Human cystathionine β-synthase (CBS) contains two classes of binding sites for S-adenosylmethionine (SAM): complex regulation of CBS activity and stability by SAM

Biochem J. 2013 Jan 1;449(1):109-21. doi: 10.1042/BJ20120731.


CBS (cystathionine β-synthase) is a multidomain tetrameric enzyme essential in the regulation of homocysteine metabolism, whose activity is enhanced by the allosteric regulator SAM (S-adenosylmethionine). Missense mutations in CBS are the major cause of inherited HCU (homocystinuria). In the present study we apply a novel approach based on a combination of calorimetric methods, functional assays and kinetic modelling to provide structural and energetic insight into the effects of SAM on the stability and activity of WT (wild-type) CBS and seven HCU-causing mutants. We found two sets of SAM-binding sites in the C-terminal regulatory domain with different structural and energetic features: a high affinity set of two sites, probably involved in kinetic stabilization of the regulatory domain, and a low affinity set of four sites, which are involved in the enzyme activation. We show that the regulatory domain displays a low kinetic stability in WT CBS, which is further decreased in many HCU-causing mutants. We propose that the SAM-induced stabilization may play a key role in modulating steady-state levels of WT and mutant CBS in vivo. Our strategy may be valuable for understanding ligand effects on proteins with a complex architecture and their role in human genetic diseases and for the development of novel pharmacological strategies.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites / genetics
  • Binding Sites / physiology
  • Cystathionine beta-Synthase / chemistry*
  • Cystathionine beta-Synthase / genetics
  • Cystathionine beta-Synthase / pharmacokinetics*
  • Enzyme Activation / genetics
  • Enzyme Activation / physiology
  • Homocystinuria / enzymology
  • Homocystinuria / genetics
  • Homocystinuria / metabolism
  • Humans
  • Protein Binding / genetics
  • Protein Stability
  • S-Adenosylmethionine / chemistry*
  • S-Adenosylmethionine / pharmacokinetics
  • S-Adenosylmethionine / physiology*


  • S-Adenosylmethionine
  • Cystathionine beta-Synthase