title: Structure of soybean β-cyanoalanine synthase and the molecular basis for cyanide detoxification in plants.
abstract: Plants produce cyanide (CN-) during ethylene biosynthesis in the mitochondria and require β-cyanoalanine synthase (CAS) for CN- detoxification. Recent studies show that CAS is a member of the β-substituted alanine synthase (BSAS) family, which also includes the Cys biosynthesis enzyme O-acetylserine sulfhydrylase (OASS), but how the BSAS evolved distinct metabolic functions is not understood. Here we show that soybean (Glycine max) CAS and OASS form α-aminoacrylate reaction intermediates from Cys and O-acetylserine, respectively. To understand the molecular evolution of CAS and OASS in the BSAS enzyme family, the crystal structures of Gm-CAS and the Gm-CAS K95A mutant with a linked pyridoxal phosphate (PLP)-Cys molecule in the active site were determined. These structures establish a common fold for the plant BSAS family and reveal a substrate-induced conformational change that encloses the active site for catalysis. Comparison of CAS and OASS identified residues that covary in the PLP binding site. The Gm-OASS T81M, S181M, and T185S mutants altered the ratio of OASS:CAS activity but did not convert substrate preference to that of a CAS. Generation of a triple mutant Gm-OASS successfully switched reaction chemistry to that of a CAS. This study provides new molecular insight into the evolution of diverse enzyme functions across the BSAS family in plants.
authors: Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
authors: Hankuil
authors: H
authors: Yi
authors: Matthew
authors: M
authors: Juergens
authors: Joseph M
authors: JM
authors: Jez
mesh_terms: D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D003486:Cyanides / Q000493:pharmacokinetics*; D003547:Cysteine Synthase / Q000737:chemistry / Q000378:metabolism; D008658:Inactivation, Metabolic; D008190:Lyases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D009154:Mutation; D011487:Protein Conformation; D013025:Glycine max / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D003486:Cyanides; D003547:Cysteine Synthase; D008190:Lyases; C019955:beta-cyanoalanine synthase