Zusammenfassung
Taxon-specific epsilon-crystallin (epsilon C) from duck eye lens is identical to duck heart muscle lactate dehydrogenase. It forms a dimer of dimers with a dissociation constant of 2.2 x 10(-7) M, far beyond the value observed for other vertebrate lactate dehydrogenases. Comparing the characteristics of wild-type epsilon-crystallin with those of three mutants, G115N, G119F and 115N/119F, ...
Zusammenfassung
Taxon-specific epsilon-crystallin (epsilon C) from duck eye lens is identical to duck heart muscle lactate dehydrogenase. It forms a dimer of dimers with a dissociation constant of 2.2 x 10(-7) M, far beyond the value observed for other vertebrate lactate dehydrogenases. Comparing the characteristics of wild-type epsilon-crystallin with those of three mutants, G115N, G119F and 115N/119F, representing the only significant peripheral sequence variations between duck epsilon C and chicken or pig heart muscle lactate dehydrogenase, no significant conformational differences are detectable. Regarding the catalytic properties, the Michaelis constant of the double mutant 115N/119F for pyruvate is found to be decreased; for wild-type enzyme, the effect is overcompensated by the high expression level of epsilon C in the eye lens. As taken from spectral analysis of the guanidine-induced and temperature-induced denaturation transitions, epsilon C in its dimeric state is relatively unstable, whereas the native tetramer exhibits the high intrinsic stability characteristic of common vertebrate heart and muscle lactate dehydrogenases. The denaturation mechanism of epsilon C is complex and only partially reversible. In the case of thermal unfolding, the predominant side reaction competing with the reconstitution of the native state is the kinetic partitioning between proper folding and aggregation. alpha-Crystallin, the major molecular chaperone in the eye lens, inhibits the aggregation of epsilon C by trapping the misfolded protein.
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