O-Acetylhomoserine

This enzyme [EC 4.2.99.10], also referred to as O-acetyl-homoserine sulfhydrylase, catalyzes the reaction of O-acetylhomoserine with methanethiol to generate methionine and acetate. The enzyme can also act on other thiols or H2S, producing homocysteine or thioethers. The enzyme isolated from baker s yeast will also catalyze the reaction exhibited by O-acetylserine (thiol)-lyase [EC 4.2.99.8], albeit more slowly. 

This enzyme [EC 2.3.1.31] catalyzes the reaction of ace-tyl-CoA with homoserine to produce coenzyme A and O-acetylhomoserine. 

From the above work, two unexpected findings emerged. First, crude extracts of spinach also catalyzed a direct reaction of HjS with O-acetylhomoserine to form homocysteine (Giovanelli and Mudd, 1%7). Thus the sulfur moiety of homocysteine could possibly be derived not only from cysteine via transsulfuration, but also directly from HjS via sulfhydra-tion. Second, a variety of O-acylhomoserine esters, but not homoserine itself, were active in the synthesis of both homocysteine and cystathionine (Giovanelli and Mudd, 1966, 1%7) the question arose as to which, if any, of these compounds is of physiological significance as the precursor of the four-carbon portion of homocysteine. 

Members of at least three bacterial families and all fungi examined utilize O-acetylhomoserine in the cystathionine-homocysteine biosynthetic pathway, whereas several members of the family of enteric bacteria utilize O-succinylhomoserine (Flavin, 1975). The findings now to be discussed establish that green plants use for this purpose yet another O-ester of homoserine, O-phosphohomoserine. 

For the sulfhydration reaction, C>-phosphohomoserine is the most active substrate under assay conditions optimal for activity with this compound. When the activities were determined under conditions optimal for O-acetylhomoserine, activity with O-acetylhomoserine was increased to 17% of that with O-phosphohomoserine, while the relative activity with O-succinylhomoserine remained unchanged. Sulfhydrase activities with O-phosphohomoserine and O-acetylhomoserine showed different patterns of development during early growth of excised barley embryos, suggesting that activities with the two substrates may be catalyzed by separate enzymes (Datko et al., 1977). Purification of these activities will be required to determine whether this suggestion is correct. 

Extracts of a variety of plants were assayed by a sensitive and specific technique for their ability to synthesize O-acetyl-, O-succinyl-, 0-oxalyl-, 0-malonyl-, and O-phosphohomoserine (Giovanellieta/., 1974). Synthesis of 0-acylhomoserine derivatives was detected only in Pisum sativum and Ldthyrus sativus, plants which are unusual in accumulating 0-acetyl-homoserine (Grobbelaar and Steward, 1958) or 0-oxalylhomoserine (Przybylska and Pawelkiewicz, 1965), respectively. Extracts of Pisum fruits catalyzed the synthesis of 0-acetylhomoserine, but not of the other 0-acylhomoserine derivativesf extracts of Lathyrus seedlings catalyzed the... 

Together, these data strongly suggest that O-phosphohomoserine is the physiologically important a-aminobutyryl donor for synthesis of both cystathionine and homocysteine in plants. This finding is consistent with the in vivo experiments of Dougall and Fulton (1967), which showed that neither O-succinyl nor 0-acetylhomoserine was utilized in preference to homoserine for cystathionine synthesis by cells of Paul s Scarlet Rose. 

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