effect of ph on the stability and structure of yeast hexokinase a acidic amino acid residues in the cleft region are critical for the opening and the closing of the structure ph and salts have a marked effect on the stability structure and function of many globular proteins due to their ability to influence the electrostatic interactions in this work calorimetry cd and fluorescence studies have been carried out to understand the phdependent conformational changes of the twodomain protein yeast hexokinase a in conjunction with the crystal structural data available the present results have enabled the complete characterization and analysis of the phdependent conformational changes of the enzyme that have strong implications in understanding its structurefunction relationship the calorimetric profiles show a single thermal transition in the acidic ph range whereas two independent transitions were observed in the alkaline ph range suggesting the structural merger of the domains at the acidic ph comparison of the thermal transitions at ph 85 studied by different techniques suggests that the first transition corresponds to the smaller domain and the second transition corresponds to the larger domain the aciddenatured state of hexokinase a has high secondary structure content with little or no tertiary interactions and binds to the hydrophobic dye 8anilinonaphthalene1sulfonic acid suggesting that it is a molten globulelike state whereas the alkalidenatured state is less structured than the aciddenatured state but more structured than the ureadenatured state suggestive of a premolten globulelike state structural analysis using the published hexokinase b structure as well as the hexokinase a structure with the revised amino acid sequence in conjunction with the results obtained by us suggests that the ionization state of the acidic residues at the active site could regulate domain movements that are responsible for the opening and the closure of the cleft between the two domains and in turn affect the structure and function of the enzyme