Regulation of glucose-6-phosphate dehydrogenase and hexokinase in anoxia-tolerant mollusks: role of reversible phosphorylation
Metabolic rate depression is key to animal survival without oxygen and requires coordinated suppression of ATP-generating and ATP-consuming cellular functions by stable regulatory mechanisms. This thesis examined the possible role of reversible protein phosphorylation in metabolic suppression in organs of the anoxia-tolerant intertidal marine mollusc, Littorina littorea. Studies focused on glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme of the pentose phosphate pathway (PPP), and hexokinase (HK), an important enzyme at the forefront of carbohydrate metabolism. The data show that hepatopancreas G6PDH is regulated by phosphorylation and entry into anoxia leads to a more dephosphorylated form, whose properties suggest a more active enzyme. This would favour enhanced carbon flow through the PPP to sustain NADPH production for antioxidant defense. Furthermore, the study provides one of the first demonstrations of coordinated regulation of the PPP between active and hypometabolic states and implicates specific protein kinases and phosphatases in G6PDH regulation. Studies of HK showed that it is also regulated by reversible protein phosphorylation, in a tissue-specific manner. In the hepatopancreas, it appears that HK affinity for one of its substrates is enhanced during anoxia whereas in foot muscle, HK activity is suppressed. Differences between control and anoxic HK were also observed in their susceptibility to urea denaturation, response to specific protein kinase and protein phosphatase incubations as well as their elution profiles from an ion-exchange column. Overall, these studies confirm an integral role of reversible protein phosphorylation in the suppression and reorganization of L.littorea metabolism for anoxia survival.