epsilon-crystallin from duck eye lens - Comparison of its quaternary structure and stability with other lactate dehydrogenases and complex formation with alpha-crystallin

Taxon-specific epsilon-crystallin (epsilon C) from duck eye lens is identic al to duck heart muscle lactate dehydrogenase. It forms a dimer of dimers w ith a dissociation constant of 2.2 x 10(-7) M, far beyond the value observe d for other vertebrate lactate dehydrogenases. Comparing the characteristic s of wild-type epsilon-crystallin with those of three mutants, G115N, G119F and 115N/119F, representing the only significant peripheral sequence varia tions between duck epsilon C and chicken or pig heart muscle lactate dehydr ogenase, no significant conformational differences are detectable. Regardin g the catalytic properties, the Michaelis constant of the double mutant 115 N/119F for pyruvate is found to be decreased; for wild-type enzyme, the eff ect is overcompensated by the high expression level of epsilon C in the eye lens. As taken from spectral analysis of the guanidine-induced and tempera ture-induced denaturation transitions, epsilon C in its dimeric state is re latively unstable, whereas the native tetramer exhibits the high intrinsic stability characteristic of common vertebrate heart and muscle lactate dehy drogenases. The denaturation mechanism of epsilon C is complex and only par tially reversible. In the case of thermal unfolding, the predominant side r eaction competing with the reconstitution of the native state is the kineti c partitioning between proper folding and aggregation. alpha-Crystallin, th e major molecular chaperone in the eye lens, inhibits the aggregation of ep silon C by trapping the misfolded protein.