Gluconeogenesis

Two gluconeogenesis-specific phosphatases then successively cleave off the phosphate residues from fructose 1,6-bisphos-phate. In between these reactions lies the isomerization of fructose 6-phosphate to glucose 6-phosphate—another glycolytic reaction. 

Glycerol initially undergoes phosphorylation at C-3 [7]. The glycerol 3-phosphate 

Koolman, Color Atlas of Biochemistry, 2nd edition 2005 Thieme All rights reserved. Usage subject to terms and conditions of license. 

The synthesis of glucose from noncarbohydrate sources is referred to as the gluconeogenesis. It is feasible only in certain organism tissues. The major site for gluconeogenesis is the liver. To a lesser extent, the kidneys and intestinal mucosa are involved in this process. 

Mechanism for Gluconeogenesis. Since the glycolysis involves three energetically irreversible steps at the pyruvate kinase, phosphofructokinase, and hexokinase levels, the production of glucose from simple noncarbohydrate materials, for example, pyruvate or lactate, by a reversal of glycolysis ( from bottom upwards ) is impossible. Therefore, indirect reaction routes are to be sought for. 

The Jirst indirect route in glucose synthesis involves the formation of phosphoenolpyruvate from pyruvate without the intervention of pyruvate kinase. This route is catalyzed by two enzymes. At first, pyruvate is converted into oxaloacetate. This reaction occurs in the mitochondria as the pyruvate molecules enter them, and is catalyzed by pyruvate carboxylase according to the scheme 

This enzyme, similar to all C02 assimilating enzymes, contains biotin for a cofactor. Oxaloacetate is released from the mitochondria into the cytoplasm to enter gluconeogenesis. In the cytoplasm, oxaloacetate converts to phosphoenolpyruvate via a reaction catalyzed by phosphoenolpyruvate carboxylase  

The reaction equilibrium is shifted to the right. The major supplier of phosphate groups is GTP, but for this purpose, ATP may also be available. 

Question It has been stated above that tissues can break down glucose, but can they synthesize it too  

but only certain tissues have this ability. 

In mammalian cells, glucose is the most abundant carbohydrate energy source. It is metabolized in all cells as a glycolytic fuel and is stored in liver and muscle as the polymer glycogen. But certain cells have the enzymes to catalyze the synthesis of glucose under certain conditions. The requirements are (1) the availability of specific carbon skeletons (carbon backbone structures of various types), (2) energy, in the form of ATP, necessary to accomplish the sequence of reactions, and (3) the enzymes to catalyze reactions of the sequence. 

The carbon skeletons used for the synthesis of glucose are not of carbohydrate origin but are derived from particular amino acids. One exception to this is the carbon skeleton of lactate, itself a product of carbohydrate metabolism, which can be incorporated into a new glucose molecule. This process, the synthesis of new glucose from essentially noncarbohydrate precursors, is called gluconeogenesis. 

Question Can the glycolytic pathway operate in the reverse direction that is, can pyruvate be converted into glucose  

Blood glucose levels must be maintained within a relatively constant range to supply critical organs and tissues (such as brain, RBCs, cornea, lens, kidney medulla, and testes), even when intake of dietary carbohydrates is low. 

During a prolonged fast, glucose can be synthesized from various precursors, 

Because three of the reactions of the glycolytic pathway are irreversible, it is not possible to simply run glycosis in reverse to manufacture glucose. 

The critical and irreversible glycolytic steps that must be bypassed follow  

However, seven of the reactions of glycolysis are reversible and can be used for gluconeogenesis. 

Phosphofructokinase, and/or hexokinase must also be bypassed in converting other hexoses to glucose. 

Pyruvate converted to PEP without using the pyruvate kinase reaction Formally, pyruvate is first converted to oxaloacetate, which is in turn converted to PEP. In the first reaction of this process P3mivate carboxylase adds carbon dioxide to pyruvate with the expenditure of one ATP equivalent of energy. Biotin, a carboxyl-group transfer cofactor in animals, is required by this enz)one  

The reaction takes place in two parts on two different sub-sites on the enzyme. In the first part biotin attacks bicarbonate with a simultaneous attack/hydrolysis by bicarbonate on ATP, resulting in the release of ADP and inorganic phosphate (note the coupling by the enzyme of independent processes in this reaction)  

Note that the 14 Angstrom arm of biocytin allows biotin to move between the two sites, in this case carrying the activated carboxyl group. In the second site a pyruvate carbanion then attacks the activated carboxyl group, regenerating the biotin cofactor and releasing oxaloacetate  

Pyruvate carboxylase is followed by the Phosphoenolpyruvate carboxykinase (PEPCK) reaction. In this reaction oxaloacetate is decarboxylated with a simultaneous phosphorylation by GTP to give GDP  

Gluconcogenesis occurs primarily in the liver. It can also occur in the kidney, but this contribution is relatively small. The carbon skeletons used for glucose synthesis can be derived from lactate, glycerol, or amino adds. During exercise, the lactate produced and released by fermentative muscle is taken up by the liver cind 

TABLE 4.6 Metabolisin of the Liver before and during Exercise 

CK tirall pmditclion (mmol/min) Uptalw of glucpgcnic nutnents (glucose ei uivatEniH. mmoL/mm) 0.S2 t.S6 1.4  

Net breakdown of muscle can occur with either exercise or prolonged fasting. The mechanisms that control the breakdown of the various types of protein found in muscle are not well understood. It has, however, been established that while the branched-chain amino acids (BCAAs) released tend to be oxidized for energy in the muscle cell, other released amino acids enter the bloodstream for catabolism, and perhaps gluconeogenesis, in the liver Examination of the amino acids released from skeletal muscle reveals an apparent anomaly alanine accounts for or ly about 6% of the amino acids of muscle, but for about 35% of the amino acids released from muscle during exerdse. 

FIGURE 439 First step in catabolism of BCAAs and iramfer of a-amino groups to pyruvate. 

3- Phosphoglycerate 1 3 Diphosphoglycerate Glyceraldehyde-3-phosplmte Dihydroxyacetone phosplmte Fructose-l 6-diphosplmte Fructose-6-phosplmte Glucose-6-phosphate Glucose-6-phosphate 

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Pyruvate carboxylase is the most important of the anaplerotie reactions. It exists in the mitochondria of animal cells but not in plants, and it provides a direct link between glycolysis and the TCA cycle. The enzyme is tetrameric and contains covalently bound biotin and an Mg site on each subunit. (It is examined in greater detail in our discussion of gluconeogenesis in Chapter 23.) Pyruvate carboxylase has an absolute allosteric requirement for acetyl-CoA. Thus, when acetyl-CoA levels exceed the oxaloacetate supply, allosteric activation of pyruvate carboxylase by acetyl-CoA raises oxaloacetate levels, so that the excess acetyl-CoA can enter the TCA cycle. 

Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway... 

Nearly All Gluconeogenesis Occurs In the Liver and Kidneys in Animals... 

The complete route of gluconeogenesis is shown in Figure 23.1, side by side with the glycolytic pathway. Gluconeogenesis employs three different reactions, catalyzed by three different enzymes, for the three steps of glycolysis that are... 

FIGURE 23.1 The pathways of gluconeogenesis and glycolysis. Species in blue, green, and peach-colored shaded boxes indicate other entry points for gluconeogenesis (in addition to pyruvate). 

FIGURE 23.5 Pyruvate carboxyl compartmentalized reaction. Pyruva verted to oxaloacetate in the mitoci Because oxaloacetate cannot be trai across the mitochondrial membrant reduced to malate, transported to tl and then oxidized back to oxaloace gluconeogenesis can continue. 

Gluconeogenesis Is Regulated by Allosteric and Substrate-Level Control Mechanisms... 

Acetyl-CoA is a potent allosteric effector of glycolysis and gluconeogenesis. It allosterically inhibits pyruvate kinase (as noted in Chapter 19) and activates pyruvate carboxylase. Because it also allosterically inhibits pyruvate dehydrogenase (the enzymatic link between glycolysis and the TCA cycle), the cellular fate of pyruvate is strongly dependent on acetyl-CoA levels. A rise in... 

As described in Chapter 19, Emile Van Schaftingen and Henri-Gery Hers demonstrated in 1980 that fructose-2,6-bisphosphate is a potent stimulator of phosphofructokinase. Cognizant of the reciprocal nature of regulation in glycolysis and gluconeogenesis. Van Schaftingen and Hers also considered the... 

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