Human Metabolism. Keith N. Frayn
synthesis, and lipogenesis, respectively, and split into pyruvate for further metabolism. It can be synthesised by gluconeogenesis, or by breaking down glycogen.
Figure 1.14 Pathways of glucose metabolism inside the cell. The pathways of glucose (carbohydrate) metabolism are shown with the key (regulatory and energy-yielding) steps marked. Glycolysis (splitting of glucose) is the major top-to-bottom pathway, and it results in two pyruvate molecules: i.e. fructose 1,6-bisphosphate is split, and the products are doubled. G 6-P, glucose 6-phosphate; F 6-P, fructose 6-phosphate; F 1,6-BP, fructose 1,6-bisphosphate; PEP, phosphoenolpyruvate; OAA, oxaloacetate.
1.3.2.1.1 Glucose phosphorylation
Following uptake into the cell by glucose transporters, the first step of glucose metabolism within cells is always phosphorylation to glucose 6-phosphate (G6-P), brought about by a member of a family of enzymes (hexokinases) that use ATP, again expressed in a tissue-specific manner. The form expressed in liver and pancreatic β-cells, hexokinase Type IV, is generally known as glucokinase; that expressed in skeletal muscle, Type II, is generally known simply as hexokinase. Phosphorylation ensures that the molecule does not diffuse again out of the cell, locking it into the cell, maintaining its inward concentration (and augmenting further glucose influx), and activating the molecule for further metabolism. G6-P is used by glycolysis (glucose breakdown, next section) and glycogen synthesis as well as the pentose phosphate pathway (Section 1.3.2.1.7 below); it may also be derived from glycogen breakdown (glycogenolysis) and gluconeogenesis (glucose synthesis: Section 1.3.2.1.5 below) depending on tissue and prevailing metabolic state. Thus, G6-P may be seen as lying at a major crossroads in carbohydrate metabolism (Figure 1.14).
1.3.2.1.2 Glycolysis
Glucose (6 carbons: molecular formula C6H12O6) is broken down by the pathway of glycolysis to pyruvate (3 carbons: C3H4O3, showing that H has been lost relative to C and O; i.e. overall this is a partial oxidation); the term glycolysis refers to this splitting of the glucose molecule. A small amount of ATP is generated by substrate-level phosphorylation in glycolysis, hence some usable energy is generated. Indeed, this is a cytosolic pathway and occurs even in cells that lack mitochondria, such as red blood cells (it is their only route for making ATP). Glycolysis ends at pyruvate. This important intermediate can be subsequently: 1, reduced to lactate (C3H6O3, so exactly half a glucose molecule with no redox changes – a true fermentation reaction); 2, oxidised (and decarboxylated) to form acetyl-CoA; 3, carboxylated to form oxaloacetate (anaplerosis); or 4, transaminated to form the amino acid alanine (see Section 1.3.4 below).
Glycolysis is a primitive but vital pathway that occurs in the cytosol of all cells and comprises an initial energy investment phase (priming – phosphorylation) followed by splitting (6 carbons into 2 × 3 carbons), and an energy generation phase of oxido-reduction and phosphorylation. During this phase NAD+ is reduced to NADH and ATP is produced by substrate-level phosphorylation. Glycolysis of one molecule of glucose therefore yields two molecules of pyruvate, 2 ATP and 2 NADH without requiring oxygen. Glycolysis is a vital pathway because of its multiple functions. In muscle, glycolysis splits glucose in order to provide energy, but in liver, excess glucose remaining once glycogen stores have been repleted is broken down by glycolysis to pyruvate, then acetyl-CoA, for lipid synthesis.
1.3.2.1.3 Lactate and ethanol metabolism
The reduction of glycolysis-derived pyruvate to lactate by the near-equilibrium (freely reversible) enzyme lactate dehydrogenase is an important mechanism to permit glycolysis to proceed. Glycolysis involves one redox step that is linked to NADH formation from NAD+. Normally, when oxygen is present in the cell, the NADH is reoxidised back to NAD+ by the electron transport chain within the mitochondria: this is vital because the NAD+ is required for further electron capture to permit glycolysis to proceed. In the absence 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