Quinone oxidoreductase, ζ-crystallin, glucose dehydrogenase, and alcohol dehydrogenase belong to a superfamily of medium-chain dehydrogenase/reductases. The crystal structures ofEscherichia coliquinone oxidoreductase (QOR) andThermoplasma acidophilumglucose dehydrogenase have recently been determined and are compared here with the well-known structure of horse liver alcohol dehydrogenase. A structurally based comparison of these three enzymes confirms that they possess extensive overall structural homology despite low sequence identity. The most significant difference is the absence of the catalytic and structural zinc ions in QOR. A multiple structure-based sequence alignment has been constructed for the three enzymes and extended to include ζ-crystallin, an eye lens structural protein with quinone oxidoreductase activity and high sequence identity toE. coliquinone oxidoreductase. Residues which are important for catalysis have been altered and the functions and activities of the enzymes have diverged, illustrating a classic example of divergent evolution among a superfamily of enzymes.
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