CHAPTER 1 - Adenine Nucleotides as Stoichiometric Coupling Agents in Metabolism and as Regulatory Modifiers: The Adenylate Energy Charge

Abstract

This chapter provides an overview of adenylate energy charge. It describes adenine nucleotides as stoichiometric coupling agents in metabolism and as regulatory modifiers. Metabolic sequences are stoichiometrically coupled. Because of the importance of ATP in such coupling, it is proposed that metabolic sequences may usefully be characterized by their ATP coupling coefficients—the number of units of ATP or ATP equivalents regenerated or used in a specific metabolic conversion. The role of ATP in stoichiometric coupling appears to be accompanied by an equally ubiquitous participation of adenine nucleotides in the kinetic regulation of metabolism. The energy charge of the adenylate pool has been proposed as a convenient parameter for comparing and relating the effects of adenylates on regulatory enzymes in diverse metabolic sequences. At a high value of energy charge, sequences that lead to regeneration of ATP are inhibited and those that use ATP are stimulated; a decrease of the charge to a value lower than normal reverses both effects. These modulations are also affected by the concentrations of relevant metabolites, such as the end products of biosynthetic sequences. It seems likely that the responses to variation in energy charge of several ATP-regenerating sequences and of many ATP-utilizing sequences, as modulated in each case by other metabolite modifiers, are of central importance in the maintenance of metabolic homeostasis.