STUDY OF CYSTEINE RESIDUES IN THE ALPHA-SUBUNIT OF ESCHERICHIA-COLI TRYPTOPHAN SYNTHASE .1. ROLE IN CONFORMATIONAL STABILITY

To eludicate the role in conformational stability of Cys residues buri ed in the interior of a protein, the thermodynamic properties of denat uration of mutant alpha subunit of Escherichia coli tryptophan synthas e, in. which Ser, Ala, Val or Gly was substituted for each of the thre e Cys residues, were analyzed using calorimetry. The mutants were less stable than the wild type, indicating that Cys residues contribute gr eatly to the stability of the alpha subunit, In most cases, a large de crease in denaturation enthalpy was observed, compensated for by the d enaturation entropy to a major extent, The extent of changes in the de naturation Gibbs energy and denaturation enthalpy varied greatly depen ding on both substituting residues and positions, Of all the mutant pr oteins, the Cys154Ser mutant showed the greatest decrease in denaturat ion enthalpy; its denaturation enthalpy was half that of the wild type , and was considerably repaired by adding a ligand of the alpha subuni t. Because the enthalpy of ligand binding to Cysl54Ser in the native s tate did not change, it seems that the decrease in the denaturation en thalpy of Cysl54Ser and its recovery by ligand binding are caused by c onformational changes in the denatured state due to the mutation.