Chain elongation process

The stepwise polymerisation of activated amino acids leads to the formation of activated dimers, which very often cyclise to diketopiperazines and are thus removed from the chain elongation process (Orgel, 1989). 

P. putida grown with hexanoic acid contained approximately 75, 11, and 10 mol% of 3HHx,3HO, and 3HD units and also small amounts of four unsaturated repeating units. The mechanism for the formation of 3HO unit was investigated by 13C NMR study, which showed that the most of 3HO units found in this PHA were formed by the reaction of hexanoic acid with acetyl-CoA [53]. These results confirmed that P. putida produces 3HA units by fatty acid synthesis pathway, through a -oxidation and chain elongation process. 

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. 

Figure 3.6. Polyketlde chains use an iterative process akin to the fatty acid chain elongation process. However, an extra layer of chemical diversity can be generated by feeding different starter units into the system consequently, the polyketlde biosynthetic system has been compared to a Lego system where a few modules can be joined together in many different ways.

In principle, bifunctional aldehydes should be able to engage in twofold enzymatic aldol additions to both of their acceptor carbonyls in a fashion to be classified as a tandem reaction, that is, without the need for isolation of intermediates. Depending on the specificity of the enzyme used and on the functionalization in the starting material, the isomeric constitution as well as the absolute and relative stereochemistry should be deliberately addressable. Therefore, we engaged in a program to evaluate the scope and the Hmitations of such two-directional chain elongation processes for the construction of extended poly functional molecules [36]. 

Polycarboxylic acid synthases. Several enzymes, including citrate synthase, the key enzyme which catalyzes the first step of the citric acid cycle, promote condensations of acetyl-CoA with ketones (Eq. 13-38). An a-oxo acid is most often the second substrate, and a thioester intermediate (Eq. 13-38) undergoes hydrolysis to release coenzyme A.199 Because the substrate acetyl-CoA is a thioester, the reaction is often described as a Claisen condensation. The same enzyme that catalyzes the condensation of acetyl-CoA with a ketone also catalyzes the second step, the hydrolysis of the CoA thioester. These polycarboxylic acid synthases are important in biosynthesis. They carry out the initial steps in a general chain elongation process (Fig. 17-18). While one function of the thioester group in acetyl-CoA is to activate the methyl hydrogens toward the aldol condensation, the subsequent hydrolysis of the thioester linkage provides for overall irreversibility and "drives" the synthetic reaction. 

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

Oxoacid chain elongation process. Used three times... 

Herpes viruses are almost exclusively treated with nucleoside analogues that inhibit viral DNA polymerase. Just as with HIV NRTIs, the nucleoside analogues used against herpes viruses lack a functional 3 -OH and terminate the chain elongation process of nucleotide polymerases. The nucleoside analogues in Figure A.49 all have a role in treating... 

The de novo synthesis of fatty acids in the mammary gland utilizes mainly acetate and some (3-hydroxybutyrate. These precursors arise from the microbial fermentation of cellulose and related materials in the rumen. Once in the mammary gland, acetate is activated to acetyl-CoA. The mechanism of fatty acid synthesis essentially involves the carboxylation of acetyl-CoA to malonyl-CoA, which is then used in a step-wise chain elongation process. This leads to a series of short-chain and medium-chain length fatty acids, which differ by two CH2 groups (e.g., 4 0, 6 0, 8 0, etc.) (Hawke and Taylor, 1995). These are straight-chain, even-numbered carbon fatty acids. However, if a precursor such as propionate, valerate or isobutyrate, rather than acetate, is used, branched-chain or odd-numbered carbon fatty acids are synthesised (Jenkins, 1993 see Chapter 2). 

Most of the resins used in batch synthesis are derivatives of 1% crosslinked copoly-(styrene-divinylbenzene) resin in the form of beads 50 to 100 p in diameter. These beads swell four to six times their dry volume in the solvents used in peptide assembly and during the chain elongation process the resin again may double its swollen volume. In batch synthesis, provided the proper size vessel is chosen, the increase in swollen volume is not a major concern. 

For anti-Hib vaccine, s)mthetic chemistry was compelled to reach the efficiency level attained during the production of the glycoconjugate vaccine from natural CPS. The contribution of several groups [149] to the synthesis of Hib-oligosaccharides paved the way for further improvement. A simplified synthesis of the repeating disaccharide unit and chain elongation process in one step through a polycondensation reaction were essential achievements before further developments and clinical evaluation were possible (see O Scheme 6). The synthetic... 

Linoleic acid can be converted in mammalian liver to y-linolenic acid and arachidonic acid by the microsomal desaturation and chain elongation process (Figure 18-14). Thus, the requirement for arachidonic acid may be dispensed with when the diet contains adequate amounts of linoleic acid. Similarly, a-linolenic acid is converted by desaturation and chain elongation to EPA and DCHA (Figure 18-15). 

The Oxoadd Chain Elongation Process Decarboxylation as a Driving Force in Biosynthesis Stabilization and Termination of Chain Growth by Ring Formation... 

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