General structure and classification of lipoproteins

Lipids absorbed from the diet and synthesized by the liver and adipose tissue must be transported between various cells and organs for utilization and storage. Since lipids are insoluble in water, the problem of transportation in the aqueous plasma is solved by associating nonpolar lipids (triacylglycerols and cholesteryl esters) with amphipathic lipids(phospholipids and cholesterol) and proteins to make water-miscible Lipoproteins.

General Structure of Lipo protein

 Lipoproteins Consist of a Nonpolar Core & a Single Surface Layer of Amphipathic Lipids

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules (Figure-1). These are oriented so that their polar groups face outward to the aqueous medium. The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein,constituting nearly 70% of some HDL and as little as 1% of Chylomicons. Some apolipoproteins are integral and cannot be removed, whereas others can be freely transferred to other lipoproteins.

 

Figure-1- showing general structure of lipoprotein

 Classification of Lipoproteins

 Lipoproteins can be classified in three ways-

 1) Based on density

Because fat is less dense than water, the density of a lipoprotein decreases as the proportion of lipid to protein increases.  Lipoproteins with high lipid content will have low density and so float on centrifugation. Those with high protein content sediment easily and have a high density. They are separated by Ultracentrifugation. Depending upon the floatation constant (Sf), Five major groups of lipoproteins have been identified that are important physiologically and in clinical diagnosis. These are

 (i) Chylomicons, derived from intestinal absorption of triacylglycerol and other lipids; Density is generally less than0.95 while the mean diameter lies between 100- 500 nm

 (ii) Very low density lipoproteins(VLDL), derived from the liver for the export of triacylglycerol; density lies between 0.95- 1.006 and the mean diameter lies between 30-80 nm.

 (iii) Intermediate density lipoproteins (IDL) are  derived from the catabolism of VLDL,with a density  ranging intermediate between Very low densityand Low density lipoproteins i.e. ranging between 1.006-1.019 and the meandiameter ranges between 25-50nm.

 (iv)Low-density lipoproteins (LDL), representing a final stage in thecatabolism of VLDL; density lies between 1.019-1.063 and mean diameter lies between 18-28 nm

 (iv) High-density lipoproteins (HDL),involved in cholesterol transport and also in VLDL and chylomicron metabolism. Density ranges between 1.063-1.121 and the mean diameter varies between 5-15 nm. (Table)

Figure- 2-showing the relationship of density and mean diameter of lipoproteins

Triacylglycerol is the predominant lipid in chylomicron and VLDL, whereas cholesterol and phospholipid are the predominant lipids in LDL and HDL, respectively. (Table)

 2) Based on electrophoretic mobilities

Lipoproteins may be separated according to their electrophoretic properties into alpha , beta, pre-beta,and broad beta lipoproteins. The mobility of a  lipoprotein is mainly dependent upon protein content. Those with higher protein content will move faster towards the anode and those with minimum protein content will have minimum mobility.

 HDL are alpha , LDLbeta, VLDL pre-beta, and IDL are broad beta lipoproteins. Free fattyacids and albumin complex although not a lipoprotein is an important lipid fraction in serum and is the fastest moving fraction. Chylomicons remain at the origin since they have more lipid content. VLDL with less protein content than LDL move faster than LDL, this is due to nature of apoprotein present.

Table- showing the composition of lipoproteins. As the lipid content increases, density decreases and size increases, that is why Chylomicons are least dense but biggest in size, while HDL are rich in proteins , hence most dense but smallest in size.

 3)Based on nature of Apo- protein content

 One or more apolipoproteins (proteins or polypeptides) are present in each lipoprotein. The major apolipoproteins of HDL Alpha Lipoproteins) are designated A.The main apolipoprotein of LDL (beta -lipoprotein) is apolipoprotein B(B-100), which is found also in VLDL. Chylomicons contain a truncated form of apo B (B-48) that is synthesized in the intestine, while B-100 is synthesized in the liver. Apo B-100 is one of the longest single polypeptide chains known,having 4536 amino acids and a molecular mass of 550,000 Da. Apo B-48 (48% ofB-100) is formed from the same mRNA as apo B-100 after the introduction of a stop signal by an RNA editing enzyme. Apo C-I, C-II, and C-III are smaller polypeptides (molecular mass 7000–9000 Da) freely transferable between several different lipoproteins. Apo E is found in VLDL, HDL, Chylomicons, andchylomicron remnants; it accounts for 5–10% of total VLDL apolipoproteins in normal subjects.

 Functions of Apoproteins- Apolipoproteins carry out several roles: 

 (1) They can form part of the structure of the lipoprotein, eg, apo B is a structural component of VLDL and Chylomicons

 (2)They are enzyme cofactors, e.g. C-II for lipoprotein lipase, A-I for lecithin: cholesterolacyl transferase (LCAT), 

 (3) They act as enzyme inhibitors, eg, apo A-II and apo C-III for lipoprotein lipase, apo C-I for cholesteryl ester transfer protein;

 (4)They act as ligands for interaction with lipoprotein receptors in tissues, eg,apo B-100 and apo E for the LDL receptor, apo A-I for the HDL receptor. The functions of apo A-IV and apo D, however, are not yet clearly defined, although apo D is believed to be an important factor in human neuro degenerative disorders and acts as cholesteryl ester transfer protein required for the exchange of triglycerides and cholesteryl esters between VLDL,chylomicron remnants and HDL.

 All apoproteins are synthesized mainly in liver but small amounts can be synthesized in almost all organs.

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Semester Paper-Significance and details of the reaction catalyzed by Acetyl co A carboxylase

 a)  Acetyl co A carboxylase-Fatty acid synthesis starts with the carboxylation of acetyl CoA to malonylCo A catalyzed by Acetyl co A carboxylase . This irreversible reaction is the committed step in fatty acidsynthesis.

 Acetyl CoA carboxylase, contains a biotin prosthetic group. The carboxyl group of biotin is covalently attached to the  epsilon amino group of a lysine residue, as in pyruvate carboxylase and propionyl CoA carboxylase .The enzyme is a multienzyme protein containing a variable number of identical subunits, each containing biotin, biotin carboxylase, biotin carboxyl carrier protein, and transcarboxylase, as well as a regulatory allosteric site.

 The reaction takes place in two steps: (1) carboxylation of biotin involving ATP and (2) transfer of the carboxyl to acetyl-CoA to form malonyl-CoA.

 Regulation of Acetyl co A carboxylase- This enzyme is also the essential regulatory enzyme for fatty acid metabolism.

 a) Hormonal control- The carboxylase is controlled by three global signals glucagon, epinephrine, and insulin that correspond to the overall energy status of the organism. Insulin stimulates fatty acid synthesis by activating the carboxylase, whereas glucagon and epinephrine have the reverse effect.

 b) Allosteric modification-The levels of citrate, palmitoyl Co A, and AMP within a cell also exert control. Citrate, a signal that building blocks and energy are abundant,activates the carboxylase. Palmitoyl CoA and AMP, in contrast, lead to the inhibition of the carboxylase. Citrate facilitates the polymerization of the inactive octamers into active filaments. The level of citrate is high when both acetyl CoA and ATP are abundant.

The stimulatory effect of citrate on the carboxylase is antagonized by palmitoyl CoA, which is abundant when there is an excess of fatty acids. Palmitoyl CoA causes the filaments to disassemble into the inactive octamers.

Palmitoyl CoA also inhibits the translocase that transports citrate from mitochondria to the cytosol, as well as glucose 6-phosphate dehydrogenase, which generates NADPH in the pentose phosphate pathway

 c) Covalent Modification- is carried out by means of reversible phosphorylation and dephosphorylation mediated by hormonal action. AcetylCoA carboxylase is switched off by phosphorylation and activated by dephosphorylation. Epinephrine and glucagon activate protein kinase A to bring about phosphorylation. Hence, these catabolic hormones switch off fattyacid synthesis by keeping the carboxylase in the inactive phosphorylated state.

Insulin stimulates the carboxylase by causing its dephosphorylation by stimulating phosphatase enzyme.

 d)Response to Diet

Fatty acid synthesis and degradation are reciprocally regulated so that both are not simultaneouslyactive. In starvation, the level of free fatty acids rises because hormones such as epinephrine and glucagon stimulate adipose-cell lipase.  In well fed state, Insulin, in contrast, inhibits lipolysis. Acetyl CoA carboxylase also plays a role in the regulation of fatty acid degradation. Malonyl CoA, the product of the carboxylase reaction, is present at a high level when fuel molecules are abundant. Malonyl CoA inhibits carnitine acyl transferaseI, preventing access of fatty acyl CoA s to the mitochondrial matrix in times of plenty.

 e) Long-term control is mediated by changes in the rates of synthesis and degradation of the enzymes participating in fatty acid synthesis.Animals that have fasted and are then fed high-carbohydrate, low-fat diets show marked increases in their amounts of acetyl CoA carboxylase and fatty acid synthase (Another multienzyme complex of fatty acid bio synthetic pathway) within a few days. This type of regulation is known as adaptive control.

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