Cytochrome single-metal redox

We have investigated in-situ STM of the small single-metal redox proteins cytochrome c (MW 12 kDa) and azurin ( 14 kDa), and the larger four copper-enzyme laccase (MW 64 kDa), all involved in natural ET [51, 54, 55]. The choice rested on the following considerations ... 

In this chapter, we overview first some recent examples of interfacial electrochemical ET of composite metalloproteins where molecular mechanistic detail has in some way been achieved. We discuss next some theoretical issues regarding in situ STM of large molecules, where resonance or environmentally activated tunnel channels are opened by the redox metal centre. This is followed by an overview of some recent achievements in the area of in situ STM/AFM of the single-metal proteins cytochrome c and azurin on polycrystalline and single-crystal platinum and gold surfaces. Such an integrated approach offers new perspectives for experimental and theoretical characterization of metalloproteins at solid surfaces in contact with the natural aqueous medium for metalloprotein function. 

Superoxide complexes are also invoked as intermediates in the reactions of bleomycin, catechol dioxygenases, and cytochrome P450. They can be expected to occur in cases where the redox cofactor consists of a single metal ion in the absence of other readily oxidizable cofactors. 

Not all cytochromes from sulfate-reducing bacteria reduce Fe(III) or other metals. D. vulgaris produces a cyt C553, which has a molecular mass of 9 kDa, midpoint redox potential of OmV, and a single heme and the iron atom is coordinated by histidine methionine. It is unclear at this time if the inability of this cyt C553 to reduce metals is due to lack of a bishistidinyl iron coordination or to some other factor, such as steric hinderance owing to orientation of heme in the protein. 

From considerations of these new clusters, as well as other, better-known ones such as those found in cytochrome oxidase (185) and di-and trinuclear copper clusters (186), one can almost get the impression that some lonely metals do not want to be single and need a (redox) partner. This partner could in some cases be an iron ion, and they are linked by a /n-oxo or /x-hydroxo bridge More seriously, as discussed in,... 

The following consideration of cytochrome c oxidase reviews (1) composition, (2) structure, (3) overall reaction, (4) the electron transfer steps of cytochrome c oxidase, (5) status of proton translocation and proposed aqueous D- and K-channels for proton ingress, (6) the redox Bohr effect and its correlation with electrochemical transduction in elastic-contractile model proteins, and (7) possible molecular sources of protons for translocation with an abundant and uniquely positioned functional side chain that exhibits interesting parallels to the states of QH2 with, however, single rather than double proton changes and coordination to a metal ion required for electron transfer capability. 

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