Chain-elongating enzyme systems

Experiments with chloroplasts showed an apparent inhibition of fatty acid synthesis by PAN (at 72 ppm for 10 min). The result is difficult to interpret the inhibition could be attributed to inactivation of one of the enzymes of the multistep system or to oxidation of the reductant (reduced NADP, or NADPH) required in the chain elongation process. 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

Several architectural paradigms are known for polyketide and fatty acid synthases. While the bacterial enzymes are composed of several monofunctional polypeptides which are used during each cycle of chain elongation, fatty acid and polyketide synthases in higher organisms are multifunctional proteins with an individual set of active sites dedicated to each cycle of condensation and ketoreduction. Peptide synthetases also exhibit a one-to-one correspondence between the enzyme sequence and the structure of the product. Together, these systems represent a unique mechanism for the synthesis of biopolymers in which the template and the catalyst are the same molecule. 

We found the principal FAs upon infection to be oleic acid (18 1) and saturated FAs. The most characteristic increases in FAs were in palmitic (16 0), which went from 19% to 28%, stearic (18 0) from 10% to 14% and oleic (18 1) from 18% to 28%. The relative paucity of polyunsaturates in PLs led us to suggest that there were defects in appropriate desaturases, chain elongation systems and acylation enzymes and we concluded that the malaria parasite, though it may have no capacity for de novo biosynthesis of lipids from acetate, can regulate its use of host cell lipids and lipid precursors in such a manner as to establish and maintain a lipid composition distinct in many respects from that of the erythrocyte (Beach et al., 1977). 

Thus we may conclude that only the C20- and C22-polyenoic acids of the oleic, linoleic, and linolenic acid types are built into tissue phospha-tides. It cannot yet be decided whether the specificity of the chain elongating and dehydrogenating enzyme system is responsible, or whether a particular selective principle during the formation of the lipid cell structures is active. Possibly both are true. There may also be a relation... 

Current views on the compartmentation of the enzyme systems that promote chain elongation and decarbonylation are represented in Figure 4. This scheme is based on electron microscopy and on studies of enzyme location and inhibitionThe chain length of alkanes formed probably depends on the aldehydes available rather than on the specificity of the decarbonylation thus crude extracts of leaves generated n-alkanes from n-Ci6 to n-C32 fatty acids with no particular preference for substrate. The restricted ranges of n-alkanes observed in vivo presumably result from the specificity of products from the fatty-acid elongation system that produce the substrate for decarbonylation. 

The biosynthesis of a variety of biologically active peptides proceeds nucleic acid-free on protein templates (IK Peptide synthetases generally activate an acceptor amino acid by formation of amino-acyl adenylates or phosphates, which will be stabilized in an enzyne-aminoacylation step, similar as in tRNA-aminoacylation. Reaction with a donor peptide, which may be covalently bound, leads to a specific chain elongation. While small peptides like glutathione are formed by "one-step"-synthetases, more complex structures like gramicidin S are produced by multienzvme systems, which may contain multifunctional polypeptides. Characteristic features of such systems are 1.)activation as aminoacyl adenylates, 2.) aminoacylation of enzyme thiol-groups, 3.) covalently bound peptide intermediates and 4.) a specific intrinsic transport mechanism similar to the biosynthesis of fatty acids. 

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