Regulation of hexokinase by reversible phosphorylation in skeletal muscle of a freeze-tolerant frog

Hexokinase (HK) was isolated from hind leg skeletal muscle of the wood frog, Rana sylvatica, a freeze tolerant species that uses glucose as a cryoprotectant. Analysis of kinetic parameters (Km and Vmax) of HK showed significant increases in Km glucose (from 144 ± 4.4 to 248 ± 12.0 μM) and Km ATP (from 248 ± 8.5 to 330 ± 20.9 μM), as well as a decrease in Vmax (from 86.1 ± 0.40 to 52 ± 0.49 mU mg− 1 of protein) in frogs following freezing exposure, indicating lower affinity for HK substrates and lower enzyme activity in this state. Subsequent analyses indicated that differential phosphorylation of HK between the two states was responsible for the altered kinetic properties. HK was analyzed by SDS-PAGE; phosphoprotein staining revealed a 33% decrease in phosphate content of HK from frozen frogs but immunoblotting showed no change in total HK protein content. Muscle extracts from control and frozen frogs were incubated with ions and second messengers to stimulate the actions of protein kinases and protein phosphatases, with results indicating that HK can be phosphorylated by protein kinases A and C, and AMP-activated protein kinase, and can be dephosphorylated by protein phosphatases 1, 2A and 2C. The data indicate that in control frogs, HK is in a higher phosphate form and displays a high substrate affinity and high activity, whereas in frozen frogs HK is less phosphorylated, with lower substrate affinity and lower activity. Studies also showed that HK affinity for ATP decreases further in response to low temperature, but that high cryoprotective glucose concentrations can prevent these changes in affinity. Finally, the activity and structure of HK from frozen frogs is more sensitive to non-compatible osmolytes than the enzyme in control frogs.