ABSTRACT
Metabolic arrest strategies are prominent among bivalves and gastropods as mechanisms for surviving periods of harsh environmental stress. For marine mollusks, metabolic arrest is a key part of anaerobiosis whereas terrestrial gastropods show metabolic rate depression during aerobic estivation. One of the most powerful mechanisms of metabolic arrest is the downregulation of enzymes via reversible protein phosphorylation. In marine mollusks, coordinated control of glycolytic rate depression during anoxia is achieved via covalent modification of glycogen phosphorylase, 6-phosphofructo-1-kinase (PFK-1), 6-phosphofructo-2-kinase, and pyruvate kinase (PK), producing less active enzyme forms during the aerobic-anoxic transition. Studies with isolated tissues of the whelk Busycon canaliculatum have shown that anoxia-induced phosphorylation of PK is independent of changes in cellular pH and not influenced by hormones or the second messengers of cAMP-dependent protein kinase or protein kinase C. However, phosphorylation is stimulated by cGMP indicating that cGMP-dependent protein kinase is key to coordinating the anoxia-induced inactivation of glycolytic enzymes. Estivating land snails Otala lactea use the same reversible phosphorylation mechanism for aerobic metabolic arrest with specific phosphorylation targets including PK, PFK-1, and pyruvate dehydrogenase (PDH). Control over PDH highlights the importance of targeting mitochondrial enzymes to achieve metabolic arrest in an aerobic system.
