Interactions between serine acetyltransferase and O-acetylserine (thiol) lyase in higher plants--structural and kinetic properties of the free and bound enzymes

Eur J Biochem. 1998 Jul 1;255(1):235-45. doi: 10.1046/j.1432-1327.1998.2550235.x.


The last steps of cysteine synthesis in plants involve two consecutive enzymes. The first enzyme, serine acetyltransferase, catalyses the acetylation of L-serine in the presence of acetyl-CoA to form O-acetylserine. The second enzyme, O-acetylserine (thiol) lyase, converts O-acetylserine to L-cysteine in the presence of sulfide. We have, in the present work, over-produced in Escherichia coli harboring various type of plasmids, either a plant serine acetyltransferase or this enzyme with a plant O-acetylserine (thiol) lyase. The free recombinant serine acetyltransferase (subunit mass of 34 kDa) exhibited a high propensity to form high-molecular-mass aggregates and was found to be highly unstable in solution. However, these aggregates were prevented in the presence of O-acetylserine (thiol) lyase (subunit mass of 36 kDa). Under these conditions homotetrameric serine acetyltransferase associated with two molecules of homodimeric O-acetylserine (thiol) lyase to form a bienzyme complex (molecular mass approximately 300 kDa) called cysteine synthase containing 4 mol pyridoxal 5'-phosphate/mol complex. O-Acetylserine triggered the dissociation of the bienzyme complex, whereas sulfide counteracted the action of O-acetylserine. Protein-protein interactions within the bienzyme complex strongly modified the kinetic properties of plant serine acetyltransferase: there was a transition from a typical Michaelis-Menten model to a model displaying positive kinetic co-operativity with respect to serine and acetyl-CoA. On the other hand, the formation of the bienzyme complex resulted in a very dramatic decrease in the catalytic efficiency of bound O-acetylserine (thiol) lyase. The latter enzyme behaved as if it were a structural and/or regulatory subunit of serine acetyltransferase. Our results also indicated that bound serine acetyltransferase produces a build-up of O-acetylserine along the reaction path and that the full capacity for cysteine synthesis can only be achieved in the presence of a large excess of free O-acetylserine (thiol) lyase. These findings contradict the widely held belief that such a bienzyme complex is required to channel the metabolite intermediate O-acetylserine.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetyltransferases / chemistry
  • Acetyltransferases / genetics
  • Acetyltransferases / metabolism*
  • Arabidopsis / enzymology*
  • Chloroplasts / enzymology
  • Cysteine / biosynthesis*
  • Cysteine Synthase / chemistry
  • Cysteine Synthase / genetics
  • Cysteine Synthase / metabolism*
  • Enzyme Stability
  • Escherichia coli / enzymology
  • Multienzyme Complexes / chemistry
  • Multienzyme Complexes / genetics
  • Multienzyme Complexes / metabolism*
  • Protein Binding
  • Protein Conformation
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism
  • Serine / analogs & derivatives
  • Serine / metabolism
  • Serine O-Acetyltransferase
  • Species Specificity
  • Titrimetry


  • Multienzyme Complexes
  • Recombinant Proteins
  • Serine
  • Acetyltransferases
  • Serine O-Acetyltransferase
  • Cysteine Synthase
  • O-acetylserine
  • Cysteine