Succinylhomoserine

Several PLP-dependent enzymes catalyze elimination and replacement reactions at the y-carbon of substrates, an unusual process which provides novel routes for mechanism-based inactivation. An example of this class of enzymes is cystathionine y-synthase [0-succinylhomoserine (thiol)-lyase], which converts (7-succinyl-L-homoserine and L-cysteine to cystathionine and succinate as part of the bacterial methionine biosynthetic pathway (Walsh, 1979, p. 823). Formation of a PLP-stabilized o-carbanion intermediate activates the )8-hydrogen for abstraction, yielding j8-carbanion equivalents and allowing elimination of the y-substituent. The resulting j8,y-unsaturated intermediate serves as an electrophilic acceptor for the replacement nucleophile. Suitable manipulation of the j8-carbanion intermediate allows strategies for the design of inactivators which do not affect enzymes which abstract only the a-hydrogen. 

Finally, synthetic samples of (4R)- and (4S)-[4- H,]-0-succinylhomoserine 122, Hg = H, and 122, H = H, respectively, were incubated with the enzyme in the presence of L-cysteine (128, 129), and the H NMR spectra of the resultant samples of cystathione 99a were compared with those of the samples from the vinylglycine incubations. It was evident from these that the /-substitution reaction had occurred with retention of configuration. 

Homoserine kinase 2 homoserine acyltransferase 3 0-succinylhomoserine (thiol)-lyase 4 cystathionine -lyase 5 tetrahydropteroylglutamate methyltransferase... 

This enzyme [EC 2.3.1.46], also referred to as homoserine O-transsuccinylase, catalyzes the reaction of succinyl-CoA with homoserine to produce coenzyme A and O-succinylhomoserine. 

A /3,y-elimination of succinylhomoserine to a-ketobutyrate is also catalyzed by cystathionine-y-synthase from Salmonella in the absence of L-cysteine. 

Fig. 38. The demonstration that in the reaction catalysed by cystathionine-y-synthetase with (45 H - H)succinylhomoserine, the (Z)-vinylglycyl species (3) is involved, and that the overall transformation occurs with retention of stereochemistry at C.,.

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. 

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