Structural similarities within groups of pyridine nucleotide-linked dehydrogenases

If the pyridine nucleotide-linked dehydrogenases are related by evolution from one or a few common ancestral genes, it might be possible to suggest special evolutionary relationships among particular dehydrogenases by a comparison of structural and functional properties, even before extensive comparative primary sequence information is available. The dehydrogenases are compared using their amino acid compositions, isoelectric points, subunit molecular weights, and some functional properties. Amino acid compositions of dehydrogenases are tabulated per thousand total residues. The comparison of compositions is facilitated by using the difference index method developed by Metzger. The methods of comparison are evaluated using cytochromes c, proteases, and glyceraldehyde-3-phosphate dehydrogenases. Glutamate and glucose-6-phosphate dehydrogenases appear to be similar in many respects and mutually distinct from the other dehydrogenases. Similarities also appear to exist among the low molecular weight dehydrogenases: liver alcohol, mitochondrial malate, glycerol-3-phosphate, and NADP-linked isocitrate dehydrogenases. The Metzger difference index suggests that the primary sequence homology between H- and M-type lactic dehydrogenase subunits may be very extensive, perhaps being 75% identical.