Metabolic utilization of carbohydrate
Peripheral tissues utilize glucose and the above-men-tioned intermediates from fructose and galactose via glycolysis and the citric acid or Krebs cycle pathways. Glycolysis, a sequence of reactions in which glucose is converted to pyruvate, with concomitant produc-tion of ATP, is the prelude to the citric acid cycle and electron transport chain, which together release the energy contained in glucose. Under aerobic condi-tions pyruvate enters mitochondria, where it is com-pletely oxidized to carbon dioxide and water. If the supply of oxygen is limited, as in actively contracting muscle, pyruvate is converted to lactate. Therefore, complete oxidation of glucose to carbon dioxide and water occurs under aerobic conditions through the reactions of the glycolytic pathway (in the cell’s cyto-plasm), the Krebs cycle, and oxidative phosphoryla-tion (in the mitochondrion). The overall reaction can be summarized stoichiometrically as:
C6H12O6 + 6O2 → 6CO2 + 6H2O
Approximately 40% of the free energy ( G) released by this transformation is captured by the production of ATP (38 moles of ATP per mole of glucose oxi-dized), which is used for a wide variety of purposes, including powering muscle contraction, transporting substances across membranes against a concentration gradient, and synthesis of cell macromolecules. The remainder of the free energy is released as heat.
When the demand for oxygen exceeds supply, as in muscle during intense exercise, anaerobic glycolysis produces lactic acid as a major end-product. The rela-tive lack of oxygen means that oxidative phosphoryla-tion cannot keep up with the supply of reduced dinucleotides and, for glycolysis to proceed, NADH must be recycled back to NAD+. This is achieved by the reaction:
Pyruvate + NADH + H+ → Lactate + NAD+
which is catalyzed by the enzyme lactate dehydro-genase. Anaerobic glycolysis provides some or all of the ATP needs for some cells and tissues; for example, erythrocytes, white blood cells, lymphocytes, the kidney medulla, and eye tissues. The lactate released from tissues undergoing anaerobic glycolysis is taken up by other tissues that have a high number of mito-chondria per cell, such as heart muscle, in which the lactate is converted back to pyruvate and then enters the Krebs cycle via acetyl coenzyme A.
In hepatic and muscle cells some glucose is con-verted to glycogen in the glycogenesis pathway. Gly-cogen is a readily mobilized storage form of glucose residues linked with α-1,4-glycosidic bonds into a large, branched polymer. Glycogen is a reservoir of glucose for strenuous muscle activity and its synthesis and degradation are important for the regulation of blood glucose concentrations.