Enzymology
H2S oxidation by nanodisc-embedded human sulfide quinone oxidoreductase
Buildup of hydrogen sulfide (H2S), which functions as a signaling molecule but is toxic at high concentrations, is averted by its efficient oxidation by the mitochondrial sulfide oxidation pathway. The first step in this pathway is catalyzed by a flavoprotein, sulfide quinone oxidoreductase (SQR), which converts H2S to a persulfide and transfers electrons to coenzyme Q via a flavin cofactor. All previous studies on human SQR have used detergent-solubilized protein. Here, we embedded human SQR in nanodiscs (ndSQR) and studied highly homogenous preparations by steady-state and rapid-kinetics techniques.Kinetics of Nitrite Reduction and Peroxynitrite Formation by Ferrous Heme in Human Cystathionine β-Synthase
Cystathionine β-synthase (CBS) is a pyridoxal phosphate-dependent enzyme that catalyzes the condensation of homocysteine with serine or with cysteine to form cystathionine and either water or hydrogen sulfide, respectively. Human CBS possesses a noncatalytic heme cofactor with cysteine and histidine as ligands, which in its oxidized state is relatively unreactive. Ferric CBS (Fe(III)-CBS) can be reduced by strong chemical and biochemical reductants to Fe(II)-CBS, which can bind carbon monoxide (CO) or nitric oxide (NO•), leading to inactive enzyme.Transient Kinetic Analysis of Hydrogen Sulfide Oxidation Catalyzed by Human Sulfide Quinone Oxidoreductase
Background: Sulfide quinone oxidoreductase is a flavoprotein that catalyzes the first step in H2S oxidation.Results: Transient kinetic studies reveal the presence of an equally intense charge-transfer (CT) band in the presence of sulfite as with sulfide.Conclusion: Only the CT intermediate formed in the presence of sulfide is kinetically competent.Significance: An off-pathway reaction could be promoted under pathologically high sulfite concentrations.
