Glucose in gluconeogenesis

Glycerol is an important by-product of fat metabolism. Fatty acids, from triacylglycerols, are metabolised in the liver and peripheral tissue via 8-oxidation into acetyl-CoA the remaining glycerol is an important source of glucose (in gluconeogenesis) in the liver. Glycerol enters the reaction sequence ... 

Figure 26.2 Converting glucose 6-phosphate to glucose in gluconeogenesis. The ATP expended in glycolysis is not claimed back, helping the process to work uphill as well as downhill. ...

The citric acid cycle is the final common pathway for the aerobic oxidation of carbohydrate, lipid, and protein because glucose, fatty acids, and most amino acids are metabolized to acetyl-CoA or intermediates of the cycle. It also has a central role in gluconeogenesis, lipogenesis, and interconversion of amino acids. Many of these processes occur in most tissues, but the hver is the only tissue in which all occur to a significant extent. The repercussions are therefore profound when, for example, large numbers of hepatic cells are damaged as in acute hepatitis or replaced by connective tissue (as in cirrhosis). Very few, if any, genetic abnormalities of citric acid cycle enzymes have been reported such ab-normahties would be incompatible with life or normal development. 

In the brain, when ketones are metabolized to acetyl CoA, pyruvate dehydrogenase is inhibited. Glycolysis and subsequently glucose uptake in brain decreases. This important switch spares body protein (which otherwise would be catabolized to form glucose by gluconeogenesis in the liver) by allowing the brain to indirectly metabolize fetty acids as ketone bodies. 

Figure 8.13 The central role of transdeamination in metabolism of amino adds and further metabolism of the oxoacids in the liver. The box contains the reactions for conversion of the amino acids to their respective oxoacids. Processes are as follows (1) digestion of protein in the intestine and absorption of resultant amino acids, (2) degradation of endogenous protein to amino acids (primarily but not exclusively muscle protein), (3) protein synthesis, (4) conversion of amino acid to other nitrogen-containing compounds (see Table 8.4), (5) oxidation to CO2, (6) conversion to glucose via gluconeogenesis, (7) conversion to fat.

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