Relationships between enzymatic flux capacities and metabolic flux rates: Nonequilibrium reactions in muscle glycolysis

Abstract

The rules that govern the relationships between enzymatic f lux capacities (V max ) and maximum physiological f lux rates (v) at enzyme-catalyzed steps in pathways are poorly understood. We relate in vitro V max values with in vivo f lux rates for glycogen phosphorylase, hexokinase, and phosphofructokinase, enzymes catalyzing nonequilibrium reactions, from a variety of muscle types in fishes, insects, birds, and mammals. Flux capacities are in large excess over physiological f lux rates in low-f lux muscles, resulting in low fractional velocities (%V max ‫؍‬ v͞V max ؋ 100) in vivo. In high-f lux muscles, close matches between f lux capacities and f lux rates (resulting in fractional velocities approaching 100% in vivo) are observed. These empirical observations are reconciled with current concepts concerning enzyme function and regulation. We suggest that in high-f lux muscles, close matches between enzymatic f lux capacities and metabolic f lux rates (i.e., the lack of excess capacities) may result from space constraints in the sarcoplasm.Studies of structural and functional design in animals have led to valuable insights into the relationships between functional capacities and maximum physiological requirements or loads (1-3). At the biochemical level, biological design has been studied in terms of relationships between protein structure and function (4), pathway stoichiometry (5, 6), and mechanisms of regulation (5, 7). However, the synthesis and turnover of the thousands of metabolic enzymes possessed by each cell type is a costly enterprise (8), and the various compartments in cells appear to be highly crowded (9, 10). Current evidence suggests that there are probably upper limits to the design of biochemical capacities (11,12). Despite this, the rules that govern the design of functional capacities at the biochemical level are poorly understood. What are the relationships between enzymatic capacities for flux and maximum physiological flux rates through pathways? How much enzyme, in Diamond's words (13), is ''enough but not too much?'' Locomotory and cardiac muscles are ideally suited for the examination of capacity͞load relationships at the biochemical level because the maximum rate at which they do mechanical work defines the maximum rate at which ATP is hydrolyzed and, therefore, the maximum rate at which ATP resynthesizing pathways must operate. Herein, we present patterns of relationships, thus far unrecognized, between enzymatic flux capacities at nonequilibrium reactions in glycolysis and rates of glycolytic flux in muscles during exercise. Sources and Analysis of DataData concerning flux rates (v) and flux capacities (enzyme V max values) at three nonequilibrium steps in the glycolytic pathway, the hexokinase, glycogen phosphorylase, and phosphofructokinase reactions, are considered. These are taken from our own studies and published work from other laboratories. Most of the reactions in glycolysis lie close to equilibrium in vivo and are catalyzed by enzymes whose V max values may exceed p...

Flux control and excess capacity in the enzymes of glycolysis and their relationship to flight metabolism inDrosophila melanogaster

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Partly Ordered Synthesis and Degradation of Glycogen in Cultured Rat Myotubes

“…Fetal calf serum (FCS) 1 and horse serum were purchased from Biological Industries. Penicillin was obtained from Løvens Kemiske Fab-* This work was supported by grants from the Danish Medical Research Council, Grant 9502885, Vera og Carl Michaelsens Legat, the Novo-Nordisk Foundation, and from the University of Copenhagen.…”

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