Structure of the active ternary complex of pig heart lactate dehydrogenase with S-lac-NAD at 2.7 Å resolution

Abstract

The structure of pig heart lactate dehydrogenase complexed with the active coenzyme substrate analog (3 S)-5-(3-carboxy-3-hydroxypropyl) NAD + was solved to high resolution. The results show: 1. (1) The substrate and active site arrangements resemble closely the earlier proposed “active” ternary complex of lactate dehydrogenase and differ significantly from those in abortive ternary complexes of dogfish M 4 lactate dehydrogenase. 2. (2) Small differences with respect to ternary inhibitor complexes of dogfish M 4 lactate dehydrogenase occur in the coenzyme conformation as well as in the spatial arrangement of protein side-chains at the coenzyme binding site. 3. (3) The “loop” is in an overall closed conformation typical for ternary complexes of lactate dehydrogenase, although it differs slightly from the conformation found in dogfish M 4 lactate dehydrogenase ternary complexes. 4. (4) The stronger binding of NAD to the heart isoenzyme cannot be accounted for by an additional hydrogen bond to the pyrophosphate group via glutamine 31. 5. (5) The anion binding sites found between P axis-related subunits in dogfish M 4 lactate dehydrogenase are also present in pig H 4 lactate dehydrogenase. It is proposed that in lactate dehydrogenase catalysis the p K value of the internal acid/base catalyst His195 is modulated by “loop” residue Arg109, leading to substrate activation.