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Induced fit in yeast hexokinase.

In the nucleoside triphosphatase activity that yeast hexokinase shows in the absence of acceptor substrate the Km values for MgATP and for MgITP, as well as the Ki for MgGTP, are markedly different from those in the phosphotransferase reaction. In the presence of lyxose or xylose, two non-phosphorylable analogues of the sugar substrates, these Km or Ki values change to those corresponding to the hexokinase reaction. Besides this effect on the apparent affinities, lyxose and xylose can enhance by about 20 times the maximal rate of hydrolysis of the terminal phosphoryl group of the nucleoside triphosphates. In addition, xylose slowly induces a specific, apparently irreversible, inactivation of the enzyme, both as hexokinase and as nucleoside triphosphatase. The relative hydrolytic activities on ATP, ITP, and GTP resemble the relative efficiencies of these nucleotides as phosphate donors, as well as those in the inactivation of the enzyme in the presence of xylose. The presence of a phosphate group on carbon 6 of glucose does not prevent the sugar effects on the Km values of hexokinase for the nucleoside triphosphates, although it reduces their rates of hydrolysis. These effects indicate the occurrence of an “induced fit” by which certain sugars specifically modify the binding of the nucleoside triphosphate substrates and enhance the labilizing action of the enzyme on their terminal phosphoryl bond. Moreover, hexokinase seems to induce a peculiar configuration of the pyranose ring of the sugar substrate, probably a boat form. When bulky substituents prevent the latter to be formed, sugars, even if they are still able to bind to the enzyme, do not induce an increased labilization of the nucleoside triphosphates as glucose or lyxose do.