O-Acetyl-L-homoserine

In some organisms L-homoserine is initially acyla-ted to O-acetyl-L-homoserine (by acetyl-CoA), and in plants it is converted to oxalyl-L-homoserine by reaction with oxalyl-CoA. 

Kase H, Nakayama K. Production of O-acetyl-L-homoserine by methionine analog-resistant mutants... 

Selenium is present in biological materials in a number of compounds that correspond in their structure to sulfur compounds. The amino acid L-selenocysteine (see Section 2.2.1.1.1) bound in proteins arises from selane and O-acetyl-L-serine, methyla-tion yields Se-methyl-L-selenocysteine, reaction with O-succinyl-L-homoserine leads to L,L-selenocystathionine, hydrolysis of the last compound provides i-selenohomocysteine and methylation... 

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 cell free extract which contained 720 mg protein was mixed with C labeled methyl-mercaptan of specific activity 2.4 x 10 CPM/m mole. O-acetyl homoserine was also added. The total volume of the system did not exceed 1.5 ml. Since the cell-free extract was from a microorganism, it contained en mes required for methionine biosynthesis. Aliquots were drawn and fractionated for L-methlonine at different intervals. It could be determined that the rate of incorporation of radioactivity in methionine was 2240 CPM/min. Calculate the rate of en nnatlc reaction. 

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. ... 

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