Extensive variations and basic features in the alcohol dehydrogenase-sorbitol dehydrogenase family

Structural comparisons of sorbitol dehydrogenase with zinc-containing 'long' alcohol dehydrogenases reveal distant but clear relationships. An alignment suggests 93 positional identities with horse liver alcohol dehydrogenase (25% of 374 positions) and 73 identities with yeast alcohol dehydrogenase (20%). Sorbitol dehydrogenase forms a link between these distantly related alcohol dehydrogenases and is in some regions more similar to one of them that they are to each other. 43 residues (11%) are common to all three enzymes and include a heavy over-representation of glycine (half of all glycine residues in sorbitol dehydrogenase), showing the importance of space restrictions in protein structures. Four regions are well conserved, two in each domain of horse liver alcohol dehydrogenase. They are two segments close to the active-site zinc atom of the catalytic domain, and two in the central beta-pleated sheet strands of the coenzyme-binding domain. These similarities demonstrate the general importance of internal and central building units in proteins. Large variations affect a region adjacent to the third protein ligand to the active-site zinc atom in horse liver alcohol dehydrogenase. Such changes at active sites of related enzymes are unusual. Other large differences concern the segment around the non-catalytic zinc atom of horse liver alcohol dehydrogenase; three of its four cysteine ligands are absent from sorbitol dehydrogenase. Three segments with several exchanges correspond to a continuous region with superficial areas, inter-domain contacts and inter-subunit interactions in the catalytic domain of alcohol dehydrogenase. They may correlate with the altered quaternary structure of sorbitol dehydrogenase. Regions corresponding to top and bottom beta-strands in the coenzyme-binding domain of the alcohol dehydrogenase are also little conserved. Within sorbitol dehydrogenase, a large segment shows an internal similarity. The two distantly related alcohol dehydrogenases and sorbitol dehydrogenase form a triplet of enzymes illustrating basic protein relationships. They are ancestrally close enough to establish similarities, yet sufficiently divergent to illustrate changes in all but fundamental properties.