Structure of soybean β-cyanoalanine synthase and the molecular basis for cyanide detoxification in plants

Plant Cell. 2012 Jun;24(6):2696-706. doi: 10.1105/tpc.112.098954. Epub 2012 Jun 26.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Catalytic Domain
  • Crystallography, X-Ray
  • Cyanides / pharmacokinetics*
  • Cysteine Synthase / chemistry
  • Cysteine Synthase / metabolism
  • Glycine max / drug effects
  • Glycine max / enzymology
  • Glycine max / metabolism*
  • Inactivation, Metabolic
  • Lyases / chemistry*
  • Lyases / genetics
  • Lyases / metabolism*
  • Models, Molecular
  • Mutation
  • Protein Conformation
  • Substrate Specificity

Substances

  • Cyanides
  • Cysteine Synthase
  • Lyases
  • beta-cyanoalanine synthase

Associated data

  • PDB/3VBE
  • PDB/3VC3