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.
Human cystathionine β-synthase (CBS) contains two classes of binding sites for S-adenosylmethionine (SAM): complex regulation of CBS activity and stability by SAM
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Angel L. Pey, Tomas Majtan, Jose M. Sanchez-Ruiz, Jan P. Kraus; 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 1 January 2013; 449 (1): 109–121. doi: https://doi.org/10.1042/BJ20120731
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