Enzymes gluconeogenic pathway

COMPARTMENTALIZED PYRUVATE CARBOXYLASE DEPENDS ON METABOLITE CONVERSION AND TRANSPORT The second interesting feature of pyruvate carboxylase is that it is found only in the matrix of the mitochondria. By contrast, the next enzyme in the gluconeogenic pathway, PEP carboxykinase, may be localized in the cytosol or in the mitochondria or both. For example, rabbit liver PEP carboxykinase is predominantly mitochondrial, whereas the rat liver enzyme is strictly cytosolic. In human liver, PEP carboxykinase is found both in the cytosol and in the mitochondria. Pyruvate is transported into the mitochondrial matrix, where it can be converted to acetyl-CoA (for use in the TCA cycle) and then to citrate (for fatty acid synthesis see Figure 25.1). /Uternatively, it may be converted directly to 0/ A by pyruvate carboxylase and used in glu-... 

The pathway for gluconeogenesis is shown in Figures 6.23 and 6.24. Some of the reactions are catalysed by the glycolytic enzymes i.e. they are the near-equilibrium. The non-equilibrium reactions of glycolysis are those catalysed by hexokinase (or glucokinase, in the liver), phosphofructokinase and pyruvate kinase and, in order to reverse these steps, separate and distinct non-equilibrium reactions are required in the gluconeogenic pathway. These reactions are ... 

The reaction involves biotin as a carrier of activated HCO3 (Fig. 14-18). The reaction mechanism is shown in Figure 16-16. Pyruvate carboxylase is the first regulatory enzyme in the gluconeogenic pathway, requiring acetyl-CoA as a positive effector. (Acetyl-CoA is produced by fatty acid oxidation (Chapter 17), and its accumulation signals the availability of fatty acids as fuel.) As we shall see in Chapter 16 (see Fig. 16-15), the pyruvate carboxylase reaction can replenish intermediates in another central metabolic pathway, the citric acid cycle. 

FIGURE 14-22 Nonoxidative reactions of the pentose phosphate pathway, (a) These reactions convert pentose phosphates to hexose phosphates, allowing the oxidative reactions (see Fig. 14-21) to continue. The enzymes transketolase and transaldolase are specific to this pathway the other enzymes also serve in the glycolytic or gluconeogenic pathways, (b) A schematic diagram showing the pathway... 

Answer In the liver, lactate is converted to pyruvate and then to glucose by gluconeogenesis (see Figs 14-15, 14-16). This pathway includes the glycolytic bypass step catalyzed by fructose 1,6-bisphosphatase (FBPase-1). A defect in this enzyme would prevent the entry of lactate into the gluconeogenic pathway in hepatocytes, causing lactate to accumulate in the blood. 

The product of this reaction, oxaloacetate, can either enter the gluconeogenic pathway (Chap. 11) by way of malate or condense with acetyl-CoA to yield citrate. Pyruvate carboxylase is an allosteric enzyme, and it is activated by the heterotropic effector, acetyl-CoA. Thus, pyruvate in the mitochondria is the substrate for either pyruvate dehydrogenase or pyruvate carboxylase, the activities of which, in turn, are controlled by reactants associated with the citric acid cycle. The interplay among pyruvate dehydrogenase, pyruvate carboxylase, pyruvate, and the citric acid cycle is shown in Fig. 12-9. 

GAPDH is common to both the glycolytic and gluconeogenic pathways. The muscle and liver enzymes are similar in structure and properties (194, 09), and the different behavior of the enzyme in muscle and liver must therefore be ascribed to differences in the cellular environment. 

In our consideration of the glycolytic and gluconeogenic pathways, we shall examine the mechanisms of selected enzymes in some detail. Of particular interest will be the enzymes that play the most central roles in converting one type... 

Thus, the equivalent of glucose 6-phosphate can be completely oxidized to CO 2 with the concomitant generation of NADPH. In essence, ribose 5-phosphate produced by the pentose phosphate pathway is recycled into glucose 6-phosphate by transketolase, transaldolase, and some of the enzymes of the gluconeogenic pathway. 

Pyruvate carboxylase is an important enzyme in the gluconeogenic pathway converting pyruvate and bicarbonate into oxaloacetate. AcCoA is required for allosteric activation of the enzyme, and in the diabetic state where fatty acid oxidation is elevated, the high concentrations of AcCoA result in significant activation and gluconeogenesis overactivity. Phenylalkanoic... 

The third study involves the purified enzymes fmctose-l,6-bisphosphatase and phosphofmctokinase. This study brings discussion of "2-6" to a conclusion by demonstrating its effects on these enzymes. Fmctose-l,6-bisphosphatase, an enzyme of the gluconeogenic pathway, catalyzes the conversion of F-l,6-P2 to F-6-P. Ekdahl and Ekman (1984) showed that the activity of this enzyme is maximal in the absence of or presence of low concentrations of "2-6." These researchers showed that its activity is inhibited by "2-6" at concentrations of 10 micromolar and higher. The running rat experiment showed that the resting rat liver contained "2-6" at a concentration of about 12 [iM. Their data also indicated that this level of "2-6" is sufficient to inhibit the activity of the enzyme, but that the levels found in the running rat would be expected to permit near-maximal activity. 

The liver is the major site of biosynthetic reactions in the body. In addition to those pathways mentioned previously, the liver synthesizes fatty acids from the pymvate generated by glycolysis. It also synthesizes glucose from lactate, glycerol 3-phosphate, and amino acids in the gluconeogenic pathway, which is principally a reversal of glycolysis. Consequently, in liver, many of the glycolytic enzymes exist as isoenzymes with properties suited for these functions. 

Fig. 31.1. Glycolysis and gluconeogenesis in the hver. The gluconeogenic pathway is almost the reverse of the glycolyrtic pathway, except for three reaction sequences. At these three steps, the reactions are catalyzed by different enzymes. The energy requirements of these reactions differ, and one pathway can be activated while the other is inhibited.

Both of these dietary states are 1 characterized by an elevation of glucagon. Glucagon stimulates amino acid transport into the liver, stimulates gluconeogenesis through decreasing levels of fructose 2,6-bisphosphate, and induces the synthesis of enzymes in the urea cycle, the gluconeogenic pathway, and the pathways for degradation of some of the amino acids. 

Several enzymes are common to both the glycolytic and gluconeogenic pathways, but four enzymes catalyse steps that only occur in gluconeogenesis pyruvate carboxylase (reaction 3.6), phosphoenol-... 

The metabolic steps in gluconeogenesis occur in two intracellular compartments (Fig. 3.2) the cytosol and the mitochondrial matrix. The enzymes of the tricarboxylic acid cycle reside in the mitochondrial matrix, apart from succinate dehydrogenase which is present in the inner mitochondrial membrane, whereas most of the enzymes of the gluconeogenic pathway are present in the cytosol. Transaminases, such as alanine aminotransferase and aspartate aminotransferase, are present both in mitochondria and cytosol of the domestic fowl liver (Sarkar, 1977). One of the control enzymes in gluconeogenesis, PEPCK, has a different intracellular distribution in avian liver compared with mammalian liver (Table 3.3). PEPCK in both pigeon and domestic fowl liver is present almost exclusively (> 99%) in mitochondria (Soling et al.. 1973), whereas in most mammals that have been studied, it is present mainly in the cytosol, and only present, if at all, in smaller amounts in... 

The metabolic role of many minerals and vitamins is as prosthetic groups or coenzymes in different enzyme systems. Consequently, mineral and vitamin deficiencies can cause a breakdown of the processing system and precipitate metabolic disease. For example, methylmalonyl-CoA isomerase (see p. 203) is an important vitamin Bi2-dependent enzyme in the gluconeogenic pathway. A deficiency of vitamin B12 (or cobalt) may reduce enzyme activity, decrease the efficiency of glucose synthesis and predispose the animal to ketosis. Similarly, ceruloplasmin is a copper-dependent enzyme responsible for releasing iron from cells into blood plasma. A copper deficiency may reduce ceruloplasmin activity, decrease the efficiency of iron utilisation for haemoglobin synthesis and predispose the animal to anaemia. 

The addition of phosphoenolpyruvate alone is almost sufficient to obtain the maximal synthesis of PP-ribose-P from ribose-5-P, as extracts of liver cells contain a significant amount of pyruvate kinase. However, all the enzymes of the pentose phosphate, glycolytic, and gluconeogenic pathways are also present in these extracts. Thus,... 

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