Amino acid conversion

Table 8.2 shows the amino acid conversions which led to contemporary amino acids, as suggested by Wong. The evolutionary relationships between the code words for the various amino acids are shown in Table 8.1. 

Stein, W. H., and Moore, S., Electrolytic desalting of amino acids. Conversion of arginine to ornithine. J. Biol. Chem. 190, 103-106 (1951). 

Fig. 16.21. RNA editing. In liver, the apoprotein B (ApoB) gene produces a protein that contains 4,563 amino acids. In intestinal cells, the same gene produces a protein that contains only 2,152 amino acids. Conversion of a C to a U (through deamination) in the RNA transcript generates a stop codon in the intestinal mRNA. Thus, the protein produced in the intestine (B-48) is only 48% of the length of the protein produced in the liver (B-lOO).

Decarboxylases are known for their roles in a wide variety of catabolic and anabolic pathways, including decarboxylation of a- and p-keto acids, amino acid conversions, and carbohydrate biosynthesis. Mechanistically, a decarboxylation has parallels to retro-aldol cleavage reactions (Figure 1.32). 

In spite of the large number of folate derivatives, all the folate coenzymes have a similar function in metabolism. The folate coenzyme activates 1-carbon units (methyl, methylene, and formyl) and facilitates their transfer from one metabolite to another. A 1-carbon unit is transferred principally in amino acid conversion and in purine and pyrimidine biosynthesis. Many of the reactions involving folate coenzymes are discussed in other chapters, so these reactions are reviewed only briefly here. 

The target amino acids are usually chosen based on an a priori knowledge of some of the relevant structural details [162]. These methods require a carefijUy selected labeling scheme to counteract the effects of isotope scram-bhng and amino acid conversion, which might randomize the isotopic label within the protein [162,163]. 

Among the most interesting of the biological reactions of tryptophan is its conversion to nicotinic acid. In the vertebrates, at any rate, this is a truly anabolic process which serves the end of providing source material for the synthesis of DPN and TPN. The discussion of this pathway of metabolism is contained in the chapter. Synthetic Processes Involving Amino Acids. Conversion to nicotinic acid can not be the major pathway for the catabolism of tryptophan. Based on the capacity of fed tryptophan to prevent symptoms of niacin deficiency in animals, it can be calculated that only 1% or 2% is thus converted in man and the primates. In the rat the conversion may amount to as much as 10%. 

The facile formation of XX from amino acid XI led Wiesner etal. to consider structure I-A for annotinine and XI-A for the amino acid. Conversion of XI-A to XX-A in a series of simple steps is easily envisaged. 

Vitamin B-6 As a coenzyme for transamination, decarboxylation, deamination, transsulfuration, absorption of amino acids, conversion of glycogen to glucose, and fatty acid conversions. 

Consumption of citric acid Causes a rapid start-up of the fermentation process under low-pH conditions, avoiding competition with sucrose- (glucose-) consuming, citrate-negative, depectinizing yeasts Causes a pH increase (replacement of citric acid by lactic acid and acetic acid), in turn controlling bacterial growth Beneficial for the production of pyruvate metabolites Stimulates amino acid conversions... 

Decarboxylation reactions are common in Nature and they are involved in many pathways, including decarboxylation of keto acids, amino acid conversions, and carbohydrate synthesis. Many decarboxylases use cofactors such as metal ions, pyridoxal 5 -phosphate, biotin, and flavin, but a small subset, for example, orotidine 5 -phosphate decarboxylase (ODCase) and methyhnalonyl CoA decarboxylase do not utilize any cofactor. ODCase catalyzes the decarboxylation of orotic acid (shown in Figure 8), and it generates one of the largest rate enhancements known to be produced by any enzyme (rate of the reaction is enhanced by a factor of Several... 

Bendall SC et al (2008) Prevention of amino acid conversion in SILAC experiments with embryonic stem cells. Mol Cell Proteomics 7 1587-1597... 

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