Crystal structure of Escherichia coli cystathionine gamma-synthase at 1.5 angstrom resolution

The transsulfuration enzyme cystathionine gamma-synthase (CGS) catalyses th e pyridoxal 5'-phosphate (PLP)- dependent gamma-replacement of O-succinyl-L -homoserine and L-cysteine, yielding L-cystathionine. The crystal structure of the Escherichia coli enzyme has been solved by molecular replacement wi th the known structure of cystathionine beta-lyase (CBL), and refined at 1. 5 Angstrom resolution to a crystallographic R-factor of 20.0 %, The enzyme crystallizes as an alpha(4) tetramer with the subunits related by non-cryst allographic 222 symmetry. The spatial fold of the subunits, with three func tionally distinct domains and their quarternary arrangement, is similar to that of CBL, Previously proposed reaction mechanisms for CGS can be checked against the structural model, allowing interpretation of the catalytic and substrate-binding functions of individual active site residues, Enzyme-sub strate models pinpoint specific residues responsible for the substrate spec ificity, in agreement with structural comparisons with CBL, Both steric and electrostatic designs of the active site seem to achieve proper substrate selection and productive orientation. Amino acid sequence and structural al ignments of CGS and CBL suggest that differences in the substrate-binding c haracteristics are responsible for the different reaction chemistries. Beca use CGS catalyses the only known PLP-dependent replacement reaction at C ga mma of certain amino acids, the results will help in our understanding of t he chemical versatility of PLP.