Amicyanin and Cytochrome

Structure of the MADH-Amicyanin-Cytochrome c-551i Complex 

Unlike the MADH-amicyanin protein interface which was discussed earlier, the amicyanin-cytochrome c-551i interface revealed in the crystal structure of the complex is relatively hydrophilic. The association between proteins appears to be stabilized by several hydrogen bond and ionic interactions (Chen et al., 1994). The shortest gap between the proteins, for consideration of a possible site for interprotein electron transfer, is from the backbone O of Glu of amicyanin to the backbone N of Glyof cytochrome c-551i, a distance of less than 3.  

FIGURE 6. Orientation of redox cofactors in the MADH-amicyanin-cytochrome c-551i complex. A portion of the crystal structure is shown with the direct distances between the cofactors indicated. Coordinates are available in the Brookhaven Protein Data Bank, entry 2MTA. 

Pathways Analysis of the Electron Transfer Protein Complex 

For electron transfer from copper to heme, two dominant sets of pathways of comparable efficiency were predicted. In each set of pathways, the point of intermolecular electron transfer was from the backbone O of Glu i of amicyanin to the backbone N of Gly of cytochrome c-551i, and the entry of electrons to iron occurred either via the porphyrin ring or the His ligand. In one set of pathways the exit of electrons from copper occurred via the Cys copper ligand, and the phenolic side chain of Tyr was an intermediate between Cys and Glu. In the other set of pathways the exit of electrons from copper occurred via the Met copper ligand, and the backbone of Lys was an intermediate between Met and Glu i 

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Chen, L., Durley, R., Mathews, F. S., and Davidson, V. L., 1994, Structure of an electron transfer complex Methylamine dehydrogenase, amicyanin and cytochrome c-551i. Science 264 86990. 

Figure 16 Structure of the ternary electron-transfer complex of MADH, amicyanin, and cytochrome c-551 (pdb code 2MTA). The copper atom of amicyanin is 9.4 A from the edge of Trp 108 in the electron donor MADH and 24.7 A from the electron acceptor iron atom in cytochrome c-551...

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

The three-dimensional structures of MADH from P. versutus and P. denitrificans have been determined in the presence and absence of amicyanin, and in the presence of amicyanin and cytochrome Cssk (57, 59-61, 92, 159). The architecture of the tetramer is best described as consisting of two a/3 catal3d ic units (Fig. 15). Each a-subunit interacts with the -subunit in the a)8 catalidic units and with the a-subunit of the other a/S catal5dic unit, but there are no interactions between the two )8-subunits of the tetramer. The large subunit consists of seven... 

A crystal structure is also available for a ternary complex of MADH, amicyanin, and cytochrome C55U (89, 90). The latter protein... 

A complex of these proteins has been crystallized as a hetero-octamer comprised of one MADH tetramer, two amicyanins and two cytochromes (Chen et al., 1994). The direct distances between redox centers are 9.4 from TTQ to copper, and 23 from copper to heme (Figure 6). 

Davidson, V. L., and Jones, L. H., 1995, Complex formation with methylamine dehydrogenase affects the pathway of electron transfer from amicyanin to cytochrome c-551i. J. Biol. Chem. 270 23941A23943. 

Gray, K. A., Knaff, D. B., Husain, M., and Davidson, V. L., 1986, Measurement of the oxidation-reduction potentials of amicyanin and c-type cytochromes from Paracoccus denitrificans, FEES Lett. 207 39n242. 

Amicyanin is found in methylotrophic bacteria that can use methylated amines as an energy source. The inactivation of the amicyanin gene in Paracoccus denitrificans results in complete loss of its ability to grow on methylamine, a direct indication that amicyanin is a key component of the methylamine driven electron-transfer chain. Amicyanin accepts an electron from methylamine dehydrogenase and transfers it to a c-type cytochrome (see Section 5.4.5). Currently, more than a dozen amicyanin and pseudoazurin sequences are available. 

Three-dimensional structures. The TPQ-con-taining amine oxidase from E. coU is a dimer of 727-residue subunits with one molecule of TPQ at position 402 in each subunit. 7458 Methylamine dehydrogenase is also a large dimeric protein of two large 46.7-kDa subunits and two small 15.5-kDa subunits. Each large subunit contains a TTQ cofactor Reduced TTQ is reoxidized by the 12.5-kDa blue copper protein amicyanin. Crystal structures have been determined for complexes of methylamine dehydrogenase with amicyanin and of these two proteins with a third protein, a small bacterial cytochrome... 

The availability of the crystal structure of the binary complex of these two proteins (92) adds to the impact of these studies, and provides a model to which current electron transfer theories can be applied. The theoretical results can then be compared with kinetic studies of electron transfer within the complex. Additional studies 185, 186) have focused on the electron transfer reaction between MADH, amicyanin, and c5rtochrome Cssu from P. denitrificans (as pointed out in Section II, MADH and amicyanin are induced by growth on methylamine, and cytochrome Cm is induced by growth on methanol). Again, a crystal structure is available for the complex of... 

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... 

The first class is cupredoxins—single-domain blue copper proteins composed of only one BCB domain. These proteins include plastocyanin, azurin, pseudoazurin, amicyanin, auracyanins, rusticyanin, halocyanin, and sulfocyanin (see Section IV). Plantacyanin of the phytocyanin family (Section V), subunit II of the cytochrome c oxidase, and the recently characterized nitrosocyanin also fall into this class. The last two are single BCB domain polypeptides closely related structurally to cupredoxins, but harboring, respectively, a binuclear copper site known as CuA and a novel type of copper-binding site called red (see Sections IX and X). 

In P. denitrificans, amicyanin is an obligatory mediator of electron transfer from MADH to soluble c-type cytochromes (Husain and Davidson, 1986). Each protein is induced in this bacterium during growth on... 

A binary complex of MADH and amicyanin (Chen et al., 1992) and a ternary protein complex of these proteins plus cytochrome c-551i (Chen et al., 1994) from P. denitrificans have been crystallized and their structures have been determined. The structures of the crystallized complexes of these proteins indicate that the interface between MADH and amicyanin is... 

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