A RELATIONSHIP BETWEEN PROTEIN STABILITY AND PROTEIN FUNCTION

Enzymes are thought to use their ordered structures to facilitate cata lysis. A corollary of this theory suggests that enzyme residues involv ed in function are not optimized for stability. We tested this hypothe sis by mutating functionally important residues in the active site of T4 lysozyme, Six mutations at two catalytic residues, Glu-11 and Asp-2 0, abolished or reduced enzymatic activity but increased thermal stabi lity by 0.7-1.7 kcal(.)mol(-1). Nine mutations at two substrate-bindin g residues, Ser-117 and Asn-132, increased stability by 1.2-2.0 kcal(. )mol(-1), again at the cost of reduced activity, X-ray crystal structu res show that the substituted residues complement regions of the prote in surface that are used for substrate recognition in the native enzym e, In two of these structures the enzyme undergoes a general conformat ional change, similar to that seen in an enzyme-product complex, These results support a relationship between stability and function for T4 lysozyme, Other evidence suggests that the relationship is general.