Cytochrome structure-function correlation

In a very broad overview of the structural categories one can state several statistical correlations with type of function. Hemes are almost always bound by helices, but never in parallel a//3 structures. Relatively complex enzymatic functions, especially those involving allosteric control, are occasionally antiparallel /3 but most often parallel a//3. Binding and receptor proteins are most often antiparallel /3, while the proteins that bind in those receptor sites (i.e., hormones, toxins, and enzyme inhibitors) are most apt to be small disulfide-rich structures. However, there are exceptions to all of the above generalizations (such as cytochrome cs as a nonhelical heme protein or citrate synthase as a helical enzyme), and when one focuses on the really significant level of detail within the active site then the correlation with overall tertiary structure disappears altogether. For almost all of the dozen identifiable groups of functionally similar proteins that are represented by at least two known protein structures, there are at least... 

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. 

Although it may be too early to feel absolutely secure about the functional structural correlations, these studies establish that steroids, depending upon their structure, can guide the NADPH hydrogen generated by the hexose monophosphate shunt into either the hydroxylation pathway through the cytochrome P450 electron transport chain or into biosynthetic pathways. 

When scientists examine the primary structures of proteins that carry out the same function in different organisms, they can correlate the number of amino acid differences in the proteins to the closeness of the taxonomic relationship between the species. For exarr5)le, c)h ochrome c, a protein that transfers electrons in biological oxidations, has about 100 amino acids. Yeast cytochrome c differs by 48 amino acids from horse cytochrome c, whereas duck cytochrome c differs by only two amino acids from chicken cytochrome c. Ducks and chickens, therefore, have a much closer taxonomic relationship than horses and yeast. Likewise, the cytochrome c in chickens and turkeys have identical primary structures. Human cytochrome c and chimpanzee cytochrome c are also identical and differ by one amino acid from the C54ochrome c of the rhesus monkey. 

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