Regulation of Metabolic Pathways:- Regulatory Mechanisms Evolved under Strong Selective Pressures
In the course of evolution, organisms have acquired a remarkable collection of regulatory mechanisms for maintaining homeostasis at the molecular, cellular, and organismal level. The importance of metabolic regulation to an organism is reflected in the relative proportion of genes that encode regulatory machinery—in hu mans, about 4,000 genes (~12% of all genes) encode regulatory proteins, including a variety of receptors, regulators of gene expression, and about 500 different protein kinases! These regulatory mechanisms act over different time scales (from seconds to days) and have different sensitivities to external changes. In many cases, the mechanisms overlap: one enzyme is subject to regulation by several different mechanisms. After the protection of its DNA from damage, perhaps nothing is more important to a cell than maintaining a constant supply and concentration of ATP. Many ATP-using enzymes have Km values between 0.1 and 1mM, and the ATP concentration in a typical cell is about 5 mM. If [ATP] were to drop significantly, the rates of hundreds of reactions that involve ATP would decrease, and the cell would probably not survive. Furthermore, because ATP is converted to ADP or AMP when “spent” to accomplish cellular work, the [ATP]/[ADP] ratio profoundly affects all reactions that employ these cofactors. The same is true for other important cofactors, such as NADH/NAD and NADPH/NADP+. For example, consider the reaction catalyzed by hexokinase:
ATP+ glucose → ADP + glucose 6-phosphate
Note that this expression is true only when reactants and products are at their equilibrium concentrations, where ΔG=0. At any other set of concentrations, ΔG is not zero. Recall (from Chapter 13) that the ratio of products to substrates (the mass action ratio, Q) determines the magnitude and sign of ΔG and therefore the amount of free energy released during the reaction
Because an alteration of this mass action ratio profoundly influences every reaction that involves ATP, or ganisms have been under strong evolutionary pressure to develop regulatory mechanisms that respond to the [ATP]/[ADP] ratio. Similar arguments show the importance of maintaining appropriate [NADH]/[NAD+] and [NADPH]/[NADP+] ratios.
AMP concentration is a much more sensitive indicator of a cell’s energetic state than is ATP. Normally cells have a far higher concentration of ATP (5 to 10 mM) than of AMP (<0.1 mM). When some process (say, muscle contraction) consumes ATP, AMP is produced in two steps. First, hydrolysis of ATP produces ADP, then the reaction catalyzed by adenylate kinase produces AMP:
2 ADP → AMP+ATP
If [ATP] drops by 10%, producing ADP and AMP in the same amounts, the relative change in [AMP] is much greater (Table 15–1). It is not surprising, therefore, that many regulatory processes hinge on changes in [AMP]. One important mediator of regulation by AMP is AMP dependent protein kinase (AMPK), which responds to an increase in [AMP] by phosphorylating key proteins, thereby regulating their activities. The rise in [AMP] may be caused by a reduced nutrient supply or increased exercise. The action of AMPK (not to be confused with the cyclic AMP–dependent protein kinase; see Section 15.4) increases glucose transport and activates glycolysis and fatty acid oxidation, while suppressing energy requiring processes such as the synthesis of fatty acids, cholesterol, and protein. We discuss this enzyme fur ther, and the detailed mechanisms by which it effects these changes In addition to the cofactors ATP, NADH, and NADPH, hundreds of metabolic intermediates also must be present at appropriate concentrations in the cell. For example, the glycolytic intermediates dihydroxyacetone phosphate and 3-phosphoglycerate are precursors of tri acylglycerols and serine, respectively. When these products are needed, the rate of glycolysis must be adjusted to provide them without reducing the glycolytic pro duction of ATP. Priorities at the organismal level have also driven the evolution of regulatory mechanisms. In mammals, the brain has virtually no stored source of energy, de pending instead on a constant supply of glucose from the blood. If glucose in the blood drops from its normal concentration of 4 to 5 mM to half that level, mental con fusion results, and a fivefold reduction in blood glucose can lead to coma and death. To buffer against changes in blood glucose concentration, release of the hormones insulin and glucagon, elicited by high or low blood glu cose, respectively, triggers metabolic changes that tend to return the blood glucose concentration to normal. Other selective pressures must also have operated throughout evolution, selecting for regulatory mechanisms that
1. maximize the efficiency of fuel utilization by preventing the simultaneous operation of pathways in opposite directions (such as glycolysis and gluconeogenesis);
2. partition metabolites appropriately between alternative pathways (such as glycolysis and the pentose phosphate pathway);
3. draw on the fuel best suited for the immediate needs of the organism (glucose, fatty acids, glycogen, or amino acids); and
4. shut down biosynthetic pathways when their products accumulate.
The importance of effective metabolic regulation is clear from the consequences of failed regulation: in many cases, serious disease (as described in Box 15–1, for example).
