Model of the coordination site for cobalt-substituted cytochrome P450cam

Cobalt cystathionine β-synthase: a cobalt-substituted heme protein with a unique thiolate ligation motif

Human cystathionine β-synthase (hCBS), a key enzyme in the trans-sulfuration pathway, catalyzes the condensation of serine with homocysteine to produce cystathionine. CBS from higher organisms is the only known protein that binds pyridoxal-5'-phosphate (PLP) and heme. Intriguingly, the function of the heme in hCBS has yet to be elucidated. Herein, we describe the characterization of a cobalt-substituted variant of hCBS (Co hCBS) in which CoPPIX replaces FePPIX (heme). Co(III) hCBS is a unique Co-substituted heme protein: the Co(III) ion is 6-coordinate, low-spin, diamagnetic, and bears a cysteine(thiolate) as one of its axial ligands. The peak positions and intensities of the electronic absorption and MCD spectra of Co(III) hCBS are distinct from those of previously Co-substituted heme proteins; TD-DFT calculations reveal that the unique features arise from the 6-coordinate Co bound axially by cysteine(thiolate) and a neutral donor, presumably histidine. Reactivity of Co(III) hCBS with HgCl(2) is consistent with a loss of the cysteine(thiolate) ligand. Co(III) hCBS is slowly reduced to Co(II) hCBS, which contains a 5-coordinate, low-spin, S = 1/2 Co-porphyrin that does not retain the cysteine(thiolate) ligand; this form of Co(II) hCBS binds NO((g)) but not CO((g)). Co(II) hCBS is reoxidized in the air to form a new Co(III) form, which does not contain a cysteine(thiolate) ligand. Canonical and alternative CBS assays suggest that maintaining the native heme ligation motif of wild-type Fe hCBS (Cys/His) is essential in maintaining maximal activity in Co hCBS. Correlation between the coordination structures and enzyme activity in both native Fe and Co-substituted proteins implicates a structural role for the heme in CBS.

Unusual hyperporphyrin spectrum by bis(glutathione dimethyl ester)-hemin complex, a model of cytochrome P-450-thiolate complexes

A bis(thiolato) complex of iron-porphyrin exhibiting a "split Soret band" with peaks at 374 and 466 nm, classified as a "hyperporphyrin (HP) spectrum", was prepared with Fe(III)protoporphyrin dimethyl ester (FePPIXDME), glutathione dimethyl ester (GSHDME) and tetrabutylammonium hydroxide in acetone solvent. The structure of the complex was characterized by comparing their electronic absorption and electron spin resonance (ESR) spectra with those of thiolate adducts of cytochrome P-450 and their chemical model complexes. Under the same conditions as those for the formation of the HP spectrum, the generation of the free superoxide anions was detectable by ESR spectrometry at 77K. On the basis of the results, the possible reaction mechanism generating the superoxide anions by the bis(GSHDME)-FePPIXDME complex was proposed.