation of ATP from by transferring a Phosphate group to ADP from NAD or FAD in the electron transport chain is called Oxidative phosphorylation.

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Step 8:

a.   Event: (Conversion of 3-phosphoglycerate to 2­-phosphoglycerate)

Shifting the phosphate group from the carbon number 3 to the carbon number 2

a.   Enzymes stimulating this step: phosphoglycerate mutase.

The importance of this step is to make the compound suitable for the active site of the enolase enzyme.

NOTE: Mutase shifting of a functional group from one position to another within the same molecule

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ٍStep 9:

a.  Event: (2-Phosphoglycerate is dehydrated to form phosphoenolpyruvate)

The dehydration of the alcohol produces a double bond between carbons 2 and 3 and creates a high-energy enol phosphate linkage.

b.  Enzymes stimulating this step: enolase.

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Step 10:

a.  Event: (Conversion of phosphoenol pyruvate to pyruvate)

Conversion of phosphoenol pyruvate to pyruvate producing one ATP molecule.

It is irreversible step.

b.  Enzymes Stimulating this step: Pyruvate Kinase (transfer Phosphate group from phosphoenolo-pyruvate to ADP molecule to produce one ATP Molecule)

فالخطوة دى برضو اتكون عندنا kinase يعنى هينقل مجموعة فوسفات وبالفعل هو نقل مجموعة فوسفات من الphosphoenolo-pyruvate الى جزئ ADP عشان يدينا جزء واحد من الATP.

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D)  Calculation of ATP molecules from glycolysis:

We know that we get from STAGE (I) 2 molecules of glyceraldehyde-3-phosphate and we know that each molecules when continue glycolysis in the STAGE(II) give 2 ATP molecule, thus the 2 molecule of glyceraldehyde-3-phosphate give 4 ATP molecules.

Also we know that the steps number 1 and 3 consume 2 molecule of ATP

Thus, the net product of ATP during glycolysis is 2 ATP

4 ATP form Stage (II) – 2 ATP from Stage (I) = 2 ATP

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E)    Regulation of Glycolysis:

Glycolysis pathway is regulated by:

A) Control of 3 enzymes that catalyze the 3 irreversible steps of glycolysis:

1. Hexokinase (step 1),

2. Phosphofructokinase (step 3)

3. Pyruvate Kinase (step 10).

B) Energy regulation:

1. High level of ATP inhibits  phosphofructokinase (PFK-1) and pyruvate kinase.

2. High level of ADP and AMP stimulate PFK.

C) Substrate regulation:

1.  Glucose-6-phosphate inhibits hexokinase (and not glucokinase).

2. Fructose 1,6 bisphosphate stimulates phosphofructokinase-1.

3. Citrate inhibits phosphofructokinase-1.

4. Fructose 1,6 bisphosphate stimulates pyruvate kinase.

D) Hormonal regulation:

1. Insulin: Stimulates synthesis of all key enzymes of glycolysis. It is secreted after meal (in response to high blood glucose level).

2. Glucagon: Inhibits the activity of all key enzymes of glycolysis. It is secreted in response to low blood glucose level.

------------------------------------------ Best Wishes: Dr.Ehab Aboueladab, Tel:01007834123 Email:ehab10f@gmail.com,ehababoueladab@yahoo.com ------------------------------------------