Isoleucine occurrence

The occurrence of alanine in proteins was first shown by Schutzen-berger, who did not actually identify his product with the synthetical one Weyl in l88i obtained it as a decomposition product of silk and showed that his preparation was similar in properties to Strecker s synthetical alanine. He thus established it as a constituent of a protein molecule. The researches of Emil P ischer have shown that alanine is a constant constituent of all proteins. It is worthy of note that of the eighteen definitely determined units of a protein molecule, six of them, namely, isoleucine, phenylalanine, tyrosine, serine, histidine and tryptophane, are derivatives of a-aminopropionic acid. 

The co-occurrence of these three methoxypyrazines is consistent with a biosynthetic pathway (Figure 2) proposed over 20 years ago 14), The amino acid leucine is envisaged as the source of the C4 side chain of the methoxypyrazine, through condensation of its amino amide with an unspecified C2 component, and methylation of the initial pyrazinone condensation product. This proposed biosynthetic pathway readily accommodates all three methoxypyrazines through incorporation of either leucine, isoleucine or valine, all of which are commonly available amino acids in plants. Although the validity of this pathway in vines or other plant material is unknown, the major features of this proposed pathway have been shown to apply to the biosynthesis of isopropylmethoxypyrazine by certain bacteria (75, 16). 

Amino acids with a second asymmetric center, threonine, isoleucine, and hydroxyproline, must also be taken into account, but in view of the low rotations of the first (Schellman, 1960) and the relatively infrequent occurrence of all three, they do not affect protein dispersions to any large extent. 

How does the amino acid sequence of a protein specify its three-dimensional structure How does an unfolded polypeptide chain acquire the form of the native protein These fundamental questions in biochemistry can be approached by first asking a simpler one What determines whether a particular sequence in a protein forms an a helix, a (3 strand, or a turn One source of insight is to examine the frequency of occurrence of particular amino acid residues in these secondary structures (Table 2.3). Residues such as alanine, glutamate, and leucine tend to be present in a helices, whereas valine and isoleucine tend to be present in (3 strands. Glycine, asparagine, and proline have a propensity for being present in turns. 

Spencer, K. C., and D. S. Seigler, Co-occurrence of valine/isoleucine-derived and cyclopentenoid cyanogens in a Passiflora species, Biochem. Syst. Ecol., 13, 303-304 (1985e). 

Witten et al. (1973) identified adipic and 3-methyladipic acids and also reported the presence in urine, using GC-MS, of aconitic and isocitric acids in addition to citrate. Mamer et al, (1971) reported the occurrence of several hydroxyaliphatic acids in addition to those already identified by other workers, and Mamer and Tjoa have identified 2-ethylhydracrylic acid in urine derived from isoleucine metabolism (Mamer and Tjoa, 1974). Urine from healthy children and adults may contain low amounts of aliphatic dicarboxylic acids of chain length C4-C8 (Lawson et ai, 1976). Pettersen and Stokke (1973) reported a series of 3-methyl-branched C4-C8 dicarboxylic acids in urine from normal subjects, and Lindstedt and co-workers have identified other dicarboxylic acids with cyclopropane rings and acetylenic bonds as well as a series of cis and trans mono-unsaturated aliphatic dicarboxylic acids (Lindstedt et al., 1974,1976 Lindstedt and Steen, 1975). 

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