1. Substrate-saturation curves of brain hexokinase for MgATP(2-) were sigmoidal at sub-saturating concentrations of @entity413 when the @entity1025 / @entity855 ratio was maintained at 1:1. Under identical conditions, except that @entity1025 was present in excess, hyperbolic curves were observed. 2. The number of binding sites (calculated from Hill plots) is 1.8 at a @entity1025 / @entity855 ratio 1:1, and 1.0 with excess of @entity1025 . The apparent K(m) for @entity51742 is 6.5x10(-4)m at a @entity1025 / @entity855 ratio 1:1, and 3.5x10(-4)m with excess of @entity1025 . 3. Interdependence between substrate-binding sites was indicated by the effects of varying the concentration of @entity413 . The sigmoidality and deviation from Michaelis-Menten kinetics at a @entity1025 / @entity855 ratio 1:1 became less pronounced with increasing @entity413 concentration. Also, although substrate-saturation curves for @entity413 were hyperbolic when the @entity1025 / @entity855 ratio was 1:1, reciprocal plots were non-linear. These were linear with excess of @entity1025 . 4. High concentrations of @entity1025 ( @entity1025 / @entity855 ratios above 5:1) were inhibitory. 5. The results are taken to indicate homotropic co-operative binding of @entity51742 and that @entity1025 is an allosteric activator. Possible implications in regulation are discussed.

Allosteric activation of brain hexokinase by magnesium ions and by magnesium ion-- XXXX complex.

@entity51742 :: ('CHEBI:30617', 'Chemical') :: ['MgATP(2-)'] @entity413 :: ('MESH:D005947', 'Chemical') :: ['glucose'] @entity855 :: ('MESH:D000255', 'Chemical') :: ['ATP'] @entity1025 :: ('CHEBI:18420', 'Chemical') :: ['Mg(2+)']
@entity855 :: (MESH:D000255,Chemical) :: ['adenosine triphosphate']
