Histidine ligation

The carbonic anhydrase (Cam) in M. thermophila cells is elevated several fold when the energy source is shifted to acetate, suggesting a role for this enzyme in the acetate-fermentation pathway. It is proposed that Cam functions outside the cell membrane to convert CO2 to a charged species (reaction A4) thereby facilitating removal of product from the cytoplasm. Cam is the prototype of a new class (y) of carbonic anhydrases, independently evolved from the other two classes (a and P). The crystal structure of Cam reveals a novel left-handed parallel P-helix fold (Kisker et al. 1996). Apart from the histidines ligating zinc, the activesite residues of Cam have no recognizable analogs in the active sites of the a- and P-classes. Kinetic analyses establish that the enzyme has a zinc-hydroxide mechanism similar to that of Cab (Alber et al. 1999). 

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... 

Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the exact cancellation limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state. 

Figure 7. Schematic view of the push-pull mechanism for 0-0 bond cleavage of an iron-bound peroxide in thiolate-ligated (left) and histidine-ligated (right) systems such as P450 and HRP, respectively. (Adapted from Ref. [6].)...

The resonance Raman spectra of wild-type, natural-abundance and uniformly 15N-labelled archaeal Rieske-type ferredoxin show significant mixing of vFeN and vFeS for an oxidised biological [2Fe-2S] cluster with partial histidine ligation.295... 

Comparative geometric and electronic structure of low-spin ferro- and ferrihemes as models of the fe-histidine-ligated electron-transferring cytochromes 04CRV589. 

The redox sensitivity of hemopexin-encapsulated heme to electrolyte composition and pH illustrate the importance of first coordination shell (bis-histidine ligation and heme structure) and second coordination shell (protein structure/folding and environment) effects in these heme proteins. These observations also suggest a possible role for Fe " /Fe redox in hemopexin-mediated heme transport/recycling, as high chloride anion concentration and low pH are known conditions for the endosome where the heme is released. 

Cytochrome c peroxidase (CcP) and horseradish peroxidase (HRP) contain axial histidine ligation and the oxo group from compound I is released as water (Equation 3.5). Cytochrome c peroxidase oxidizes two molar equivalents of cytochrome c. HRP reacts with a number of electron donor substrates such as alkylamines... 

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