Two-carbon unit elongation

Indeed, PDC catalyzes mainly the reaction of pyruvate with benzaldehyde for the formation of (R)-PAC 27. This C—C bond formation, a two-carbon unit elongation, is coupled to the concomitant decarboxylation of pyruvate 26. This reaction is industrially developed for the synthesis of (—)-ephedrine by adding a second step, a reductive amination. Ephedrine (marketed by Merck especially) is a sympathomimetic amine commonly used as a stimulant, appetite suppressant, concentration aid, and decongestant, and it is used to treat hypotension associated with anesthesia. It is similar in structure to the (semisynthetic) derivatives amphetamine and meth-amphetamine. Chemically, it is an alkaloid derived from various plants in the genus Ephedra (family Ephedra-ceae). It works mainly by increasing the activity of noradrenaline on adrenergic receptors. ... 

As seen already, palmitate is the primary product of the fatty acid synthase. Cells synthesize many other fatty acids. Shorter chains are easily made if the chain is released before reaching 16 carbons in length. Longer chains are made through special elongation reactions, which occur both in the mitochondria and at the surface of the endoplasmic reticulum. The ER reactions are actually quite similar to those we have just discussed addition of two-carbon units... 

The first step is carboxylation of acetyl CoA to malonyl CoA. This reaction is catalyzed by acetyl-CoA carboxylase [5], which is the key enzyme in fatty acid biosynthesis. Synthesis into fatty acids is carried out by fatty acid synthase [6]. This multifunctional enzyme (see p. 168) starts with one molecule of ace-tyl-CoA and elongates it by adding malonyl groups in seven reaction cycles until palmi-tate is reached. One CO2 molecule is released in each reaction cycle. The fatty acid therefore grows by two carbon units each time. NADPH+H is used as the reducing agent and is derived either from the pentose phosphate pathway (see p. 152) or from isocitrate dehydrogenase and malic enzyme reactions. 

In non-ruminants, the malonyl CoA is combined with an acyl carrier protein (ACP) which is part of a six-enzyme complex (molecular weight c. 500 kDa) located in the cytoplasm. All subsequent steps in fatty acid synthesis occur attached to this complex through a series of steps and repeated cycles, the fatty acid is elongated by two carbon units per cycle (Figure 3.8, see also Lehninger, Nelson and Cox, 1993). 

Although palmitate, a 16-carbon, fully saturated LCFA (16 0), is to primary end-product of fatty acid synthase activity, it can be further I elongated by the addition of two-carbon units in the endoplasmic] reticulum (ER) and the mitochondria. These organelles use separate enzymic processes. The brain has additional elongation capabilities, allowing it to produce the very-long-chain fatty acids (up to 24 car bons) that are required for synthesis of brain lipids. 

It is of interest to compare two chain elongation processes by which two-carbon units are combined. 

Enzyme complexes occur in the endoplasmic reticulum of animal cells that desaturate at A5 if there is a double bond at the A8 position, or at A6 if there is a double bond at the A9 position. These enzymes are different from each other and from the A9-desaturase discussed in the previous section, but the A5 and A6 desaturases do appear to utilize the same cytochrome b5 reductase and cytochrome b5 mentioned previously. Also present in the endoplasmic reticulum are enzymes that elongate saturated and unsaturated fatty acids by two carbons. As in the biosynthesis of palmitic acid, the fatty acid elongation system uses malonyl-CoA as a donor of the two-carbon unit. A combination of the desaturation and elongation enzymes allows for the biosynthesis of arachidonic acid and docosahexaenoic acid in the mammalian liver. As an example, the pathway by which linoleic acid is converted to arachidonic acid is shown in figure 18.17. Interestingly, cats are unable to synthesize arachidonic acid from linoleic acid. This may be why cats are carnivores and depend on other animals to make arachidonic acid for them. Also note that the elongation system in the endoplasmic reticulum is important for the conversion of palmitoyl-CoA to stearoyl-CoA. 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

This first round of elongation produces the four-carbon butyryl-ACP. The cycle now repeats with malonyl-ACP adding two-carbon units in each cycle to the lengthening acyl-ACP chain. This continues until the 16-carbon palmitoyl-ACP is formed. This molecule is not accepted by the acyl-malonyl-ACP condensing enzyme, and so cannot be elongated further by this process. Instead it is hydrolyzed by a thioesterase to give palmitate and ACP. 

Polyketides are made by the sequential activity of domains of large, multifunctional enzymes called polyketide synthases (PKSs) (Fig. 6a and b). Polyketides are formed by the condensation and modification of acyl units derived from acyl-CoA precursors. Domains are organized in modules and each module carries out the series of steps necessary for one cycle of polyketide chain elongation. A single protein can have more than one module, and several different proteins together can make up a PKS. The number of modules determines the size of the polyketide. A growing polyketide chain is tethered to the enzyme as a thiol ester and moves sequentially from the N- to the C-terminus of a module, lengthened by two carbon units per module. The first module in a PKS... 

See Fig. 13-9. The carbon chain of linoleic acid is desaturated at position 6. y-Linolenic acid is elongated by two carbon units, and then another double bond is introduced in the C2o chain at position 5. 

The growing fatty acid chain is elongated by the sequential addition of two-carbon units derived from acetyl CoA. The activated donor of twocarbon units in the elongation step is malonyl ACP. The elongation reaction is driven by the release of CO2. 

The major product of the fatty acid synthase is palmitate. In eukaryotes, longer fatty acids are formed by elongation reactions catalyzed by enzymes on the cytosolic face of the endoplasmic reticulum membrane. These reactions add two-carbon units sequentially to the carboxyl ends of both saturated and unsaturated fatty acyl CoA substrates. Malonyl CoA is the two-carbon donor in the elongation of fatty acyl CoAs. Again, condensation is driven by the decarboxylation of malonyl CoA. 

Further chain-elongation of saturated or unsamrated acids by two-carbon units. 

A number of plants and phytochemicals have attracted attention for their ability to reduce many of the risk factors associated with cardiovascular disease. Research into these diseases has shown the relationship between lesions, fatty streaking and plaque formation in blood vessels and the development of strokes and myocardial infarctions. These effects are linked to levels of plasma lipids which comprise triglycerides, cholesterol and other fat substances. It is known that the biosynthesis of lipids involves the condensation of several molecules of acetylcoenzyme A and malonylcoenzyme A in a gradual process of elongation of the fatty acid chain involving the sequential addition of two carbon units giving rise to fatty acids such as lauric acid (12 carbons) and eventually to palmitic acid (16 carbons). Palmitic acid is the precursor... 

ER fatty acid chain elongation, which uses two-carbon units provided by mal-onyl-CoA, is a cycle of condensation, reduction, dehydration, and reduction reactions similar to those observed in cytoplasmic fatty acid synthesis. In contrast to the cytoplasmic process, the intermediates in the ER elongation process are CoA esters. These reactions can lengthen both saturated and unsaturated fatty acids. Reducing equivalents are provided by NADPH. 

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