title: Mercuric chloride-induced spin or ligation state changes in ferric or ferrous human cystathionine beta-synthase inhibit enzyme activity.
abstract: Cystathionine beta-synthase is a key heme and pyridoxal phosphate-dependent enzyme involved in homocysteine metabolism in humans. The role of the recently discovered heme in this protein remains an important open question. The axial ligands to the heme in both the ferrous and ferric states have been assigned as cysteine and histidine residues, respectively. In this study, we have examined the effect of ligation and spin state changes in the heme on the activity of the enzyme. Treatment of the ferric enzyme with HgCl2 results in the conversion of six-coordinate low-spin heme to five-coordinate high-spin heme and is paralleled by a loss of activity. In contrast, treatment of the ferrous enzyme with HgCl2 results in replacement of the cysteine ligand by an unidentified sixth ligand and retention of the six-coordinate state, and is also accompanied by loss of enzyme activity. Treatment of the five-coordinate HgCl2-treated enzyme with thiols, such as homocysteine, results in reversion to a six-coordinate state. Resonance Raman spectroscopy with 34S-labeled enzyme reveals the return of the endogenous thiol ligand under these conditions and rules out direct coordination by the thiolate of homocysteine to the heme.
authors.affiliation: Department of Biochemistry, University of Nebraska, Lincoln, NE 68588-0664, USA.
authors.forename: S
authors.initials: S
authors.lastname: Taoka
authors.forename: E L
authors.initials: EL
authors.lastname: Green
authors.forename: T M
authors.initials: TM
authors.lastname: Loehr
authors.forename: R
authors.initials: R
authors.lastname: Banerjee
mesh_terms: D003541:Cystathionine beta-Synthase / Q000037:antagonists & inhibitors*; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D006710:Homocysteine / Q000494:pharmacology; D006801:Humans; D008627:Mercuric Chloride / Q000494:pharmacology*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman; D013438:Sulfhydryl Compounds / Q000494:pharmacology
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D013438:Sulfhydryl Compounds; D006710:Homocysteine; D006418:Heme; D008627:Mercuric Chloride; D003541:Cystathionine beta-Synthase