Gluconeogenesis scheme

Noncarbohydrate Sources for Gluconeogenesis. In addition to pyruvate and lactate, which are delivered to the liver and kidneys, other noncarbohydrate compounds serve as substrates for glucose synthesis. In accordance with the gluconeogenesis scheme, it may be anticipated that all materials of noncarbohydrate nature that are... 

Now that we know something about the effects of hormones in triggering responses within the cell, we can return to and expand on some earlier points about metabolic control. In Section 18.3, we discussed some points about control mechanisms in carbohydrate metabolism. We saw at that time how glycolysis and gluconeogenesis can be regulated and how glycogen synthesis and breakdown can respond to the body s needs. Phosphorylation and dephosphorylation of the appropriate enzymes played a large role there, and that whole scheme is subject to hormonal action. 

Oxaloacetate is formed from pyruvate by pyruvate carboxylase, located in the mitochondria. Pyruvate carboxylase is a biotin-requiring enzyme its kinetic mechanism has been studied in detail using the enzyme purified from domestic fowl liver (Attwood Graneri, 1992). In tissues where PEPCK is located in the cytosol, its substrate oxaloacetate is required in the cytosol for the formation of PEP for gluconeogenesis. The malate-aspartate shuttle is required for gluconeogenesis in avian kidney, according to the scheme in Fig. 3.3, but whether or not it is also required for gluconeogenesis in avian liver is unresolved. There is evidence for the existence of a... 

The synthesis of glucose (gluconeogenesis), which is then stored as starch (a mixture of a-glucose polymers, vide infra) and/or cellulose (a mixture of P-glucose polymers, vide infra), occurs by condensation of 3-phosphoglyceraldehyde (glycer-aldehyde 3-phosphate) with dihydroxyacetone monophosphate (Scheme 11.13). 

Scheme 11.13. A representation of a portion of the pathway of gluconeogenesis. The 6-phosphate of glucose is considered capable of hydrolysis to glucose with the appropriate hydroiyase (EC 3.1.3.9) catalyst. The anomer shown is a (because the hydroxyl on the ano-meric carbon is cis or syn to the hydroxyl at C5, Scheme 11.14). A caution concerning nomen-ciature is needed. In the acyclic form, it is clear which carbon is number six. Cyclization to the six-membered ring pyranose (oxane) (or furanose [oxolane] when the five-membered ring forms) leaves ns with a nomenclature problem since the carbon that was C-1 (the ano-meric carbon) is often referred to as C-1 in both structures that involve formation of bonds to that carbon as well as in reactions that may occur at that carbon. However, it will be recalled (Chapter 5) that the nomenclature of cyclic ethers (and other heterocycles) generally requires that the heteroatom (or a heteroatom if more than one is present) receive the lowest number. So, the lUPAC and the lUBMB numbering system is used for archiving purposes, while the trivial nomenclature is commonly used in the literature. Both systems will be found here as appropriate.

Figure 8. The pathway of gluconeogenesis in mammalian liver. This scheme shows the major reactions for gluconeogenesis and indicates the cytosol (1) and mitochondrial (2) forms of P-enolpyruvate carboxykinase. (From Tilghman, 1975).

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