Regulation of the overall flux through a pathway is important as it allows a cell to respond to a changing environment. The regulation could be dictated by changes in overall substrates available to the cell as well as by endocrine signals that stimulate or inhibit specific metabolic pathways to subserve the needs of the organism. For example, storing glycogen in the liver in the fed state and mobilizing it in the fasted setting. This control is achieved by one or more key reactions in the pathway catalyzed by regulatory enzymes and/or transport systems that shuttle metabolites across the plasma membrane or between intracellular compartments. The physicochemical factors that control the rate of an enzyme-catalyzed reaction, such as substrate concentration, are of primary importance in the control of the overall rate of a metabolic pathway.
Nonequilibrium Reactions Are Potential Control Points
In a reaction at equilibrium, the forward and reverse reactions occur at equal rates, and there is therefore no net flux in either direction.
A↔C↔D
If this were a closed system and we added a fixed quantity of “A”, the reaction would proceed to the right to make “C and D” until a new equilibrium was reached where the forward and reverse reactions are equal. The final concentration of A, C, and D would be determined by the absolute amount of A added and the properties of the enzymes. In vivo, there is a net flux from left to right because there is a continuous supply of substrate A and continuous removal of product D. The in vivo pathway is in a “steady state” if the rates of the reactions are constant and the concentration of the substrates, products, and intermediates are constant. In practice, there are normally one or more nonequilibrium reactions in a metabolic path way, where the reactants are present in concentrations that are far from equilibrium. In attempting to reach equilibrium, large losses of free energy occur, making this type of reaction essentially irreversible. Such a pathway has both flow and direction. The enzymes catalyzing nonequilibrium reactions are usually present in low concentration and are subject to a variety of regulatory mechanisms. However, most reactions in metabolic pathways cannot be classified as equilibrium or nonequilibrium, but fall somewhere between the two extremes.
The Control of Flux Through Many Pathways Is Distributed
The flux-generating reaction can be identified as a nonequilibrium reaction in which the Km of the enzyme is considerably lower than the normal concentration of substrate. The first reaction in glycolysis, catalyzed by hexokinase (, would be considered such a flux-generating step because its Km for glucose of 0.05 mmol/L is well below the nor mal blood glucose concentration of 3 to 5 mmol/L. However, glucose must first be transported into the cell by transporters. In some tissues, transport activity is very low in the resting state relative to the activity of hexokinase. Thus intracellular glucose concentration is kept relatively low because of the high affinity of hexokinase and the relatively low rate of glucose uptake allowed by the transport system. Thus in this set ting transport activity is an important (it could be considered rate limiting) determinant of glucose uptake and subsequent metabolism. In the presence of insulin (a hormone made by the endocrine pancreas) transport activity increases so trans port is no longer a significant barrier to glucose uptake. Thus, the control of glucose uptake then shifts to hexokinase. The product of the hexokinase reaction is glucose-6-phosphate. Glucose-6-phosphate is an allosteric inhibitor of hexokinase. If the downstream pathway does not have the capacity to efficiently metabolize the additional glucose-6-phosphate when transport activity is increased, glucose-6-phosphate will increase and serve as a brake on hexokinase. Then hexokinase activity will limit how much glucose uptake is increased even though transport activity is markedly enhanced. The presence of allosteric inhibition allows downstream reactions to indirectly serve an important controlling influence on the flux through the pathway. Thus, there is rarely one enzyme control ling flux through a pathway. Rather the control is distributed; this distribution of control can vary depending on the physiologic setting. This distribution of control allows for fine tuning of metabolic flux under differing physiologic states.
