The crystal structure of D-lactate dehydrogenase, a peripheral membrane respiratory enzyme

D-Lactate dehydrogenase (D-LDH) of Escherichia coli is a peripheral membrane respiratory enzyme involved in electron transfer, located on the cytoplasmic side of the inner membrane. D-LDH catalyzes the oxidation of D-lactate to pyruvate, which is coupled to transmembrane transport of amino acids and sugars. Here we describe the crystal structure at 1.9 Angstrom resolution of the three domains of D-LDH: the flavin adenine dinucleotide (FAD)-binding domain, the cap domain, and the membrane-binding domain. The FAD-binding domain contains the site of D-lactate reduction by a noncovalently bound FAD cofactor and has an overall fold similar to other members of a recently discovered FAD-containing family of proteins. This structural similarity extends to the cap domain as well. The most prominent difference between D-LDH and the other members of the FAD-containing family is the membrane-binding domain, which is either absent in some of these proteins or differs significantly. The D-LDH membrane-binding domain presents an electropositive surface with six Arg and five Lys residues, which presumably interacts with the negatively charged phospholipid head groups of the membrane. Thus, D-LDH appears to bind the membrane through electrostatic rather than hydrophobic forces.