Heme Iron Coordination Research Articles

Reaction of chlorocruorin with heme iron ligands and carbonyl reagents.

Chlorocruorin was purified from Potamilla leptochaeta and the spectral properties of its derivatives wwere investigated. Ferri- or ferrochlorocruorin did not exhibits a ferrihemochrome or ferrohemochrome spectrum, respectively. Oxy- and carbonmonoxy-ferrochlorocruorin did show ferrohemochrome-type spectra. Ferrihemochromes were formed, however, when oxy-or ferrichlorocruorin was treated with 0.02-0.05% SDS, and they were transformed to ferrohemochromes by reduction with sodium dithionite. Ferrihemochrome formation was also brought about by increasing the pH of a ferrichlorocruorin solution to 9, or by liganding of extrinsic imidazole or cyanide to the ferric pigment. Therefore, it is apparent that at least one of the coordination positions on the heme iron in ferri-and ferrochlorocruorin is vacant or occupied by a weak-field ligand. Titration studies of ferrichlorocruorin with imidazole indicated that this supposedly vacant coordination position was occupied first by the imidazole, and that the intrinsic ligand of protein orgin was replaced finally at higher concentrations. The extrinsic ligands in the cyanide and imidazole complexes of ferrichlorocruorin were excluded from their coordination positions as the protein moiety assumed conformations inherent to the reduced pigment. Spectral analyses indicated that the intrinsic ligand is an imidazole moiety of a histidyl residue. When chlorocruorin was intact, carbonyl reagents such as cyanide and sodium bisulfite did not add to the formyl group of chlorocruoreheme. When the protein conformation was perturbed by SDS, addition to ferrichlorocruorin occurred appreciably. This addition was accelerated if the heme iron coordination position had been occupied by strong field ligands,and was reversed to some extent as the chlorocruorin complexes were reduced.

The pH-induced transition of iodinated ferricytochrome c: electron paramagnetic resonance and absorption spectra.

The heme-associated, pH-induced transition in iodinated ferricytochrome c has been studied by electron paramagnetic resonance (E.P.R.) and absorption spectroscopy. The conversion from type III to type IV (as defined by Theorell, H., and Åkesson, Å.: J. Am. Chem. Soc. 63, 1812 (1941)) ferricytochrome c forms was dramatically altered by iodination of tyrosyl residues. Both absorption and E.P.R. spectra suggested that a transition between similar heme-iron coordination structures occurred with a pK of about 6 as compared with about 9 for native ferricytochrome c. At pH 4.4 the E.P.R. spectrum (liquid nitrogen temperature) of a frozen solution of iodinated ferricytochrome c was similar to the native type III at pH 7, except for an increased g = 6.0 signal from high-spin heme iron. At neutral pH the E.P.R. spectrum of the iodinated derivative was similar to type IV ferricytochrome c. The results give further support to the hypothesis that the pK of tyrosine 67 plays an important role in determining the pK of the III to IV transition.