Studies on pituitary follitropin. III. Functional role of the amino groups in subunit and receptor interactions of the ovine hormone.

The effectiveness of Glc, mannose, 2-deoxyglucose, fructose, galactose, arabinose, and N-acetylglucosamine at protecting rat brain hexokinase (ATP: d-hexose 6-phosphotransferase, EC 2.7.1.1) from inactivation by chymotrypsin, glutaraldehyde, heat, and Ellman's reagent have been compared. The relative effectiveness at protecting against these inactivating agents decreases in the order Glc > mannose > 2-deoxyglucose > fructose, galactose, and arabinose, the last three providing no significant protection at all. The nonphosphorylatable substrate analog, N-acetylglucosamine, provides substantial protection against heat inactivation, but is ineffective against inactivation by the other agents. Similar inactivation studies were conducted using several hexose 6-phosphates. Glc-6-P and 1,5-anhydroglucitol-6-P provided substantial protection while 2-deoxyglucose-6-P, fructose-6-P, mannose-6-P, and galactose-6-P were all relatively ineffective at protecting hexokinase activity. The protective effect of these ligands is taken as an indication of ligand-induced conformational changes in brain hexokinase. The results are interpreted in terms of, and considered to support, a recently proposed model (J. E. Wilson, 1978, Arch. Biochem. Biophys.185, 88–99) in which the suitability of a carbohydrate as a substrate depends directly on its ability to induce specific conformational changes prerequisite for subsequent catalytic events while the inhibitory effectiveness of a hexose 6-phosphate is likewise related to its ability to evoke appropriate conformational change in the enzyme. Synergistic interactions between hexose and hexose-6-P binding sites, first reported for Glc and Glc-6-P by Ellison et al. (1975, J. Biol. Chem.250, 1864–1871), have been confirmed. Thus, although fructose and galactose were themselves quite ineffective at providing protection against inactivation of hexokinase by chymotrypsin or glutaraldehyde, they greatly increased the protection afforded by suboptimal (with respect to degree of protection in the absence of added hexose) levels of Glc-6-P. Conversely, the 6-phosphates of fructose, galactose, mannose, and 2-deoxyglucose, which were themselves ineffective at protecting the enzyme activity, markedly enhanced the protection provided by suboptimal levels of Glc or mannose. Based on the relationship between this enhancement of Glc-dependent protection and the hexose-6-P concentration, the dissociation constants for the complexes of hexokinase with 2-deoxyglucose-6-P and mannose-6-P were estimated to be ?0.5 mm.