title: Relationships between enzymatic flux capacities and metabolic flux rates: nonequilibrium reactions in muscle glycolysis.
abstract: The rules that govern the relationships between enzymatic flux capacities (Vmax) and maximum physiological flux rates (v) at enzyme-catalyzed steps in pathways are poorly understood. We relate in vitro Vmax values with in vivo flux 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 flux rates in low-flux muscles, resulting in low fractional velocities (%Vmax = v/Vmax x 100) in vivo. In high-flux muscles, close matches between flux capacities and flux 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-flux muscles, close matches between enzymatic flux capacities and metabolic flux rates (i.e., the lack of excess capacities) may result from space constraints in the sarcoplasm.
authors.affiliation: Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106-9610, USA. suarez@lifesci.lscf.ucsb.edu
authors.forename: R K
authors.initials: RK
authors.lastname: Suarez
authors.forename: J F
authors.initials: JF
authors.lastname: Staples
authors.forename: J R
authors.initials: JR
authors.lastname: Lighton
authors.forename: T G
authors.initials: TG
authors.lastname: West
mesh_terms: D000818:Animals; D002384:Catalysis; D004798:Enzymes / Q000378:metabolism*; D005399:Fishes / Q000378:metabolism; D006019:Glycolysis; D006593:Hexokinase / Q000378:metabolism; D007700:Kinetics; D009132:Muscles / Q000201:enzymology / Q000378:metabolism*; D010732:Phosphofructokinase-1 / Q000378:metabolism; D006005:Phosphorylases / Q000378:metabolism
chemical_list: D004798:Enzymes; D006005:Phosphorylases; D006593:Hexokinase; D010732:Phosphofructokinase-1