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mitochondria where it is a substrate for the enzyme PDH (PDH is actually a complex of three enzymes, sometimes called pyruvate dehydrogenase complex, PDC). PDC not only further oxidises pyruvate, but also removes one carbon, resulting in the formation of (2 carbon) acetyl-CoA which, as described earlier, can be fully oxidised in the TCA cycle. This reaction is essentially irreversible. The importance of this process is illustrated by looking at the energy yield of these pathways: glycolysis yields 2 ATPs by substrate-level phosphorylation, but much energy remains within the pyruvate molecule; full oxidation of pyruvate, via formation of acetyl-CoA and oxidation in the TCA cycle, yields a further 36 ATPs. This number is a theoretical maximum and allowing for some inefficiency the real figure is probably slightly lower than this, but it illustrates how much energy can be derived from oxidation, and hence how important mitochondria (TCA cycle, electron transport chain) are for producing ATP.

      Breakdown of glucose as far as acetyl-CoA can also be part of a synthetic process. Acetyl-CoA produced from glucose is the starting point for the pathways of lipid synthesis: lipogenesis, which usually refers to the synthesis of fatty acids from glucose, and cholesterol synthesis. These pathways, like most biosynthetic pathways, are cytosolic, and the acetyl-CoA must be transferred out of the mitochondria (to be expanded later – Box 5.4).

      1.3.2.1.5 Gluconeogenesis

      1.3.2.1.6 Glycogen metabolism

      Carbohydrate is stored in limited amounts as cytoplasmic glycogen granules in most tissues as an energy resource available within the tissue (and hence independent of blood supply) for rapid utilisation when required. Glycogen is a polymer of glucose whose structure was described earlier (Figure 1.8). Glycogen synthesis starts with glucose 6-phosphate (Figure 1.14) and involves sequential polymerisation of glucose units on a glycogenin protein backbone. Glucose units are added as UDP-glucose (derived from glucose 6-phosphate) to the enlarging glycogen molecule by the enzyme glycogen synthase. Glycogen synthase assembles the glucose units into a linear chain, but every 8–10 residues a branch point is introduced by a branching enzyme. This has the effect of producing a highly branched tree-like structure with many free (‘non-reducing’) ends (Figure 1.8). Glycogenolysis involves the reverse: sequential removal of glucose units. These multiple terminal glucose residues enable rapid glucose release during glycogen degradation, by the enzyme (glycogen) phosphorylase (and a debranching enzyme). Glucose 1-phosphate is released, and is converted into glucose 6-phosphate. In most tissues, which store glycogen for their own utilisation (e.g. muscle), the glucose 6-phosphate then enters the pathway of glycolysis for energy production. In the liver specifically (and to some extent in kidney, especially during starvation) the enzyme glucose 6-phosphatase is expressed (uniquely in these tissues) and converts glucose 6-phosphate to free glucose: thus, glucose derived from glycogenolysis or produced by gluconeogenesis may be released into the bloodstream to maintain blood glucose concentrations in the postabsorptive or the fasting state.

      1.3.2.1.7 Pentose phosphate pathway

      One further pathway of glucose metabolism will be mentioned briefly: the pentose phosphate pathway. Again, this pathway occurs in the cytosol. This involves the metabolism of glucose 6-phosphate through a complex series of reactions that generate pentose sugars, used in nucleic acid synthesis, and also reducing power in the form of NADPH (Figure 1.14).

      The pathway comprises two parts: an oxidative (irreversible) stage, initiated by the enzyme glucose-6-phosphate dehydrogenase, which generates NADPH and the pentose (5-carbon) sugar ribulose 5-phosphate, and then a non-oxidative (reversible) stage which interconverts the pentose sugar into a wide variety of 3 carbon (triose), 4 carbon (tetrose), 5 carbon (pentose), 6 carbon (hexose), and 7 carbon (heptose) sugars. These sugars are used for the synthesis of nucleotides and aromatic amino acids, whilst NADPH provides energy for many reductive biosyntheses – including lipogenesis and amination of 2-oxoacids to amino acids (glutamate dehydrogenase – see below); hence this is a pathway active in anabolic states. NADPH also maintains the antioxidant glutathione in its reduced (active) form (GSH). Because the relative requirements for the two products of the pentose phosphate pathway (pentose sugars and NADPH) varies, when NADPH demand exceeds pentose need, the sugar can be reinserted into glycolysis (hence ‘pentose phosphate shunt’).

      1.3.3 Lipid metabolism

      1.3.3.1 Pathways of lipid metabolism

Figure shows pathways of lipid metabolism in the cell. Synthesis of fatty acids from acetyl-CoA (lipogenesis) is driven by NADPH (from the pentose phosphate pathway), whilst the opposite pathway, breakdown of fatty acids to form acetyl-CoA 
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