Brain hexokinase has no preexisting allosteric site for glucose 6-phosphate

• Published in: The Journal of biological chemistry Vol. 263; no. 30; pp. 15492 - 15497
• Main Authors: Mehta, A, Jarori, G K, Kenkare, U W
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 25.10.1988; American Society for Biochemistry and Molecular Biology
• Subjects: Adenosine Triphosphate - metabolism; Animals; Binding Sites; brain; Brain - enzymology; Cattle; Glucose-6-Phosphate; glucose-b-phosphate; Glucosephosphates - metabolism; hexokinase; Hexokinase - metabolism; Kinetics; Ribosemonophosphates - metabolism; Spectrophotometry, Ultraviolet
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)37615-X

Difference spectroscopic investigations on the interaction of brain hexokinase with glucose and glucose 6-phosphate (Glc-6-P) show that the binary complexes E-glucose and E-Glc-6-P give very similar UV difference spectra. However, the spectrum of the ternary E-glucose-Glc-6-P complex differs markedly from the spectra of the binary complexes, but resembles that produced by the E-glucose-Pi complex. Direct binding studies of the interaction of Glc-6-P with brain hexokinase detect only a single high-affinity binding site for Glc-6-P (KD = 2.8 microM). In the ternary E-glucose-Glc-6-P complex, Glc-6-P has a much higher affinity for the enzyme (KD = 0.9 microM) and a single binding site. Ribose 5-phosphate displaces Glc-6-P from E-glucose-Glc-6-P only, but not from E-Glc-6-P complex. It also fails to displace glucose from E-glucose and E-glucose-Glc-6-P complexes. Scatchard plots of the binding of glucose to brain hexokinase reveal only a single binding site but show distinct evidence of positive cooperativity, which is abolished by Glc-6-P and Pi. These ligands, as well as ribose 5-phosphate, substantially increase the binding affinity of glucose for the enzyme. The spectral evidence, as well as the interactive nature of the sites binding glucose and phosphate-bearing ligands, lead us to conclude that an allosteric site for Glc-6-P of physiological relevance occurs on the enzyme only in the presence of glucose, as a common locus where Glc-6-P, Pi, and ribose 5-phosphate bind. In the absence of glucose, Glc-6-P binds to the enzyme at its active site with high affinity. We also discuss the possibility that, in the absence of glucose, Glc-6-P may still bind to the allosteric site, but with very low affinity, as has been observed in studies on the reverse hexokinase reaction.