Subunit Composition and Amino Acid Sequences

The amino acid sequences of the subunits of bovine heart enzyme have been determined primarily by Buse and his colleagues (Buse et al., 1986). Many of the sequences have been determined not only by DNA sequencing but also by peptide analysis. Peptide analysis is an indispensable method for the detection of post translational modifications, and an example of such a modification has been recently shown in this enzyme, and will be described below. Hydropathy plots for these amino acid sequences have provided an astonishingly successful predictions of which regions contain a-helices. This is quite remarkable considering the limited accuracy of this method for structural identification (Hosier et al., 1993 Tsukihara et al., 1996). 

Amino acid sequence analysis and determination of subunit composition are painstaking but these steps are usually necessary before further structural investigations are undertaken. It should not be forgotten that chemical composition and amino acid sequencing provided a foundation for recent structure-function findings in the cytochrome oxidase field. The complete amino acid sequence and a successful prediction of the number of a-helices greatly contributed to the successful and rapid crystallographic analysis of bovine heart cytochrome c oxidase at 2.8 resolution, four years ago (Tsukihara et al., 1995 Tsukihara et al., 1996). 

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Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

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