L-Alloisoleucine

Gibson KM, Lee CF, Bennett MJ, Holmes B, Nyhan WL (1993) Combined malonic, methylmalonic and ethylmalonic acid semialdehyde dehydrogenase deficiencies an inborn error of beta-alanine, L-valine and L-alloisoleucine metabolism J Inherit Metab Dis 16 563-567... 

Among the 20 proteinogenic amino acids, only L-isoleucine and L-threonine carry a symmetry center in their side chain and only (2S, 3S)-isoleucine and (2S, 3R)-threonine are found in proteins. When L-isoleucine was replaced in the culture medium by L-alloisoleucine, the (2S, 3R)-1 diastereomer of isoleucine, the cell growth slowed dramatically, without evidence of cell lysis. As western blotting did not show detectable DHFR expression in such cultures, we conclude that (2 S, 3R)-1 is incorporated into protein slowly if at all under our experimental conditions (Figure 3B, lane 5). However, small amounts of DHFR could be isolated from such culture (ca. 8% of the level of expression obtained in media supplemented with L-isoleucine, see Table 1). DHFR synthesis was not due to leaky expression before induction, as uninduced cells did not produce DHFR in media supplemented with L-isoleucine (Figure 3B, lane 3). Instead it appears that... 

Optically active acids. The a-bromo acid obtained from L-alloisoleucine by di-azotization (s. Synth, Meth. 13, 492) added to an aq. dispersion of Zn-dust, and stirred or refluxed overnight -> (R)-3-methylpentanoic acid. Y ca. 100%, overall 50% optical purity 92%. - This route is recommended if alkaloid resolution fails. C. G. Overberger and I. Cho, J. Org. Chem. 33, 3321 (1968). 

The transamination of L-isoleucine to d-a-keto-/3-methylvalerate and of L-alloisoleucine to Z-a-keto-/3-methylvalerate by a hog heart preparation and the cell-free extracts of a number of microorganisms e.g., Lacto-bacillus arabinosus) has been demonstrated by Meister. ... 

Devise a procedure for separating a mixture of the four stereoisomers of isoleucine into its four components (+)-isoleucine, (—)-isoleucine, (-l-)-alloisoleucine, and (—)-alloisoleucine (Problem 31). (Note Alloisoleucine is much more soluble in 80% ethanol at all temperatures than is isoleucine.)... 

By measuring the L-isoleucine/o-alloisoleucine ratio in the protein isolated from the eggshells of an extinct Australian bird, a team of scientists recently determined that this bird lived approximately 50,000 years ago. Radiocarbon ( " C) dating is not accurate for samples older than about 35,000 years, so AAR is a useful addition to the tools available to paleontologists. 

In an early paper describing this method [Science, 1970, Promote 170, 730-732], there is a note that at equilibrium a small critical excess of D-alloisoleucine is present (at 140°C they thinking report the ratio of alloisoleucine to isoleucine is 1.25), but skills that for most amino acids the equilibrium constant between the L and D forms is 1. Why should that be ... 

A novel technique for dating archaeological samples called amino acid racemiza-tion (AAR) is based on the stereochemistry of amino acids. Over time, the configuration at the a-carbon atom of a protein s amino acids is lost in a reaction that follows first-order kinetics. When the a carbon is the only chirality center, this process corresponds to racemization. For an amino acid with two chirality centers, changing the configuration of the a carbon from L to D gives a diastereomer. In the case of isoleucine, for example, the diastereomer is an amino acid not normally present in proteins, called alloisoleucine. 

The alloisoleucine/isoleucine (alleu/iso) ratio is determined directly on the automatic amino acid analyzer, and d/l enantiomeric ratios for additional amino acids are obtained by gas chromatography (above). 

For the structures of natural L-threonine ((2S,3R)-threonine) and L-allothreonine ((2S,3S)-threonine), replace the side-chain ethyl group (C2H5) in isoleucine and alloisoleucine by OH. 

Fig. 5.38 Epimerization (E) and racemization (R) of isoleucine (major pathway is epimerization of L-isoleucine and D-alloisoleucine large arrow indicates biogenic input).

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