Effects of temperature and pH on hexokinase from the flight muscles of Dipetalogaster maximus (Hemiptera : Reduviidae)

Effects of temperature and pH on the catalytic properties of hexokinase (HK , EC 2.7.1.1) from the flight muscles of Dipetalogaster maximus (Uhler) wer e studied. The enzyme showed a hyperbolic behavior with its two substrates (glucose and ATP). There was no inhibition by glucose. Apparent Km and Vmax increased as pH increased from 7.0 to 8.5. Catalytic efficiency was lowest at pH 7.0. Km, Vmax, and catalytic efficiency were higher at 37 degrees C than at 30 and 20 degrees C. There was marl;ed inhibition by ATP, which was dependent on pH and temperature. Km values for ATP were reduced and. catal ytic efficiency increased as pH increased. Lowest Vmax was olsen ed at pH 7 .0. At this pH there was 87.3% inhibition by ATP, whereas it was only 5.5% at pH 85 (at 30 degrees C). Km, Vmax, and catalytic efficiency were higher at 37 degrees C than at 30 and. 20 degrees C. The strong inhibition Lp ATP detected at 20 degrees C (pH 7.6) almost disappeared at 37 degrees C. There fore, temperature could regulate hexokinase activity by modulating the inhi bition produced by ATP. Glucose utilization and ATP production would be pro moted when temperature rises from 30 to 37 degrees C. Because insect thorac ic muscles increase their temperature over 30 degrees C during night, this phenomenon elucidates a mechanism enhancing energy supply for muscle activi ty.