of oxygen however, (or, indeed, in the absence of mitochondria, as in red blood cells), NADH would accumulate and NAD+ concentration would fall too low. By converting pyruvate to lactate and linking this to NAD, the NAD+ is regenerated to permit glycolysis to proceed, but at the cost of accumulating lactate (Figure 1.15a). When oxygen becomes available, lactate dehydrogenase can readily convert the lactate back to pyruvate (and NADH) for oxidation (and red blood cells export the lactate to the liver to be converted back into pyruvate).
Figure 1.15 Lactate and ethanol metabolism. Glycolysis produces NADH from NAD+. (a) In aerobic conditions in mammals the NAD+ is regenerated by the electron transport chain, but in anaerobic conditions NAD+ must be regenerated by lactate dehydrogenase, permitting glycolysis to continue, at the cost of accumulating lactate. (b) In yeast, pyruvate is instead reduced to ethanol in order to regenerate NAD+ and allow glycolysis to proceed (brown arrows). When humans ingest alcohol, the ethanol is oxidised to acetyl-CoA via acetaldehyde, providing a large influx of energy but also altering the NAD+:NADH ratio.
Yeast employ a different strategy to oxidise NADH and regenerate NAD+. Instead of reducing pyruvate to lactate, they reduce pyruvate to ethanol (CH3·CH2·OH) by the process of alcoholic fermentation, shown in Figure 1.15b. Pyruvate is first decarboxylated to acetaldehyde (CH3·CHO) and carbon dioxide is produced (anyone who has made their own wine by using yeast to ferment the sugar in grape juice will be familiar with the bubbles of CO2 gas given off during the fermentation). The acetaldehyde is then reduced to ethanol by alcohol dehydrogenase, reoxidising the NADH back to NAD+ so that glycolysis can proceed. This process has the advantage for yeasts that the ethanol is toxic to many organisms, but yeast can tolerate ethanol in high concentrations, hence their ‘waste product’ inhibits other, competing micro-organisms. When humans ingest ethanol, it is converted back into acetaldehyde by our alcohol dehydrogenase (and the acetaldehyde is probably at least partly responsible for the ‘hangover’ effects of excessive alcohol consumption), and then on to acetate and ultimately acetyl-CoA (Figure 1.15b). Acetyl-CoA can go on to be oxidised by the TCA cycle or used for lipid synthesis (see Section 1.3.3), in other words ethanol provides a large amount of ingested energy. But in addition, metabolism of ethanol reduces large amounts of NAD+ to NADH, providing more energy again, but also disrupting the NAD+:NADH ratio; this has the effect of inhibiting gluconeogenesis (Section 1.3.2.1.5) and causing the hypoglycaemia that is probably the origin of the stimulated appetite commonly seen following alcohol consumption.
1.3.2.1.4 Pyruvate oxidation
Pyruvate can also enter 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
Gluconeogenesis, despite its name (synthesis of new glucose), is a pathway typically active in catabolic states, when there is a need to make glucose from other fuels for organs that depend upon it. The pathway of glucose synthesis, gluconeogenesis, occurs primarily in liver cells (and to a lesser extent, in kidney) and is essentially a reversal of glycolysis (many of whose steps are freely reversible and shared by both pathways) although with some specific steps, circumventing the energy-yielding and largely irreversible steps of glycolysis (Figure 1.14). Reversal of the last step of glycolysis (phosphoenolpyruvate → pyruvate) requires formation of oxaloacetate, and spans the mitochondrion. The main regulatory enzyme of glycolysis, phosphofructokinase, must also be reversed, and for this gluconeogenesis uses fructose-1, 6-bisphosphatase. Finally, if free glucose is to be produced for export (liver), glucose 6-phosphate must be dephosphorylated to glucose by glucose-6-phosphatase – i.e. this is the final enzyme of both gluconeogenesis and glycogenolysis, both pathways allowing liver to export glucose and maintain blood glucose levels. The major substrate for gluconeogenesis is pyruvate; the major source of this under most conditions is lactate. Amino acids whose carbon skeletons can be converted to pyruvate (e.g. alanine, during starvation, discussed later) can also contribute, and in addition glycerol released from lipolysis of triacylglycerols in adipose tissue can enter the gluconeogenic pathway (and hence breakdown of storage lipids does yield a small amount of carbohydrate). Note that glucose is broken down to lactate (glycolysis) in red blood cells, for instance, and in anaerobic cells such as renal medulla: the lactate is transferred via the bloodstream to the liver where it is used to re-synthesise new glucose. This cycle is sometimes called the Cori Cycle (discussed further in Chapter 7). It does not result in irreversible loss of glucose from the body. Irreversible loss of glucose occurs after the action of PDH, as acetyl-CoA can no longer be reconverted to glucose (PDH is irreversible), and therefore acetyl-CoA is not a substrate for glucose synthesis.
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
