The A isozyme of yeast hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) crystallized as a complex with glucose has a conformation that is dramatically different from the conformation of the B isozyme crystallized in the absence of glucose. Comparison of the high-resolution structures shows that one lobe of the molecule is rotated by 12 degrees relative to the other lobe, resulting in movements of as much as 8 A in the polypeptide backbone and closing the cleft between the lobes into which glucose is bound. The conformational change is produced by the binding of glucose (R.C. McDonald, T.A. Steitz, and D.M. Engelman, unpublished data) and is essential for catalysis [Anderson, C.M., Stenkamp, R.E., McDonald, R.C. & Steitz, T.A. (1978) J. Mol. Biol. 123, 207-219] and thus provides an example of induced fit. The surface area of the hexokinase A-glucose complex exposed to solvent is smaller than that of native hexokinase B. By using the change in exposed surface area to estimate the hydrophobic contribution to the free energy changes upon glucose binding, we find that the hydrophobic effect alone favors the active conformation of hexokinase in the presence and absence of sugar. The observed stability of the inactive conformation of the enzyme in the absence of substrates may result from a deficiency of complementary interactions within the cavity that forms when the two lobes close together.