Amicyanin function

Amicyanins function as electron carriers in the respiratory chains of some me-thylotrophic bacteria, e.g., Thiobacillus versutus [92]. They transfer single electrons from methylamine dehydrogenase to a cytochrome c [78] which then transfers the electron to cytochrome c oxidase. Amicyanin from Pseudomonas denitrificans has a molecular mass of 11.6 kD and contains 106 amino acid residues. Amicyanin contains one /J-sheet more than the eight of plastocyanin and pseudoazurin, the result of several additional amino acids at its N-terminus [78] (Fig. 15). Like pseudoazurin, amicyanin is found exclusively in bacteria. 

Amicyanin (Husain and Davidson, 1986 Groeneveld etal., 1988) spectroscopically resembles plastocyanin more than pseudoazurin and has about the same number of amino acids, so that its classiflcation has been changed from subgroup II to III (the plastocyanin group see Table II). However, its sequence is distinctly different than the plastocyanins, and the new function may indicate yet another class. 

How is the reduced cofactor reoxidized Presumably the copper ion adjacent to the TPQ functions in this process, passing electrons one at a time to the next carrier in a chain. There is no copper in the TTQ-containing subunits. Electrons apparently must jump about 1.6 ran to the copper ion of amicyanin, then another 2.5 nm to the iron ion of the cytochrome c.472 Reoxidation of the aminoquinol formed in Eq. 15-53, step d, yields a Schiff base whose hydrolysis will release ammonia and regenerate the TTQ. Intermediate states with Cu+ and a TTQ semiquinone radical have been observed.4833... 

FIGURE 2. The reaction mechanism of MADH. Only the quinone portion of TTQ is shown. Bi to Bi 2 represent active-site residues that may function as general acids or bases in the reaction mechanism. AMI represents amicyanin and M+ is a monovalent cation. The details of the reaction mechanisms are presented in the text. 

Two amicyanins from the methylotropic bacteria Pseudomonas AMI and T. versutus have been a recent focus of attention (42, 43). Their function is to mediate electron transfer between bacterial cytochrome c and methylamine dehydrogenase in a relatively short electron transport chain. 

Aromatic residues have been found in proteins at positions that probably enhance the electronic coupling in systems that have been selected by evolution for efficient ET. Examples are the tryptophan mediated reduction of quinone in the photosynthetic reaction center (31), the methylamine dehydrogenase (MADH) amicyanin system, where a Trp residue is placed at the interface between the two proteins (32), as well as the [cytochrome c peroxidase-cytochrome c] complex, where a Trp seems to have a similar function (33). 

A. Structure and Function of Amicyanin and Cytochrome C550 1.. Amicyanin... 

The overlap contribution to decoherence is plotted in Figure 5.11. We recall that since the full electron transfer system (TTQ and the copper center) is not included in the modeling the phase term Jp ase)) the contributions of other atoms e.g- that of the amicyanin) are not taken into account. For ease of description the characteristic decay constant of the function will be... 

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