The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum reveals its substrate and reaction specificity

Cystathionine gamma-synthase catalyses the committed step of de novo methio nine biosynthesis in micro-organisms and plants, making the enzyme an attra ctive target for the design of new antibiotics and herbicides. The crystal structure of cystathionine gamma-synthase from Nicotiana tabacum has been s olved by Patterson search techniques using the structure of Escherichia col i cystathionine gamma-synthase. The model was refined at 2.9 Angstrom resol ution to a crystallographic R-factor of 20.1% (R-free 25.0%). The physiolog ical substrates of the enzyme, L-homoserine phosphate and L-cysteine, were modelled into the unliganded structure. These complexes support the propose d ping-pong mechanism for catalysis and illustrate the dissimilar substrate specificities of bacterial and plant cystathionine gamma-synthases on a mo lecular level. The main difference arises from the binding modes of the dis tal substrate groups (O-acetyl/succinyl versus O-phosphate). Central in fix ing the distal phosphate of the plant CGS substrate is an exposed lysine re sidue that is strictly conserved in plant cystathionine gamma-synthases whe reas bacterial enzymes carry a glycine residue at this position. General in sight regarding the reaction specificity of transsulphuration enzymes is ga ined by the comparison to cystathionine beta-lyase from E. coli, indicating the mechanistic importance of a second substrate binding site for L-cystei ne which leads to different chemical reaction types. (C) 1999 Academic Pres s.