The Three-Dimensional Structure of Cystathionine β-Lyase from Arabidopsis and Its Substrate Specificity

Abstract

Abstract The pyridoxal 5′-phosphate-dependent enzyme cystathionine β-lyase (CBL) catalyzes the penultimate step in the de novo biosynthesis of Met in microbes and plants. Absence of CBL in higher organisms makes it an important target for the development of antibiotics and herbicides. The three-dimensional structure of cystathionine β-lyase from Arabidopsis was determined by Patterson search techniques, using the structure of tobacco (Nicotiana tabacum) cystathionine γ-synthase as starting point. At a resolution of 2.3 Å, the model was refined to a final crystallographic R-factor of 24.9%. The overall structure is very similar to other pyridoxal 5′-phosphate-dependent enzymes of the γ-family. Exchange of a few critical residues within the active site causes the different substrate preferences betweenEscherichia coli and Arabidopsis CBL. Loss of interactions at the α-carboxyl site is the reason for the poorer substrate binding of Arabidopsis CBL. In addition, the binding pocket of Arabidopsis CBL is larger than that of E. coli CBL, explaining the similar binding of l-cystathionine andl-djenkolate in Arabidopsis CBL in contrast to E. coli CBL, where the substrate binding site is optimized for the natural substrate cystathionine.