Heme binding site

A protein, designated cyctochrome c", isolated from the methylotrophic bacterium Methylophilus methylotrophus, has been studied extensively because of its unusual properties and was found to have an average molecular mass of 14293.0 Da and to contain 124 amino acid residues. The A-terminal sequence to residue 62 had been determined and the heme binding site had been located at Cys-49 and Cys-52 [12]. Further studies were concerned with determining the remainder of the sequence. 

Inhibits all forms of P450 Competes with oxygen for heme-binding site Carbene forms generated, and these bind to heme Selective inhibitors... 

Burch, M.K. and Morgan, W.T. (1985) Preferred heme binding sites of histidine-rich glycoprotein. Biochemistry 24, 5919-5924. 

Curve D pyranine in the heme binding site of apomyoglobin, a site containing 30 water molecules or less. 

The functionalization of folded motifs is based on an understanding of secondary and tertiary structures (Fig. 2) and must take into account the relative positions of the residues, their rotamer populations and possible interactions with residues that do not form part of the site. For example, glutamic acid in position i has a strong propensity for salt-bridge formation, and thus reduced reactivity, if there is a Lys residue available i-4 in the sequence, but the probabihty is much less if the base is i-3 [60]. Fortunately, there is a wealth of structural information on the structural properties of the common amino acids from studies of natural proteins that provides considerable support for the design of new proteins. The naturally occurring amino acids have so far been used to construct reactive sites for catalysis [11-13], metal- and heme-binding sites [14,15,19,21,22] and for the site-selective functionalization of folded proteins [24,25]. 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

However, this explanation is not sufficient to accoimt for the bipha-sic CD spectrum of human ferri-protoheme—hemopexin (with 2,4-vinyl substituents), as well as the much weaker human CO-ferro-heme-hemopexin bisignate signal compared to the rabbit congener (139), and hence other factors must be involved. Several potential effectors exist (a) exciton coupling (b) the conformers produced by a 180° rotation about the a- and y-meso-carbon axis and consequent nonisometric interactions of the as5unmetric 2,4- and 9,10-substituents (c) the aromatic tryptophan residues near the heme binding site (s) and (d) two independent binding modes or sites. 

Fig. 6. Structure of truncated human HO-1 showing the location of the heme binding site (53). The heme propionic acid groups point away from the viewer, so that the 8-meso edge faces the exterior of the protein. The iron atom in the heme is not shown for clarity.

The rich spectroscopy and electrochemistry of the heme moiety yields a wealth of opportunities for the denovo heme protein design to evaluate the success of the heme binding site design. Combinations of these spectroscopic and electrochemical methods are elucidating the structure and function of de novo heme proteins and illustrating that they serve as excellent bioinorganic model complexes for simple cytochromes. 

A novel cytochrome b model compound containing two heme binding sites was tested by EPR and Mossbauer spectroscopy. Both sites were found to be occupied in a 1 1 stoichiometry. For one of the sites a normal b-hemichrome EPR signal with rhombic -tensor symmetry was found as expected for a bis-histidine ligation with parallel histidine planes. For the other site unusual EPR features (e.g. axial y-tensor) were obtained which were ascribed to a configuration with perpendicular histidine planes.267 A new type of cytochrome b was described for a preparation from the cytoplasmic fraction of an archaeon, Acidianus ambivalens. This is thus the first soluble cytochrome found in this... 

Figure 29 Possible heme-binding sites in cytochrome b (reproduced with permission from FEBS Lett., 1984, 166, 367)...

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