Enzymology
The human B12 trafficking protein CblC processes nitrocobalamin
In humans, cobalamin or vitamin B12 is delivered to two target enzymes via a complex intracellular trafficking pathway comprising transporters and chaperones. CblC (or MMACHC) is a processing chaperone that catalyzes an early step in this trafficking pathway. CblC removes the upper axial ligand of cobalamin derivatives, forming an intermediate in the pathway that is subsequently converted to the active cofactor derivatives. Mutations in the cblC gene lead to methylmalonic aciduria and homocystinuria.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.Structure of Human B12 Trafficking Protein CblD Reveals Molecular Mimicry and Identifies a New Subfamily of Nitro-FMN Reductases
Background: Mutations in CblD, involved in B12 (or cobalamin) trafficking, lead to disease.Results: The first crystal structure of human CblD is reported.Conclusion: CblD most closely resembles CblC, another cobalamin trafficking protein, and they belong in a new subclass with the nitro-FMN reductase superfamily.Significance: Disease-causing mutations that impair cobalamin oxidation kinetics can now be localized on the CblD structure.Pathogenic Mutations Differentially Affect the Catalytic Activities of the Human B12-processing Chaperone CblC and Increase Futile Redox Cycling
Human CblC catalyzes the elimination of the upper axial ligand in cobalamin or B12 derivatives entering the cell from circulation. This processing step is critical for assimilation of dietary cobalamin into the active cofactor forms that support the B12-dependent enzymes, methionine synthase and methylmalonyl-CoA mutase. Using a modified nitroreductase scaffold tailored to bind cobalamin and glutathione, CblC exhibits versatility in the mechanism by which it removes cyano versus alkyl ligands in cobalamin.
