title: The redox behavior of the heme in cystathionine beta-synthase is sensitive to pH.
abstract: Human cystathionine beta-synthase (CBS) is a unique pyridoxal-5'-phosphate-dependent enzyme in which heme is also present as a cofactor. Because the function of heme in this enzyme has yet to be elucidated, the study presented herein investigated possible relationships between the chemistry of the heme and the strong pH dependence of CBS activity. This study revealed, via study of a truncation variant, that the catalytic core of the enzyme governs the pH dependence of the activity. The heme moiety was found to play no discernible role in regulating CBS enzyme activity by sensing changes in pH, because the coordination sphere of the heme is not altered by changes in pH over a range of pH 6-9. Instead, pH was found to control the equilibrium amount of ferric and ferrous heme present after reaction of CBS with one-electron reducing agents. A variety of spectroscopic techniques, including resonance Raman, magnetic circular dichroism, and electron paramagnetic resonance, demonstrated that at pH 9 Fe(II) CBS is dominant while at pH 6 Fe(III) CBS is favored. At low pH, Fe(II) CBS forms transiently but reoxidizes by an apparent proton-gated electron-transfer mechanism. Regulation of CBS activity by the iron redox state has been proposed as the role of the heme moiety in this enzyme. Given that the redox behavior of the CBS heme appears to be controlled by pH, interplay of pH and oxidation state effects must occur if CBS activity is redox regulated.
authors.affiliation: Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: Samuel
authors.initials: S
authors.lastname: Pazicni
authors.forename: Gudrun S
authors.initials: GS
authors.lastname: Lukat-Rodgers
authors.forename: Jana
authors.initials: J
authors.lastname: Oliveriusová
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Rees
authors.forename: Ryan B
authors.initials: RB
authors.lastname: Parks
authors.forename: Robert W
authors.initials: RW
authors.lastname: Clark
authors.forename: Kenton R
authors.initials: KR
authors.lastname: Rodgers
authors.forename: Jan P
authors.initials: JP
authors.lastname: Kraus
authors.forename: Judith N
authors.initials: JN
authors.lastname: Burstyn
mesh_terms: D002942:Circular Dichroism; D019343:Citric Acid / Q000737:chemistry; D003067:Coenzymes / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology; D003541:Cystathionine beta-Synthase / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology; D004227:Dithionite / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004789:Enzyme Activation / Q000502:physiology; D005290:Ferric Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D019163:Reducing Agents / Q000737:chemistry; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman
chemical_list: D003067:Coenzymes; D005290:Ferric Compounds; D006420:Hemeproteins; D019163:Reducing Agents; C419387:titanium citrate; D004227:Dithionite; D019343:Citric Acid; D006418:Heme; D003541:Cystathionine beta-Synthase