ABSTRACT
A number of redox active enzymes of both bacterial and mammalian origin were thought at one time to contain the cofactor pyrroloquinoline quinone. This class of enzymes has collectively been called quinoproteins. The X-ray crystal structures of three of these enzymes been determined and discussed. In one of these, methylamine dehydrogenase, the cofactor is an orthoquinone and therefore a rightful member of the quinoprotein class. The structure of methylamine dehydrogenase is the most thoroughly studied of these quinoproteins, having been solved in two bacterial species. The structure of the Methylamine dehydrogenase (MADH) portion of the complex is virtually the same as that of the TV and P. denitrificans-MADH molecules. The unusual cofactors in methylamine dehydrogenase and galactose oxidase are both constructed at some as yet unknown stage of biogenesis by formation of a covalent bond between two gene-encoded side chains.
