Pyruvate carboxylase, regulation

Pyruvate carboxylase plays another important role in regulating metabolism by ensuring an adequate supply of OAA for the TCA cycle. Take a moment to stop and... 

Carbonic anhydrase (CA, also called carbonate dehydratase) is an enzyme found in most human tissues. As well as its renal role in regulating pH homeostasis (described below) CA is required in other tissues to generate bicarbonate needed as a co-substrate for carboxylase enzymes, for example pyruvate carboxylase and acetyl-CoA carboxylase, and some synthase enzymes such as carbamoyl phosphate synthases I and II. At least 12 isoenzymes of CA (CA I—XII) have been identified with molecular masses varying between 29 000 and 58 000 some isoenzymes are found free in the cytosol, others are membrane-bound and two are mitochondrial. 

Coordinate Regulation of Pyruvate Carboxylase and Pyruvate Dehydrogenase by Acetyl CoA... 

The two major mitochondrial enzymes (Figure 1-14-5) that use pyruvate, pyruvate carboxylase and pyruvate dehydrogenase, are both regulated by acetyl CoA. This control is important in these contexts ... 

The theory of regulation of the cycle is as follows. First, an increase in oxaloacetate concentration increases the activity of citrate synthase and hence the cycle. The concentration of oxaloacetate is regulated by the activity of the enzyme pyruvate carboxylase, which catalyses the reaction ... 

Figure 9.24 Control of the oxaloacetate concentration and hence the flux through the cycle by pyruvate carboxylase. The activity of pyruvate carboxylase is increased by an increase in its substrate, pyruvate, and its allosteric regulator, acetyl-CoA. Regulation of the activity is important to increase the concentration of oxaloacetate which increases the flux through the cycle. An increase in the rate of glycolysis increases the concentration of pyruvate, and an increase in the rate of fatty acid oxidation increases that of acetyl-CoA. Both result in an increase in the concentration of oxaloacetate and hence in the flux through the cycle, providing coordination between the rates of glycolysis, fatty acid oxidation and the cycle.

Gluconeogenesis is regulated at the level of pyruvate carboxylase (which is activated by acetyl-CoA) and FBPase-1 (which is inhibited by fructose 2,6-bisphosphate and AMP). 

Jitrapakdee, S. Wallace, J.C. (1999) Structure, function, and regulation of pyruvate carboxylase. Biochem. J. 340, 1-16. 

Regulation of Pyruvate Carboxylase The carboxy-lation of pyruvate by pyruvate carboxylase occurs at a very low rate unless acetyl-CoA, a positive allosteric modulator, is present. If you have just eaten a meal rich in fatty acids (tri-acylglycerols) but low in carbohydrates (glucose), how does this regulatory property shut down the oxidation of glucose to C02 and H20 but increase the oxidation of acetyl-CoA derived from fatty acids ... 

Acetyl-CoA is a critical regulator of the fate of pyruvate it allosterically inhibits pyruvate dehydrogenase and stimulates pyruvate carboxylase (see Fig. 15-20). In these ways acetyl-CoA prevents it own further production from pyruvate while stimulating the conversion of pyruvate to oxaloacetate, the first step in gluconeo-genesis. 

In bacteria and green plants PEP carboxylase (Eq. 13-53), a highly regulated enzyme, is responsible for synthesizing oxaloacetate. In animal tissues pyruvate carboxylase (Eq. 14-3) plays the same role. The latter enzyme is almost inactive in the absence of the allosteric effector acetyl-CoA. For this reason, it went undetected for many years. In the presence of high concentrations of acetyl-CoA the enzyme is fully activated and provides for synthesis of a high enough concentration of oxaloacetate to permit the cycle to function. Even so, the oxaloacetate concentration in mitochondria is low, only 0.1 to 0.4 x 10-6 M (10-40 molecules per mitochondrion), and is relatively constant.65 79... 

When the carbohydrates are being metabolized, TCA ycle intermediates are replenished by production of oxalo-etate from pyruvate. In mammals, this reaction is cata-[zed by pyruvate carboxylase, and one ATP-to-ADP concision is associated with the carboxylation. Other operties of this reaction are discussed later in this chapter l connection with regulation of the TCA cycle and related [etabolic sequences. 

ADP, acetyl-CoA, succinyl-CoA, and citrate. The major known sites for regulation of the cycle include two enzymes outside the cycle (pyruvate dehydrogenase and pyruvate carboxylase) and three enzymes inside the cycle (citrate synthase, isocitrate dehydrogenase, and a-ketoglutarate dehydrogenase). All of these sites of regulation represent important metabolic branchpoints. 

Eight enzyme-catalyzed reactions are involved in the conversion of acetyl-CoA into fatty acids. The first reaction is catalyzed by acetyl-CoA carboxylase and requires ATP. This is the reaction that supplies the energy that drives the biosynthesis of fatty acids. The properties of acetyl-CoA carboxylase are similar to those of pyruvate carboxylase, which is important in the gluconeogenesis pathway (see chapter 12). Both enzymes contain the coenzyme biotin covalently linked to a lysine residue of the protein via its e-amino group. In the last section of this chapter we show that the activity of acetyl-CoA carboxylase plays an important role in the control of fatty acid biosynthesis in animals. Regulation of the first enzyme in a biosynthetic pathway is a strategy widely used in metabolism. 

Sutton, F., Butler, E.T. Ill Smith, E.T. (1986). Isolation of the structural gene encoding a mutant form of E. coli phos-phoeno/pyruvate carboxylase deficient in regulation by fructose 1,6-bisphosphate. Journal of Biological Chemistry 261, 16078-81. 

Regulation of pyruvate kinase, pyruvate carboxylase and PEP carboxykinase... 

Pyruvate carboxylase also requires a monovalent cation for activity. The activity of the purified enzyme was measured in the presence of various monovalent cations, as indicated in Table 10.7. Similar patterns of stimulation have been fovmd for acetyl-CoA synthetase, an enzyme used in acetate metabolism (Webster, 1966), propionyl-CoA carboxylase (Giorgio and Plaut, 1967), and several other enzymes (Suelter, 1970). Maximal activity of the aforementioned enzymes usually occurs at a wide range of potassium concentrations, that is, from 50 to 150 mM. There is therefore little reason to believe that the slight changes in intracellular K concentrations that can occur under normal conditions or during K deficiency result in an impairment in the activities of these enzymes or in some t)q>e of regulation of the activities. 

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