AN INVESTIGATION OF THE THERMAL STABILITIES OF 2 MALATE-DEHYDROGENASES BY COMPARISON OF THEIR 3-DIMENSIONAL STRUCTURES

The tertiary structure of Thermus aquaticus malate dehydrogenase (MDH) was predicted based on the known crystal structure of pig heart cytos olic MDH. Guanidinium chloride (GdmCl) unfolding experiments showed th at there is only about a 4.2-kjoule/mol difference in DELTAG-degrees b etween the pig and Thermus MDH. However, the two enzymes varied greatl y 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 de termined for both Thermus MDH (34 min at 90-degrees-C) and pig MDH (1. 8 min at 60-degrees-C). The Thermus MDH model was then examined to see what effect the substituted residues and changes may have on the enzy me, particularly in relation to its high thermal stability.