Methyl group of valine

C-H vector to which the methyl symmetry axis is rigidly attached. A similar range of generalized order parameters is seen for the methyl groups of valine and leucine and reinforces the conclusion that the interior of ubiquitin is heterogeneously dynamic. 

Ring expansion of the penicillins to the cephalosporins has been examined. Following the elucidation of the structure of penicillin N and cephalosporin C, small amounts of isopenidllin N with an L-aminoadipoyl side-chain and de-acetoxycephalosporin C were found in Cephalosporium acremonium. Cell-free systems were obtained from the fungus which catalysed the ring expansion of penicillin N to deacetoxycephalosporin C (3.19) and the hydroxylation of this. A radical mechanism has been proposed in which the 3(pro-R)-methyl group of valine (marked in 3.17) becomes the ring methylene (marked in 3.19) of cephalosporin C. 

Further, the 3-pro-S methyl group of valine is incorporated as the ring carbon, C-2, of cephalosporin 143 (175) with loss of stereochemical integrity (176-178), although the exocyclic methylene group had evidently originated by stereospecific hydroxylation of the 3-pro-R methyl (176,177). 

The reduction step catalyzed by this enzyme has been shown to use the 4-pro-S hydrogen of NADPH (195), and when C-labeled a-acetolactate 217 was used, it could be shown that the migrating methyl group eventually became the 2-pro-S methyl group of valine 179. The migration thus has the... 

The stereochemistry of incorporation of the methyl groups of valine into penicillin has been studied in several laboratories. (2i S, 3i )-[4- C]-valine (262) synthesised by Baldwin etal 230) was fed to a R chrysogenum fermentation and the resulting phenoxymethylpenicillin (263) was examined by C-n.m.r. spectrometry 231). Comparison of the n.m.r. spectrum with one of unlabelled phenoxymethylpenicillin indicated that the 3-methyl group had been specifically enriched. Similarly Kluender etal. 232) synthesised (25,3 S)-[4- C]-valine and fed this compound to washed mycelium of C. acremonium. The resulting penicillin N was examined by C-n.m.r. spectrometry and the a-methyl carbon was found to be specifically enriched. Further support for this finding came from Aberhart and Lin 233) who synthesised (2i 5,3iS)-[4- C]-valine (264) and found that this specifically labelled the oc-methyl carbon of penicillin V (263). 

The biosynthesis of the p-lactam antibiotic penicillin (Fig. 65), and also of cephalosporin, involves incorporation of L-valine and the question arises as to which of the two diastereotopic terminal methyl groups of the valine occupies which position in the penicillin. (In the case of cephalosporin, the question is as to which methyl group is incorporated into the six-membered ring and which becomes the methylene group of the carbinyl acetate.) The problem has been solved by two groups 65d,141) by synthesis of specifically 13C methyl labeled valine (cf. Fig. 42, and p. 35) which was then biosynthetically incorporated in the antibiotics. The position of the 13C in the resulting antibiotic molecules was determined by 13C NMR spectroscopy. 

For the 1H/I5N experiment, uniformly labeled substance is obtainable at a reasonable cost, but uniform 13C labeled proteins are more expensive. However, in some cases selective labeling of the methyl groups in valine, leucine, or isoleucine residues of a protein proves sufficient for screening purposes (59). The employment of HSQC experiments to detect binding is especially well exemplified in the technique termed SAR by NMR (33). 

Now look up the structure of the amino acid valine. Would you expect the chemical shifts of the protons of the two methyl groups in valine to be the same or different ... 

Penicillin.—Recent investigation of the biosynthesis of penicillins, e.g. penicillin V (198), have concentrated on the stereochemistry associated with the ring closure of a probable L-cysteinyl-L-valine intermediate as it affects the valine-derived part of the molecule.In support of the ealier work, incorporation of the label from (2RS, 3S)-[4- C]valine (197) into the a-methyl group of penicillin V (198) has been reported, i.e. incorporation is with retention of configuration at C-3 of valine. This defines the stereochemistry associated with, for example, the addition to the double bond of the hypothetical intermediate (200). 

Fig. 3. a Determination of positional enrichment (24%) in the methyl groups of C-la-belled valine purified fron a biomass hydrolysate of C. glutamicum by integration of the satellites in the NMR spectrum (b) measurement of relative isotopomer abundancies from singlet, doublet (2 varieties) and doublet-of-doublets signals in the NMR spectrum of C-labelled alanine... 

Coordination of the threonine hydroxyl by an active site Zn in the threonyl-tRNA synthetase allows discrimination between threonine and the isosteric valine (Sankaranarayanan et al., Nat. Struct. Biol. 7[2000] 461-465). Given the similarity of serine and threonine (Ser lacks only the methyl group of Thr), if this is the only mechanism for amino acid discrimination available, threonyl-tRNA synthetase mistakenly couples Ser to threonyl-tRNA at a rate several-fold higher than it does threonine. Since this would lead to unacceptably high error rates in translation, how it is it avoided ... 

The C-a protons of amino acid residues resonate between 6 and 3.5 ppm (not shown because of interference with the HDO resonance). The resonances of the aliphatic side chains fall between 3.5 and -0.6 ppm. The well resolved resonances between 0.5 to -0.6 ppm, shifted outside the bulk resonance, most likely come from methyl groups of leucine, isoleuclne and/or valine, shifted by the ringcurrents of proximate aromatic residues. Since the Influence of the binding of DMA... 

Definite proof that the carboxyl and not the methyl group of isobutyrate is lost has been obtained from incubation experiments with isotopic valine. In brief, on the basis of the decarboxylation hypothesis of isobutyric acid, the catabolism of valine would proceed as follows ... 

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