A comparative analysis of the evolutionary patterning and mechanistic bases of lactate dehydrogenase thermal stability in porcelain crabs, genus Petrolisthes

The kinetic properties of orthologous homologs (orthologs) of enzymes are t ypically correlated with environmental temperatures in species adapted to d ifferent thermal regimes, but correlations between adaptation temperature a nd enzyme thermal stability are less clear. Although the thermal stability of a protein is related chiefly to its primary structure (including post-tr anslational modification), thermal stability can also be altered by extrins ic factors present in the intracellular milieu, Here, we present a comparat ive analysis of the thermal stability of lactate dehydrogenase (LDH) orthol ogs from 22 congeneric species of porcelain crab (genera Petrolisthes and A llopetrolisthes) from a broad range of thermal habitats. Interspecific dive rsity of LDH stability is high: temperatures required for a 50 % loss of ac tivity in 10 min ranged from 65 to 75.5 degreesC, corresponding to half-liv es of less than 1 min to more than 3 h at 70 degreesC, Although stability i s positively correlated with maximal habitat temperature in some sister tax a, phylogenetic comparative analysis incorporating all 22 species does not indicate that the interspecific diversity of LDH stability represents an ad aptive response to current thermal habitats. Examination of the mechanistic bases of LDH stabilization indicates that differences in stability are rel ated both to properties of the LDH molecule itself (intrinsic stability) an d to the effects of extrinsic protein(s). Intrinsic differences were shown by the unfolding of structure during heating, as measured by circular dichr oism spectroscopy. Stabilizing effects of extrinsic proteins are implied by the results of cellular fractionation experiments that removed low-molecul ar-mass solutes and proteins from the muscle homogenates. We conclude that the overall structural stability and functional properties of proteins can evolve independently and that in vivo protein-protein interactions can prov ide another means to regulate protein stability selectively.