Serine and Cysteine Derivatives

Serine and Cysteme Derivatives.-—L-Cysteine (1) is derived from L-serine (2) via O-acctyl-L-serine (3) in bacteria. Recent data indicate that the same pathway is utilized for the production of cysteine in plants. Thus, though never encountered in natural material, O-acetylserine is synthesized fi om serine and acetyl-CoA by enzyme preparations from kidney beans, lupin, barley, pea seedlings, leaves of swiss chard, turnip, and radishes. The transformation of O-acetylserine and sulphide into cysteine has been accomplished with enzyme systems from spinach, turnip, and kidney bean. Tracer experiments in turnip with [ S]sulphide and [ C]homoserine indicate that the reaction between O-acetylserine and sulphide constitutes the main entrance for sulphur into organic compounds in plants.  

Spinach contains two enzymes catalysing the sulphuration of O-acetylserine. One enzyme possesses the additional ability to produce S-methyl-L-cysteine (4) or S-ethyl-L-cysteine from 0-acetyl-L-serine and methanethiol or ethanethiol, respectively. The second enzyme also catalyses these processes but, in addition, catalyses the formation of L-methionine, L-ethionine, and L-homocysteine from 0-acetyl-L-homoserine and methanethiol, ethanethiol, and sulphide, respectively. Enzymes from turnip and kidney beans are able to catalyse the formation of S-methylcysteine from O-acetylserine and methanethiol C-labelled serine, when fed to Allium cepa and A. sativum along with thiols, is converted into S-substituted cysteines.  

A similar explanation may obtain for the degradation of dichrostachinic add (IS) by an enzyme preparation from seedlings of Leucaena leucocephala, 

Murakosbi, H. Kuramoto, J. Haginiwa, and L. Fowden, Biochem. Biophys. Res. Comm., 1970, 41,1009. 

The assumption that cysteine and not O-acetylserine is the immediate precursor for p-cyanoalanine is supported by tracer experiments with incorporation of [ RC]cyanide, [ C]serine, and [ C]cysteine into asparagine 

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The major advantage of this approach to the synthesis of unsaturated peptides lies in the fact that the elimination can be carried out as the final step. Peptides can thus be constructed with optically active serine and cysteine derivatives, and the formation of diastereoisomers is avoided. 

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