Interconvertible Enzyme Cascades in Metabolic Regulation

Abstract

Publisher Summary This chapter focuses on interconvertible enzyme cascades in metabolic regulation. The cascades involved in the cyclic covalent modification of interconvertible enzymes are fundamentally different from the irreversible, unidirectional cascades of proteolytic enzymes such as are involved in blood coagulation and complement fixation. When triggered by appropriate stimuli, the latter cascades respond in an explosive manner to produce an avalanche of product needed to meet an occasional biological emergency. However, having fulfilled this function, the cascades are terminated abruptly in response to an entirely different set of autoregulatory signals that lead to self-destruction of the catalytic process. In effect, these unidirectional cascades are contingency systems that serve as biological switches that can be turned on or off intermittently to meet emergency situations. In contrast, the covalent modification of an interconvertible enzyme is a dynamic process in which the coupling of two opposing cascades leads to continual cyclic activation and inactivation of the enzyme. Such cascades are more properly designed for controlled amplification of primary stimuli, as is needed in the regulation of key enzymes in metabolism.