In this article we will discuss about the process of biosynthesis of fatty acids, explained with the help of suitable diagrams.
Synthesis of Saturated Fatty Acids:
It must be pointed out at the very outset that the biosynthesis of fatty acids does not generally take place by the reactions — in the reverse direction — of β-oxidation; the latter are indeed reversible in mammals, except the one catalyzed by acyl-coA dehydrogenase, but there is a dehydroacyl-coenzyme A-reductase, a NADPH enzyme, which can permit reduction on the double bond; it however appears that this mode of formation of fatty acids is of relatively limited importance.
But the reverse pathway of β-oxidation is of great physiological interest, as it permits the elongation of pre-existing, medium-chain fatty acids, leading to stearic acid (C18), one of the principal saturated fatty acids of tissues, and to long-chain (C20 to C26) fatty acids. This system is intra-mitochondrial. However, in some organisms the short-chain fatty acids can be synthesized by the reactions – in the reverse direction of β-oxidation.
In mammals, the major pathway of the biosynthesis of fatty acids is an extra-mitochondrial process (cytosolic and/or microsomal). To produce fatty acids from the precursor which is acetyl-coA, the cells must be able to reduce the ketone groups: this will be achieved thanks to NADPH; they must also be able to form C— C bonds in order to condense acetyl radicals: although the methyl group of acetyl-coA is capable of binding to a carbonyl, this is not the reaction used for obtaining chains of fatty acids; the cells use a more reactive intermediate, malonyl-coA.
The synthesis of malonyl-coenzyme A consists in the binding of a molecule of CO2 to a molecule of acetyl-coA, catalyzed by acetyl-coA-carboxylase, a biotine enzyme, in presence of ATP, according to the mechanism described in figure 5-16. This is an example of CO2 binding which can be carried out by living beings (another example in connection with the transformation of pyruvic acid into oxaloacetic acid by pyruvate-carboxylase).
In procaryotes, the acyl groups of acetyl-coA and malonyl-coA are transferred, respectively by an acetyl-transferase and a malonyl-transferase, to a small protein (M.W. # 9 000) called Acyl Carrier Protein or ACP.
The prosthetic group of this protein is phosphopantetheine bound by an ester linkage between its phosphate group and the hydroxyl of a serine of the protein. Phosphopantetheine bears a close resemblance to coenzyme A; the sulphydryl group is again the active part in the binding and transfer of acyl groups (see fig. 5-17).
Acetyl-ACP and malonyl-ACP then react with acyl-synthetase, responsible for the synthesis of fatty acids. Transfer of acyl groups to the polyenzymatic complex takes place, without any free intermediate at any time.
In the case of mammals and numerous other eucaryotes studied, the process is simplified. There is no intervention of any acyl group carrier protein (ACP). The acetyl-coA and malonyl coA (the latter being synthesized by acetyl-coA carboxylase) react directly with acyl-synthetase. This enzyme is a multienzymatic complex of M.W. = 2.3 x 106, which possesses “binding” sites ending by a — SH group: a cysteine radical or a phosphopantetheine radical.
A transfer of acetyl and malonyl radicals therefore takes place from a — SH group (that of the coenzyme A) directly to another (that of the synthetase). The first condensation can take place as indicated in figure 5-18. It must be noted that this condensation is accompanied by a decarboxylation affecting the CO2 previously bound by the action of acetyl-coA-carboxylase, which is therefore not incorporated in the fatty acids.
The following reactions, presented in figure 5-19, then take place:
1. A reduction of acetoacetyl-Enz. to D-β-hydroxybutyryl-Enz.,
2. Adehydration of D-β-hydroxybutyryl-Enz. to crotonyl-Enz., an α-β unsaturated derivative in trans configuration, catalyzed by enoyl-hydratase
3. A reduction of crotonyl-Enz. to butyryl-Enz.,
This series of reactions obviously point to those constituting one turn of β-oxidation, in the reverse direction. But 3 important differences must be noted:
1. Here the intermediates are directly linked to the enzyme (and not to the coenzyme A),
2. The coenzyme of the reduction reactions is NADPH (and not FADH2 or NADH),
3. The β-hydroxylated compound has a D-configuration (not L).
The butyryl-Enz. thus formed reacts with another molecule of malonyl-coA (which is transferred to one of the SH of the enzyme) according to a process similar to the one described in fig. 5-18. Another turn of the helix will produce a chain of fatty acid lengthened by two carbon atoms (i.e. a chain in C6) and so on.
When the fatty acid formed has a particular length, it is liberated from the polyenzymatic complex by the action of deacylase, present in the acylsynthetase complex. The main fatty acid generally formed is palmitic acid (C16). The synthesized fatty acids can either be used for the synthesis of glycerides or other lipids, or carried into the mitochondria to be lengthened or catabolized.
This transport takes place in the form of an ester between the alcohol group of carnitine: COOH-CH2-CHOH-CH2-N ≡ (CH3)3 and the fatty acid, called acylcarnitine. The esterification reaction is catalyzed by carnitine palmityl transferase.
Plants possess a double system. The cytosolic system uses acetyl-coenzyme A. The system located in the chioroplast uses acetyl-ACP. The biosynthesis reactions are similar to those described above. Palmitic acid and stearic acid are formed. The latter is lengthened (probably in the endoplasmic reticulum) by a system requiring malonyl coA.
Synthesis of Unsaturated Fatty Acids:
A. Monounsaturated Fatty Acids:
There are 2 systems, one anaerobic present in some bacteria (E.coli), the other aerobic present in all other cells.
The anaerobic synthesis is carried out by the enzymatic complex synthesizing the saturated fatty acids with the following variant: the 10 carbon atom β-hydroxyacyl-ACP will be dehydrated (see fig. 5-19) to give simultaneously a α, β-dehydroacyl-ACP (C10, ∆2) and a β, γ-dehydroacyl-ACP (C10, ∆3). Only the former will be reduced by NADPH + H+, the latter maintains its double bond and will be again lengthened in the conventional manner.
One will therefore obtain successively:
The aerobic system permits the unsaturation of long-chain fatty acids. A double bond is generally introduced between the carbons 9 and 10 of palmitic and stearic acids providing palmitoleic (C16, ∆9) and oleic (C18, ∆9) acids. One of the characteristics of the unsaturation enzyme is that it requires both molecular oxygen and a reduced coenzyme (NADPH + H+).
B. Polyunsaturated Fatty Acids:
As far as polyunsaturated fatty acids are concerned, only non-bacterial microorganisms and plants are capable of synthesizing linoleic acid (C18, ∆9,12) and α-linolenic acid (C18, ∆9,12,15) by unsaturation of oleic acid. Some insects synthesize linoleic acid. The synthesis of oleic and linoleic acids takes place in the endoplasmic reticulum, whereas that of linoleic acid takes place in the chloroplasts and seems to be linked with chlorophyll synthesis.
Linoleic and linolenic acids which are not synthesized by several groups of animals (numerous insects, mammals…) are called essential fatty acids. Contrary to plants, animals can introduce new double bonds in these two essential fatty acids to give polyunsaturated fatty acids like arachidonic acid (C20 ∆5,8,11,14) or docosahexaenoic acid (C22 ∆4,7,10,13,16,19).
This biosynthesis is microsomal. It takes place by a series of reactions wherein unsaturations and elongations alternate [ex: 18:2 (9,12) → 18:3 (6,9,12) → 20:3 (8,11,14) → 20:4 (5,8,11,14) → 22.5 (7,10,13,16) → 22.6 (4,7,10,13,16)]. An identical diagram is operative if one starts from linolenic acid 18:3 (9, 12, 15). The unsaturation enzymes require both molecular oxygen and a reduced coenzyme (NADPH). Lengthening takes place by the pathway involving malonyl-coenzyme A.
Regulation of the Metabolism of Fatty Acids:
The fact that the pathway of biosynthesis of fatty acids is different from the pathway of oxidation allows — as in the case of the biosynthesis and degradation of glycogen, or in glycolysis and neoglucogenesis — an independent regulation of the 2 processes. These regulation mechanisms will not be studied here, but some important factors may be mentioned.
The biosynthesis of fatty acids requires NADPH which is mainly provided by the oxidation of glucose in the pentose-phosphates cycle. It also requires energy and therefore the presence of ATP, supplied by the oxidation of carbohydrates (or, in plants, by photosynthesis); if ATP concentration decreases (and therefore ADP concentration increases), biosynthesis slows down, but on the contrary β-oxidation is stimulated, which will lead to a rise in ATP concentration.
The reaction catalyzed by acetyl-coA-carboxylase is the limiting step of the biosynthesis of fatty acids. This enzyme is activated by citric acid or insulin, but inhibited by glucagon or fatty acids, whether they be the terminal products of acylsynthetase (feedback inhibition mechanism) or of exogenous origin, for example nutritional.
An accumulation of fatty acids may also result from a deficiency of L-α-glycerophosphoric acid which — as will be seen in the following paragraph — is the compound to which the acyl-coA bind in the biosynthesis of glycerides and glycerophospholides; but this compound is formed from triosesphosphates (see fig. 4-32).
It is important to note that the 3 factors we have just mentioned (NADPH, ATP, L-α-glycero-phosphoric acid) are provided for their major part, by the metabolism of carbohydrates, which underlines the close relations existing between carbohydrates and lipids in respect of metabolism and regulation (not forgetting the important link represented by acetyl-coA).
Synthesis and catabolism of fatty acids are 2 competitive mechanisms which are regulated, at least in mammals. In order to penetrate into the mitochondrion, the fatty acids must be in the form of acyl carnitine. Malonyl coA, an intermediate in the biosynthesis, is a powerful inhibitor of carnitine palmityl transferase, thus blocking the β oxidation. When synthesis stops, the fatty acids can be esterified by carnitine and penetrate into the mitochondrion where they will be catabolized.