Amino acids biological occurrence

D2. Dent, C. E., A study of the behaviour of some sixty amino acids and other ninhydrin reacting substances on phenol-collidine filter paper chromatograms with notes as to the occurrence of some of them in biological fluids. Biochem. J. 43, 169-180 (1948). 

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

There is also a significant occurrence of nonprotein D-amino acids in nature.These amino acids have attractive structural variations and remarkable biological properties. However, this chapter will not include details on nonprotein D-amino acids. Selenium-containing amino acids, such as selenocysteine and selenomethionine, are also considered as unusual amino acids. Recently, much interest in selenium-containing amino acids has emerged. More information about selenium-containing amino acids is available in Chapter 5.05. 

In the case of proteins or nucleic acids we do not have two, but several comonomers furthermore we are not dealing with the simple case of radical polymerization, but with the more complex polycondensation. Very little is known about the kinetics of the copolymerization of polycondensates - for example analysis of ta and re has not been done systematically for amino acids. However, a few general points can still be made on the basis of the general principles of copolymerization. One has been already mentioned that the initial composition of amino acids in the prebiotic soup may not correspond to the amino-acid composition in the chain. Thus, the fact that one given amino acid has a very small frequency of occurrence in protein chains may not necessarily mean that this amino acid was not present under prebiotic conditions the low frequency in the chains can simply be the result of the kinetics of polycondensation. Conversely, the presence of preferred residues or short sequences in protein chains might be due to the interplay of kinetic parameters, and have little to do with the initial biological constraints. 

The high affinity of zinc for mtrogeneous and sulfur-containing ligands seems chiefly responsible for the occurrence of zinc in a wide variety of biological compounds, such as proteins, amino acids, nucleic adds,... 

Twenty-two amino acids were detected by early analyses of the Murchison chondrite, 8 of which were biologically common, 10 of which liad a restricted occurrence on the Earth, and 4 of which had no natural terrestrial source [33]. Further analyses identified 75 amino acids in the Miuchison [34] 8 of them were biologically common, 11 of them were uncommon, and 55 of them had no terrestrial source [35-37]. 

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