Structural basis for altered activity of M- and H-isozyme forms of human lactate dehydrogenase

Lactate dehydrogenase (LDH) interconverts pyruvate and lactate with concomi tant interconversion of NADH and NAD(+). Although crystal structures of a v ariety of LDH have previously been described, a notable absence has been an y of the three known human forms of this glycolytic enzyme. We have now det ermined the crystal structures of two isoforms of human LDH-the M form, pre dominantly found in muscle; and the H form, found mainly in cardiac muscle. Both structures have been crystallized as ternary complexes in the presenc e of the NADH cofactor and oxamate, a substrate like inhibitor. Although ea ch of these isoforms has different kinetic properties, the domain structure , subunit association, and active-site regions are indistinguishable betwee n the two structures. The pK(a) that governs the K-M for pyruvate for the t wo isozymes is found to differ by about 0.94 pH units, consistent with vari ation in pK(a) of the active-site histidine, The close similarity of these crystal structures suggests the distinctive activity of these enzyme isofor ms is likely to result directly from variation of charged surface residues peripheral to the active site, a hypothesis supported by electrostatic calc ulations based on each structure, (C) 2001 Wiley-Liss, Inc.