Heme ligation

His/Met and the di heme ligation is probably hydroxide rather that tyrosine. Thus, solution studies, as well as analyses of primary sequences, confirm that cytochrome cd from different sources can have different ligands to the heme irons. 

Wires terminated by hydrophobic substrates are incapable of coordinating the heme directly. When these wires bind P450CAM, water may or may not be displaced from the active site. Alkyl-tethered wires with ethylbenzene substrates do not produce significant changes in heme ligation or spin state on binding [35], whereas alkyl and perfluorobiphenyl-tethered adamantyl wires cause partial displacement of coordinated water from the active site [34-36]. 

Wallace CJA, Clark-Lewis I (1992) Functional Role of heme ligation in cytochrome c effects of replacement of methionine 80 with natural and non-natural residues by semisynthesis. J Biol Chem 267 3852-3861... 

Figure 15 The X-ray structure of HasA from S. marcescens showing heme ligated by Y75 and H32. The figure was created using pdb coordinates lb2v ...

Fig 19. The NIR MCD spectra at room temperature of (a) oxidized and (b) semire-duced diheme cytochrome c peroxidase Pseudomorms aeruginosa). The assignments of the various features of the spectra are indicated in terms of a simple model of heme ligation, (c) The complete enzymatic cycle established hy MCD and EPR spectroscopy in terms of the oxidation levels of the two hemes. 

Angelis, F.D., Jarzcki, A.A., Car, R., Spiro, T.G. Quantum chemical evaluation of protein control over heme ligation CO/02 discrimination in myoglobin. J. Phys. Chem. B 2005,109, 3065-70. 

SE Martinez, D Huang, A Szczepaniak, WA Cramer and JL Smith (1994) Crystal structure of the chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation. Structure 2 95-105... 

CJ Carrell, H Zhang, WA Cramer and JL Smith (1997) Biological identity and diversity in photosynthesis and respiration structure of the lumen-side domain of the chloroplast Rieske protein. Structure 5 1613-1625 SE Martinez, D Huang, A Szczepaniak, WA Cramer and JL Smith (1994) Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation. Structure 2 95-102 SE Martinez, D Huang, M Ponomarev, WA Cramer and JL Smith (1994) The heme redox center of chloroplast cytochrome f is linked to buried five-water chain. Protein Sci 5 1081-1092... 

HisBS N H proton shift spans 0.33 ppm at 5 °C among the Mb complexes (Table 2), indicating that the AspBl—HisBS hydrogen bond depends on the heme ligation state. Since the alteration of the electronic structure of either a hydrogen-donor or -acceptor influences the NH proton signal, the HisBS N H... 

As would be expected from the reduction potential of the environment where anaerobes live, most cytochromes found in these organisms have low-reduction potentials, associated with a bis-histidinyl coordination. However, even with this type of heme ligation, cytochromes with fairly high reduction potentials have been found in anaerobes, as will be discussed below. Only a few His-Met cytochromes have so far been identified in anaerobes, which have a reduction potential of 0 mV, lower than that of analogous cytochromes from aerobes. 

In early studies of cytochromes, spectroscopic methods were relied upon for determination of spin state, coordination number, and axial-heme ligation. Model hemes in which metal-ion size, charge, and d-orbital occupancy, as well as ligand structure and composition, could be systematically varied, were spectrally characterized for comparison to cytochromes. Synthetic 6cLS Fe and Fe porphyrinates have been studied for many years with the goal of clarifying structure-function relationships observed in cytochromes. 

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