An investigation of the thermal stabilities of two malate dehydrogenases by comparison of their three-dimensional structures

Abstract

The tertiary structure of Thermus aquaticus malate dehydrogenase (MDH) was predicted based on the known crystal structure of pig heart cytosolic MDH. Guanidinium chloride (GdmCl) unfolding experiments showed that there is only about a 4.2-kjoulelmol difference in ΔG° between the pig and Thermus MDH. However, the two enzymes varied greatly in their [GdmCl] 1 2 , with Thermus MDH showing the expected increased stability (3.20 M against 0.58 M for pig MDH). The half-lives were determined for both Thermus MDH (34 min at 90°C) and pig MDH (1.8 min at 60°C). The Thermus MDH model was then examined to see what effect the substituted residues and changes may have on the enzyme, particularly in relation to its high thermal stability.