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Intermediary Carbohydrate Metabolism

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Intermediary Carbohydrate Metabolism J Clin Pathol: first published as 10.1136/jcp.s1-2.1.66 on 1 January 1969. Downloaded from J. clin. Path., 22, suppl. (Ass. clin. Path.), 2, 66-71 Intermediary carbohydrate metabolism P. J. RANDLE From the Department ofBiochemistry, University of Bristol Current interest in mammalian carbohydrate meta- may act by raising substrate Km or lowering Vmax bolism is largely centred on the operation of or both. They may conform to classical inhibitor metabolic pathways in tissues and whole animals. kinetics (see Dixon and Webb. 1958) or the rela- Recent studies have indicated that the various tionship between degree of inhibition and inhibitor pathways may be controlled by one or two key concentration may be sigmoid (ie, show a threshold). enzymes which function as pacemakers. Trans- Similar considerations may be applied to activators. porting systems in the plasma membrane and also In general it is important to distinguish between in intracellular membranes, such as the mitochon- those inhibitors which influence Vmax alone, Km drial membrane, are of increasing importance in alone, and both Vmax and Km. Where only K. is control and hence in the operation of metabolic altered inhibition may be overcome by compensatory pathways. changes in substrate concentration; this will not be the case when only Vmax is altered. CONTROL IN TISSUES The identification of pacemaker reactions in metabolic pathways has depended upon simul- The majority of enzymes and transporting systems taneous measure-ments of rates of flow and con- in carbohydrate metabolism probably conform to centrations of substrates and products of individual Michaelis-Menten kinetics. The relationship be- reactions in tissues. The concepts of mass action copyright. tween substrate concentration (or gradient) and ratios and of crossover have been particularly reaction (or transport) velocity is hyperbolic, and valuable.3 A pacemaker reaction is rate-limiting in velocity is also directly proportional to enzyme its pathway and an increase in flow is due in concentration. In terms of pathway operation re- particular to an increase in the activity of a pace- action velocity (flow) is most sensitive to substrate maker en7yme. It follows that a pacemaker reaction concentration at values below the Kin,' and capacity, must be displaced from equilibrium, under condi- ie, Vmax2 or maximum flow, is mainly dependent tions where it limits flow in the whole pathway, ie, upon enzyme or carrier concentration. Some en- the mass action ratio of products and reactant will http://jcp.bmj.com/ zymes do not conform to Michaelis-Menten kinetics differ markedly from the equilibrium constant for the and of particular interest from the point of view of reaction. It is important to know whether the equili- control are those enzymes which exhibit sigmoid brium constant is dependent on factors such as pH rate curves, ie, show a sigmoid relationship between (as in the case of kinases), or divalent ions (as in the reaction velocity and substrate concentration. From case of aconitase), in interpreting mass action ratios. the point ofview ofpathway operation these enzymes In the case of transport processes the mass action have a threshold, and reaction velocity (or flow) is ratio is given by the ratio of concentrations on the on September 29, 2021 by guest. Protected most sensitive to substrate concentration at values two sides of the membrane; and the equilibrium above the threshold and below the Km. Enzymes constant is given by the ratio of concentrations at which show sigmoid rate curves appear invariably to equilibrium. In general when flow through a path- be regulatory enzymes, though there are some regu- way is increased by activation of a pacemaker latory enzymes which show hyperbolic rate curves. reaction, the mass action ratio is changed in the Although substrate concentration may be an direction of the equilibrium constant. The identi- important determinant of rate for some regulatory fication of a pacemaker reaction may be most enzymes the concentration of effectors is of more clearly made when a crossover occurs, ie when for a general importance. Such effectors are metabolites particular reaction in a metabolic pathway an which activate or inhibit enzymes for which they increased rate of flow is accompanied by a decrease may or may not be substrates. Inhibitory effectors in substrate concentration(s) and an increase in product concentration(s) or vice versa. Unfor- 'Michaclis constant, ie, substrate concentration at which reaction velocity = half maximum velocity. ED. tunately there are instances where no clear crossover 'Vm.. =maximum velocity of an enzyme reaction. 'See Editors' Appendix. 66 J Clin Pathol: first published as 10.1136/jcp.s1-2.1.66 on 1 January 1969. Downloaded from Intermediary carbohydrate metabolism 67 occurs although an increase in mass action ratio Metabolic integration may involve both receptor may accompany an increase in flow. This may be and effector systems and is mediated in part by the seen, for example, when two or more pacemaker endocrine system and in part by the nervous system. reactions are in close apposition. The secretion of insulin is regulated in the ,-cell by An additional and necessary approach involves the blood glucose concentration. In this instance screening individual enzymes for activation or the receptor system lies within the cell secreting the inhibition by a wide variety of metabolites. This effector (insulin). The secretion of pituitary growth type of study can indicate possible regulatory hormone may also depend upon blood glucose enzymes and suggest whole tissue experiments to concentration but in this instance the receptor cells establish their regulatory role. It has been parti- may be located in the hypothalamus. Some con- cularly valuable in situations where two or more sideration is given in a later section to the gluco- regulatory enzymes are closely apposed in metabolic receptor mechanism in the pancreas. pathways. The elucidation of mechanisms of regu- lation is similarly dependent upon the detection of CONTROL OF GLYCOLYSIS IN MUSCLE effectors for individual enzymes and the demon- stration of effector mechanisms in the cell. Table I shows the enzymes and transport systems Studies along these lines have indicated probable concerned with glucose uptake, glycolysis, glycogen control mechanisms for a number of pathways of synthesis, glycogenolysis, and pyruvate oxidation in carbohydrate metabolism and have provided a muscle. Glucose uptake appears to be controlled by qualitative basis for understanding their operation. the two apposed steps of plasma membrane trans- Fuller understanding can come only from quan- port of glucose and its intracellular phosphorylation titative studies in which the operation of the pathway by hexokinase. The glycolytic pathway between may be described in the form of mathematical glucose 6-phosphate and pyruvate appears to be models based on the kinetic properties and con- controlled only by phosphofructokinase. Glycogen centrations of enzymes together with the concen- synthesis may be regulated by glycogen synthetase trations of their substrates and effectors. Some and glycogen breakdown by phosphorylase. The copyright. progress along these lines is now being made in entry of glucose into the citrate cycle may be regu- restricted segments of pathways but this approach lated by pyruvate dehydrogenase. raises special problems. Enzyme kinetics have mostly The control systems appear to subserve three been derived from studies with enzymes in dilute major physiological roles. One set of signals may solution where substrate or effector concentrations vary rates of glucose uptake, glycogen breakdown, are in vast excess. In cells the concentrations of some and glycolysis according to the state of ATP syn- enzymes may be comparable to that of their sub- thesis and breakdown. Thus glucose transport, strates or effectors and it is not known whether this phosphorylase b, and phosphofructokinase may be http://jcp.bmj.com/ will appreciably modify their kinetic properties. activated by breakdown products of ATP (ADP, The control of glycolysis in muscle is discussed in AMP, and phosphate) and inhibited by ATP. a later section as an illustration of these techniques Another set of signals may regulate rates of glucose and problems. uptake, glycogen synthesis and breakdown, gly- colysis and pyruvate oxidation according to the CONTROL IN ANIMALS: METABOLIC INTEGRATION availability of alternative respiratory fuels such as fatty acids or ketone bodies. The major signals on September 29, 2021 by guest. Protected The control mechanisms which have been outlined appear to be acetyl CoA, CoA, NADH2, and NAD for tissues are mechanisms common to both uni- which may regulate pyruvate dehydrogenase, and cellular organisms and to individual cells of animals. citrate formed from acetyl CoA which may regulate Additional control mechanisms are seen in animals phosphofructokinase. A third mechanism is con- which lead to integration of the varied metabolic cerned with hormone action. Insulin may stimulate pathways of individual tissues. These control glucose uptake by activating glucose transport and mechanisms are partly a consequence of metabolic stimulate glycogen formation by causing the con- specialization (for example gluconeogenesis in liver version of glycogen synthetase from D to I form. In and lipid synthesis and breakdown in adipose its D form the activity of glycogen synthetase is tissue); and partly a consequence of the relatively
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