CRYSTAL-STRUCTURE OF COD-LIVER CLASS-I ALCOHOL-DEHYDROGENASE - SUBSTRATE POCKET AND STRUCTURALLY VARIABLE SEGMENTS

The structural framework of cod liver alcohol dehydrogenase is similar to that of horse and human alcohol dehydrogenases. In contrast, the s ubstrate pocket differs significantly, and main differences are locate d in three loops. Nevertheless, the substrate pocket is hydrophobic li ke that of the mammalian class I enzymes and has a similar topography in spite of many main-chain and side-chain differences. The structural framework of alcohol dehydrogenase is also present in a number of rel ated enzymes like glucose dehydrogenase and quinone oxidoreductase. Th ese enzymes have completely different substrate specificity, but also for these enzymes, the corresponding loops of the substrate pocket hav e significantly different structures. The domains of the two subunits in the crystals of the cod enzyme further differ by a rotation of the catalytic domains by about 6 degrees. In one subunit, they close aroun d the coenzyme similarly as in coenzyme complexes of the horse enzyme, but form a moro open cleft in the other subunit, similar to the situa tion in coenzyme-free structures of the horse enzyme. The proton relay system differs from the mammalian class I alcohol dehydrogenases. His 51, which has been implicated in mammalian enzymes to be important fo r proton transfer from the buried active site to the surface is not pr esent in the cod enzyme. A tyrosine in the corresponding position is t urned into the substrate pocket and a water molecule occupies the same position in space as the His side chain, forming a shorter proton rel ay system.