Allosteric activation of brain hexokinase by magnesium ions and by magnesium ion--adenosine triphosphate complex.
1. Substrate-saturation curves of brain hexokinase for MgATP(2-) were sigmoidal at sub-saturating concentrations of glucose when the Mg(2+)/ATP ratio was maintained at 1:1. Under identical conditions, except that Mg(2+) was present in excess, hyperbolic curves were observed. 2. The number of binding sites (calculated from Hill plots) is 1.8 at a Mg(2+)/ATP ratio 1:1, and 1.0 with excess of Mg(2+). The apparent K(m) for MgATP(2-) is 6.5x10(-4)m at a Mg(2+)/ATP ratio 1:1, and 3.5x10(-4)m with excess of Mg(2+). 3. Interdependence between substrate-binding sites was indicated by the effects of varying the concentration of glucose. The sigmoidality and deviation from Michaelis-Menten kinetics at a Mg(2+)/ATP ratio 1:1 became less pronounced with increasing glucose concentration. Also, although substrate-saturation curves for glucose were hyperbolic when the Mg(2+)/ATP ratio was 1:1, reciprocal plots were non-linear. These were linear with excess of Mg(2+). 4. High concentrations of Mg(2+) (Mg(2+)/ATP ratios above 5:1) were inhibitory. 5. The results are taken to indicate homotropic co-operative binding of MgATP(2-) and that Mg(2+) is an allosteric activator. Possible implications in regulation are discussed.
D000255:Adenosine Triphosphate; D000494:Allosteric Regulation; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology*; D004789:Enzyme Activation; D005947:Glucose; D006168:Guinea Pigs; D006593:Hexokinase; D007700:Kinetics; D008274:Magnesium; D009942:Organometallic Compounds; D013053:Spectrophotometry; D013057:Spectrum Analysis
D009942:Organometallic Compounds; D000255:Adenosine Triphosphate; D006593:Hexokinase; D008274:Magnesium; D005947:Glucose