Drosophila glucose dehydrogenase and yeast alcohol oxidase are homologous and share N-terminal homology with other flavoenzymes.

The presence of similar nucleotide cofactor-binding domains in otherwise unrelated enzymes has been rationalized on the basis of the exon-shuffling model of protein evolution (Gilbert 1978). In the common case of enzymes which share a similar nucleotide-binding domain but exhibit different specificities for primary substrates, the degree of sequence similarity is restricted to small regions specifically demonstrated to bind to the nucleotide cofactor. This pattern of restricted similarity suggests that dissimilar regions arose from nonhomologous exons. We show herein that the sequence similarity of two unrelated flavoenzymes, Drosophila glucose dehydrogenase ( GLD ) and yeast alcohol/methanol oxidase ( AOX / MOX ) are unexpectedly similar throughout their primary sequence. In addition these enzymes share considerable similarity both with other unrelated flavoenzymes and with the ras protein for a small amino-terminal region encoding their nucleotide-binding sites. The genes encoding GLD (E.C. 1.1.99.10) and AOX/MOX (E.C. 1.1.3.13) recently have been isolated from fruit flies (Cavener et al. 1986~; Krasney et al. 1990) and yeast (Ledeboer et al. 1985; Koutz et al. 1989), respectively. The biochemical properties of Drosophila GLD are very similar to those of Aspergillus oryzue GLD, which catalyzes the oxidation of D-glucose to D-glucono-&lactone. Unlike the ubiquitous glucose oxidase and AOX/MOX, GLD does not reduce molecular oxygen to hydrogen peroxide (Bak 1967). Drosophila GLD is essential for cuticular modification during development and is secreted in adult males as part of the seminal fluid transferred to females (Cavener and MacIntyre 1983; Cavener et al. 1986b). AOX/MOX is associated with the ability of four yeast genera to utilize methanol as the sole carbon source (Lee and Komagata 1980). It catalyzes the oxidation/reduction of methanol and oxygen to formaldehyde and hydrogen peroxide, respectively. Thus, GLD and AOX/MOX exhibit remarkably different biochemical and functional properties. A search of both the National Biomedical Research foundation (NBRF) protein data base and the GenBank nucleic acid data base unexpectedly revealed a significant degree of similarity between D. melanogaster GLD and Hansenula polymorpha (yeast) MOX and Pichia pastoris (yeast) AOX 1. The statistical significance was evaluated by comparing the similarity of the primary amino acid sequence of GLD with 200 randomized sequences of AOXl and MOX by using the RDF2 program of W. Pearson (University of Virginia). The randomized sequences were of the same length and amino acid composition as AOXI and MOX. The optimized alignment scores for the comparison of AOX 1 with GLD and for the comparison of MOX with GLD were greater than 30 SD above the mean of the randomized scores and are more than threefold higher than those of the highest random sequence. Percentage similarities