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

Cystathionine γ-synthase catalyses the committed step of de novo methionine biosynthesis in micro-organisms and plants, making the enzyme an attractive target for the design of new antibiotics and herbicides. The crystal structure of cystathionine γ-synthase from Nicotiana tabacum has been solved by Patterson search techniques using the structure of Escherichia coli cystathionine γ-synthase. The model was refined at 2.9 Å resolution to a crystallographic R-factor of 20.1 % (Rfree 25.0 %). The physiological substrates of the enzyme, l-homoserine phosphate and l-cysteine, were modelled into the unliganded structure. These complexes support the proposed ping-pong mechanism for catalysis and illustrate the dissimilar substrate specificities of bacterial and plant cystathionine γ-synthases on a molecular level. The main difference arises from the binding modes of the distal substrate groups (O-acetyl/succinyl versus O-phosphate). Central in fixing the distal phosphate of the plant CGS substrate is an exposed lysine residue that is strictly conserved in plant cystathionine γ-synthases whereas bacterial enzymes carry a glycine residue at this position. General insight regarding the reaction specificity of transsulphuration enzymes is gained by the comparison to cystathionine β-lyase from E. coli, indicating the mechanistic importance of a second substrate binding site for l-cysteine which leads to different chemical reaction types.