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The structure of mouse testicular lactate dehydrogenase isoenzyme C4 at 2.9 A resolution.

The structure of lactate dehydrogenase isoenzyme C4 from mouse testes was solved at 2.9 A resolution using the technique of molecular replacement. The electron density map revealed a ternary-like configuration of the flexible loop peptide although density corresponding to the coenzyme and substrate molecules was not present. Apparently the apo-lactate dehydrogenase molecule in solution is in a dynamic equilibrium between the O (loop open as found in dogfish apo-lactate dehydrogenase M4) and C (loop closed as found in a variety of ternary complexes) conformations. During crystallization of the apoenzyme one or the other conformers is selected. The apparent stability of the closed conformation for the apo-lactate dehydrogenase C4 molecule may in part explain the low catalytic turnover number of the C isoenzyme. A possible substitution of an arginine residue at position 30 may also be a contributing factor as well as allowing NADP to act as coenzyme.