D175 discriminates between NADH and NADPH in the coenzyme binding site of Lactobacillus delbrueckii subsp. bulgaricus D-lactate dehydrogenase.

The NAD-dependent D-(-)-lactate dehydrogenase (D-LDH) from Lactobacillus delbrueckii subsp. bulgaricus (in short, L. bulgaricus) has been modified at position 175 by site-directed mutagenesis, changing a conserved aspartate residue into an alanine. The D175A mutant enzyme displays a 40-fold shift in coenzyme preference from NADH to NADPH. This demonstrates that D175 truly belongs to the amino acid consensus GXGXXGX(17)D (where X represents any residue) which is the signature of the coenzyme binding site of most NAD-dependent dehydrogenases.