Cystine stereochemistry

No cysteine is isolated because (1) most of it is present as cystine in the hair (2) any cysteine released in the hydrolysis will be oxidized to cystine by air. The stereochemistry of cysteine is preserved in cystine, which has C2 symmetry and can be best represented by diagrams that show its symmetry - two are suggested. It is not important whether you draw the zwitterion or the neutral structure. Reduction of the S-S bond by, say, NaBH4 is needed to convert cystine into cysteine. 

Our laboratory has studied the stereochemistry of methyl group formation in a number of a, 0 elimination reactions of amino acids catalyzed by pyridoxal phosphate enzymes. The reactions include the conversions of L-serine to pyruvate with tryptophan synthase 02 protein (78) and tryptophanase (79), of L-serine and l-tyrosine with tyrosine phenol-lyase (80), and l-cystine with S-alkylcysteine lyase (81). In the latter study, the stereospecific isotopically labeled L-cystines were obtained enzymatically by incubation of L-serines appropriately labeled in the 3-position with the enzyme O-acetyl serine sulfhy-drase (82). The serines tritiated in the 3-position were prepared enzymatically starting from [l-3H]glucose and [l-3H]mannose by a sequence of reactions of known stereochemistry (81). The cysteines were then incubated with 5-alkyl-cysteine lyase in 2H20 as outlined in Scheme 19. The pyruvate was trapped as lactate, which was oxidized with K2Cr202 to acetate for analysis. Similarly, Cheung and Walsh (71) examined the conversion of D-serine to pyruvate with... 

Attention has been turned to the cysteine-derived part of the penicillins. The results closely define (a) the fates of the hydrogen atoms present at C-2 and C-3 of cysteine (201) and (b) the overall stereochemistry involved in the incorporation of the amino-acid. L-[l/- C,2- H]Cystine was found to give penicillin G (199) with only slight loss of tritium. The tritium label was shown (in an experiment with singly labelled precursor) to be located exclusively at C-6, similar results being obtained with L-[2- H]cystine. These results, which confirm those obtained earlier, ... 

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