Cystathionine γ-Synthase

Cystathionine γ-synthase catalyzes replacement of the succinyl group of O-succinylhomoserine by cysteine to yield cystathionine, a step in methionine biosynthesis in Salmonella. The enzyme catalyzes rapid exchanges of both α and β hydrogen atoms in many amino acids that are not otherwise substrates. Stereospecificity was indicated by the exchangeability of all 4 hydrogen residues in alanine but of only 2 in α-aminobutyrate. Neither the replacement of a substituent on carbon 4 nor the exchange of a hydrogen on carbon 3 have been observed with any other pyridoxal phosphate enzyme. Cystathionine itself underwent exchange of 4 hydrogen residues, the α hydrogen and 1 β hydrogen in each chain. The rate of exchange in the 3-carbon chain was 6 times greater than either the exchange rate in the other chain or the Vmax for net cystathionine formation. In the 4-carbon chain, the apparent initial rate of β hydrogen exchange was twice that of α hydrogen exchange. Cystathionine was also found to undergo slow γ elimination and γ replacement reactions; the hydrogen exchange rates were about 103 times faster than the rates of cysteine exchange or elimination. The apparent Km for cystathionine was the same in all reactions. No hydrogen exchanges were catalyzed by apoenzyme (the Km for pyridoxal-P was also determined (4 x 10-8 m)). These results indicate that the γ elimination step catalyzed by cystathionine synthase is unconcerted and that the enzyme is efficiently designed for general base attack on an amino acid β hydrogen. The solvent hydrogen previously shown to be acquired at carbon 3 of α-ketobutyrate during the γ elimination reaction is introduced stereospecifically. This protonation is, therefore, catalyzed by the enzyme.