Methionine degradation/elimination

The formation of furans, thiophenes, furanones, thiophenones etc. was investigated in a series of [l(or 6)- C]-glucose and [l- C]-arabinose/ cysteine and methionine model experiments. The labeled compounds were analyzed by capillary GC/MS and NMR-spectroscopy. From their structures the degradation pathways via different reactive intermediates (e.g. 3-deoxyaldoketose, 1-deoxydiketose) and fragmentations were evaluated. Besides the transformations to flavor compounds via identical labeled precursors, major differences in the flavor compounds result from specific Strecker reaction sequences. Major unlabeled compounds e.g. 3-mercaptopropionic acid from cysteine and 4-methylthiobutyric acid from methionine demonstrate transamination/reduction, and the formation of pyruvate and 2-mercaptopropionic acid from [l-i C]-glucose/cysteine indicates B-elimination. 

The identification of unlabled pyruvate and 2-mercaptopropionic acid in [ C]-D-glucose/L-cysteine Maillard systems clearly indicates a degradation pathway via 6-elimination. In the corresponding [l-i Cj-D-glucose/L-methionine model experiment,... 

The effect of different amino acids supplements on the synthesis of PHB by recombinant E. coli was evaluated by Mahishi and Rawal. The study revealed that when the basal medium is supplemented with amino acids, except glycine and valine, all other amino acid supplements enhanced PHB accumulation in recombinant E. coli harboring PHB synthesizing genes from S. aureqfaciens. Cysteine, isoleucine, or methionine supplementation increased PHB accumulation by 60, 45, and 61%, respectively. Amino acid biosynthetic enzyme activities in several pathways are repressed by end produa supplementation. End product inhibition in the cysteine biosynthetic pathway controls the carbon flow due to sensitivity of serine transacetylase to cysteine. Hence, supplementation of cysteine favors a change in carbon flux that eliminates the requirement of acetyl-CoA for serine transacetylation which in turn provides more carbon source and acetyl-CoA for PHB synthesis. Degradation of methionine and isoleucine yields succinyl CoA, an intermediate of tricarboxylic acid cycle and allows more acetyl-CoA to enter the PHB biosynthetic pathway. 

Mercaptide Elimination. Kallio and Larson have described an oxidative degradation of methionine by a Pseudomonas. A pyridoxal phosphate enzyme eliminates methyl mercaptan and ammonia, leaving a-ketobutyrate. The methyl mercaptide is oxidized to dimethyl disulfide. 

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