Heme group c-type

One of a few cases in which electron transfer of redox enzymes is expressed directly and reversibly at an electrode is concerned with p-cresol methylhydroxylase (PCMH). This is a flavocytochrome c enzyme of 115 kDa, which catalyzes the oxidative hydroxylation of p-cresol to p-hydroxybenzyl alcohol and subsequently to p-hydroxyben-zaldehyde. The structure of PCMH has recently been determined (56) at 3 A resolution. It is an a2 2 tetramer, with one subunit containing a covalently bound FAD and the other containing a c-type heme group. 

X-ray structures of mitochondrial 6ci complexes from three different sources (113, 124, 125) have found the b- and c-type hemes at roughly identical positions, whereas the Rieske protein was seen in different places as a function of crystal space group and presence or absence of inhibitors of the enzyme. This fact was interpreted to suggest a long-range conformational movement of the Rieske protein during turnover of the complex. The range of observed positions of the Rieske protein indicated that the soluble domain can move like a... 

There is good evidence to show that the two hemes in Pseudomonas cytochrome c peroxidase are not equivalent. One is a c type heme (+320 mV), and the other is a c heme (—330 mV) and is a peroxidatic group. A number of techniques, including CD, absorption and resonance Raman spectroscopy, show that the fifth ligand in both hemes is an imidazole group. In the oxidized... 

These enzymes are homodimeric periplasmic proteins (encoded by nirB) that contain two different heme groups per monomer. One is a c-type heme bound via a Cys-X-X-Cys-His motif and has a role in electron transfer the other, a noncovalently linked d heme, is an unusual ferric-dioxoisobacteriochlorin, restricted to this class of enzyme and forms the active site. The biosynthetic pathway of heme di is unique among the tetrapyrroles in having 0x0, methyl, and acrylate side chains, involving some seven nir genes. The Em values of heme d model complexes are 200mV lower than iron porphyrins. 

Cytochrome c peroxidase (CcP) [215] isolated from yeast mitochondria uses cytochrome c(//) as electron donor. Its crystal structure has been determined [216]. It is a monomer of molecular mass 34000 containing a single h-type heme group that is largely buried within the molecule. The Fe is coordinated on one side by a histidine residue while the sixth position is occupied by a weakly bound HjO in a channel suitable for entry and exit of small substrates. 

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

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