Human Metabolism. Keith N. Frayn

in a regulated fashion and directed to specific tissues according to their metabolic requirement. These pathways are illustrated schematically in Figure 1.13.

Figure 1.13 Overall metabolic energy flux. The three energy groups (fats, carbohydrates, proteins) are stored in macromolecular form and can be broken down into small, monomolecular units prior to conversion to the common ‘fuel’ acetyl-CoA to be oxidised in the TCA (tricarboxylic acid) cycle – catabolism. At times of energy excess, the smaller units are assembled into the larger storage molecules – anabolism. Crucially, the conversion of pyruvate into acetyl-CoA (by pyruvate dehydrogenase) is irreversible, hence carbohydrates can be converted into fats, but fats cannot be converted into carbohydrates. 1, esterification; 2, lipolysis; 3, glycogenesis; 4, glycogenolysis; 5, protein synthesis/proteolysis; 6, lipogenesis; 7, β-oxidation; 8, gluconeogenesis; 9, glycolysis; 10, pentose phosphate pathway. Coloured arrows indicate direction of anabolic and catabolic flux, though glycolysis and gluconeogenesis do not always fit this paradigm, depending on nutritional state and particular tissue.

Overall metabolic strategy for energy provision depends on substrate fluxes within and between the three major substrate groups (Figure 1.13). Catabolism is represented as downward flux, anabolism as upward flux (though the situation is a little more complex than this, as we shall see – gluconeogenesis is active in catabolism, and glycolysis in anabolism. This is why the terms anabolism and catabolism are best reserved for the whole-body situation). Each group has a ‘storage’ macromolecule/polymer which can be broken down to individual, relatively small monomeric units in the first stage of metabolism; in the second stage of metabolism, these monomeric units are all converted into a common fuel molecule, acetyl-CoA; in the third stage of metabolism, the acetyl-CoA is completely oxidised to CO₂ + H₂O by the TCA cycle and electron transport chain. In intermediary metabolism it is convenient to think in terms of numbers of carbons rather than molecular weight, since carbon (C–H) represents the energy source of the substrate. Glucose (6 carbons) is stored as glycogen (hundreds of thousands of carbons). NEFAs, or ‘free’ fatty acids (typically 16 or 18 carbons) are esterified with glycerol and stored as triacylglycerols (about 60 carbons). Amino acids (typically 3–6 carbons) are not stored as an energy reserve as such but are available in reserve as proteins (again, depending on protein size, representing thousands of carbons). Acetyl-CoA comprises an acetyl group (2 carbons: CH₃·CO–) attached to a carrier molecule (Coenzyme A, CoA). Oxidation of acetyl-CoA by the TCA cycle produces two CO₂ molecules (i.e. the acetyl group is completely oxidised). This is highly efficient but means that acetyl-CoA cannot contribute to the dynamic pool of TCA cycle intermediates i.e. it cannot replete the intermediates of the TCA cycle as it is completely oxidised with each turn of the cycle – these must be derived from carbohydrate (>3 carbon) units. Carbohydrate metabolism yields pyruvate (3 carbons), which is next decarboxylated to acetyl-CoA (and CO₂) by pyruvate dehydrogenase (PDH). PDH is essentially irreversible (far from equilibrium): acetyl-CoA cannot be converted back to pyruvate. For this reason, carbohydrates cannot be synthesised from the common fuel acetyl-CoA (Figure 1.13). Lipid metabolism comprises lipolysis of triacylglycerols to three NEFAs (+ the glycerol backbone), followed by splitting the fatty acid chain into 2-carbon units: acetyl-CoA (β-oxidation). The acetyl-CoA can be readily oxidised by the TCA cycle to provide energy but it cannot be converted to pyruvate, nor, therefore, to synthesise carbohydrates. The fatty acid chain represents an assembly of 2-carbon (acetyl-CoA-equivalent) units, and the pathway of lipogenesis utilises excess acetyl-CoA (derived from glycolysis and PDH) to synthesise fatty acids and hence triacylglycerol. Indeed triacylglycerol-fatty acid simply represents a storage form of excess acetyl-CoA (hence most fatty acids have even numbers of carbons). This means that whilst excess carbohydrate (glucose) can be readily converted to lipid (through acetyl-CoA), the reverse is not true. This is important because most stored energy is in the form of energy-dense lipid (triacylglycerol), with very little stored as glycogen, (too inefficient; <1 day supply); however, certain tissues (brain, erythrocytes, renal medulla) have an absolute requirement for glucose, and in the face of limited glycogen storage in starvation this is met by protein catabolism.

      Passage of carbohydrate carbon through PDH represents an irreversible ‘gate’ through which the carbon cannot gain re-entry, committing carbohydrate to energy provision, either by immediate oxidation of acetyl-CoA, or by storage of the acetyl-CoA as lipid (fatty acid, triacylglycerol) for reconversion back to acetyl-CoA and oxidation at a later date (e.g. in subsequent starvation); this is the reason why PDH is such a highly regulated enzyme – it represents the major control point between carbohydrate and lipid metabolism (Figure 1.13).

      1.3.1.4 Tricarboxylic acid (TCA) cycle

Oxidation of the 2-carbon acetyl (CH₃·CO–) group of acetyl-CoA is achieved by the TCA cycle within the mitochondrial matrix, with the energy from the oxidation of one acetyl group released in the form of electrons (associated with a hydrogen atom as a hydride (H⁻ ion; see Section 1.2.1.3)) and carried by NAD⁺ (×3 per acetyl group, i.e. per turn of the cycle) and FAD (×1) in their reduced forms (NADH; FADH₂). In addition, one step involves a substrate-level phosphorylation – a chemical step which is directly linked to the phosphorylation of ADP (or GDP) to ATP (or GTP) without the need for mitochondrial oxidative phosphorylation – converting GDP into GTP (or ADP into ATP in some cells).

      In the TCA cycle (Box 1.5), the 2-carbon acetyl group of acetyl-CoA combines with oxaloacetate (4 carbons) to form the 6-carbon compound citrate (a TCA, hence the name of the cycle; it is also referred to as the citrate cycle or Krebs cycle). The citrate undergoes two decarboxylation reactions, yielding both the two carbon dioxides and 2 NADH (‘oxidative decarboxylation’ reactions), to form succinyl-CoA (4 carbons). The remainder of the cycle concerns regenerating oxaloacetate from the succinyl-CoA: this process involves the (substrate-level) phosphorylation of GDP to GTP and two further oxidations, yielding the FADH₂ and the third NADH, together with the oxaloacetate.

Box 1.5 Tricarboxylic acid cycle: overall scheme

      Acetyl-CoA is oxidised by losing electrons (H⁻ ions) and ends up as CO₂. The electrons are captured by NAD⁺ and FAD to become their reduced forms, NADH and FADH₂ (and they will in turn pass these electrons on to other electron carriers in the electron transport chain, becoming re-oxidised themselves and ready for further electron carriage). This is achieved in a step-wise fashion by the TCA or Krebs cycle. In addition,