cytochrome (cytochrome c) and the (non-protein) quinone CoQ10 (ubiquinone). All these components have variable redox states and act as electron acceptors (oxidising agents) and electron donors (reducing agents); they can function as electron carriers by shifting between their reduced (electron containing) and oxidised (electron deficient) forms. To facilitate this, the proteins contain transition metals (Fe, Cu), complexed in prosthetic groups (e.g. haem) or complexed to sulphur (Fe-S centres) which are readily capable of gaining or losing an electron. They are organised in a sequential arrangement within the IMM such that the electrons are passed down a gradual incremental energy gradient. Complex I oxidises NADH back to NAD+, whilst complex II oxidises FADH2 back to FAD, both passing electrons (as pairs) to CoQ10, thence to complex III, then cytochrome c, and eventually to complex IV. Complex IV, the final redox protein in the chain, combines the electrons it receives with molecular oxygen (O2, the final electron acceptor), reducing it to water. Complexes I, III, and IV use the energy of electron transfer to pump protons (H+) out of the mitochondrial matrix across the IMM and into the intermembrane space beyond. This creates an electrochemical gradient between the inside and outside of the mitochondrion. The energy of this H+ gradient is finally utilised to drive phosphorylation of ADP to ATP: the protons can only re-enter the mitochondrial matrix by passing through ATP synthase (Fo/F1 ATPase; also called complex V although it is not a part of the actual electron transport chain), another IMM-spanning protein. Proton passage through this large protein complex provides the energy for ATP synthesis (the chemiosmotic process). Hence, provided the IMM is otherwise impermeable to protons, the oxidation of NADH/FADH2 by electron transport is tightly coupled to the phosphorylation of ADP to ATP. However, if protons leak through the IMM back into the mitochondrial matrix, the gradient is dissipated (‘uncoupled’) and ATP synthesis cannot occur, leading to mitochondrial inefficiency. The final step in metabolism is the export of ATP out of the mitochondrion and into the cytosol, and this is achieved by an adenine nucleotide translocator also spanning the IMM.
In the remainder of this chapter, we will outline the major metabolic pathways of ‘energy metabolism’ that will be considered further in this book, signposting the reader to where these are discussed in more detail.
1.3.2 Carbohydrate metabolism
1.3.2.1 Pathways of glucose metabolism
Carbohydrate metabolism centres around the hexose sugar glucose (Figure 1.7). Glucose is a ubiquitous sugar which may be derived from dietary carbohydrate or synthesised in the body. It is stored in polymeric form as glycogen: this removes the osmotic problems that would arise if it were stored in cells as free sugars (Section 1.2.2.1). Glucose, as a polar molecule, cannot cross membranes composed of phospholipid bilayers by diffusion, and two families of glucose transporter proteins (GLUTs, and sodium-glucose linked transporters, SGLTs: see Chapter 2, Box 2.2), expressed in a tissue-specific manner, facilitate its rapid movement in and out of cells. The pathways of glucose metabolism inside the cell are illustrated in simplified form in Figure 1.14, which shows the key steps of the various pathways responsible for its utilisation, disposal and production. Glucose can be stored as glycogen, used to provide 5-carbon sugars and NADPH for, e.g. nucleotide 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
