Bioenergetics describes the transfer and utilization of energy in biologic systems. It makes use of a few basic ideas from the field of thermodynamics, particularly the concept of free energy. Changes in free energy (?G) provide a measure of the energetic feasibility of a chemical reaction and can, therefore, allow prediction of whether a reaction or process can take place. Bioenergetics concerns only the initial and final energy states of reaction components, not the mechanism or how much time is needed for the chemical change to take place. In short, bioenergetics predicts if a process is possible, whereas kinetics measures how fast the reaction occurs (see Catalytic efficiency).
The direction and extent to which a chemical reaction proceeds is determined by the degree to which two factors change during the reaction. These are enthalpy (?H, a measure of the change in heat content of the reactants and products) and entropy (?S, a measure of the change in randomness or disorder of reactants and products, Figure 6.1). Neither of these thermodynamic quantities by itself is sufficient to determine whether a chemical reaction will proceed spontaneously in the direction it is written. However, when combined mathematically (see Figure 6.1), enthalpy and entropy can be used to define a third quantity, free energy (G), which predicts the direction in which a reaction will spontaneously proceed.
Figure 6.1. Relationship between changes in free energy (G), enthalpy (H), and entropy (S).
T is the absolute temperature in degrees Kelvin (°K): °K = °C + 273.
Free Energy Change
The change in free energy is represented in two ways, ?G and ?G°. The first, ?G (without the superscript “o”), represents the change in free energy and, thus, the direction of a reaction at any specified concentration of products and reactants. ?G, then, is a variable. This contrasts with the standard free energy change, ?G° (with the superscript “o”), which is the energy change when reactants and products are at a concentration of 1 mol/L. [Note: The concentration of protons is assumed to be 10?7 mol/L, that is, pH = 7.]
Although ?G° represents energy changes at these nonphysiologic concentrations of reactants and products, it is nonetheless useful in comparing the energy changes of different reactions. Furthermore, ?G° can readily be determined from measurement of the equilibrium constant (see ?Go of two consecutive reactions are additive). This section outlines the uses of ?G; ?G° is described on Standard free energy change, ?Go.
A. Sign of ?G predicts the direction of a reaction
The change in free energy, ?G, can be used to predict the direction of a reaction at constant temperature and pressure. Consider the reaction:
1. Negative ?G
If ?G is a negative number, there is a net loss of energy, and the reaction goes spontaneously as written—that is, A is converted into B (Figure 6.2A). The reaction is said to be exergonic.
Figure 6.2. Change in free energy (?G) during a reaction
A.The product has a lower free energy (G) than the reactant. B.The product has a higher free energy than the reactant.
2. Positive ?G
If ?G is a positive number, there is a net gain of energy, and the reaction does not go spontaneously from B to A (see Figure 6.2B). Energy must be added to the system to make the reaction go from B to A, and the reaction is said to be endergonic.
3. ?G is zero
If ?G = 0, the reactants are in equilibrium. [Note: When a reaction is proceeding spontaneously—that is, free energy is being lost—then the reaction continues until ?G reaches zero and equilibrium is established.]
B. ?G of the forward and back reactions
The free energy of the forward reaction (A ? B) is equal in magnitude but opposite in sign to that of the back reaction (B ? A). For example, if ?G of the forward reaction is ?5 kcal/mol, then that of the back reaction is +5 kcal/mol. [Note: ?G can also be expressed in kilojoules per mole or kJ/mol (1 kcal = 4.2 kJ).]
C. ?G depends on the concentration of reactants and products
The ?G of the reaction A ? B depends on the concentration of the reactant and product. At constant temperature and pressure, the following relationship can be derived:
?G° is the standard free energy change (see below)
R is the gas constant (1.987 cal/mol · degree)
T is the absolute temperature (°K)
[A] and [B] are the actual concentrations of the reactant and product
In represents the natural logarithm
A reaction with a positive ?G° can proceed in the forward direction (have a negative overall ?G) if the ratio of products to reactants ([B]/[A]) is sufficiently small (that is, the ratio of reactants to products is large). For example, consider the reaction:
Figure 6.3A shows reaction conditions in which the concentration of reactant, glucose 6-phosphate, is high compared with the concentration of product, fructose 6-phosphate. This means that the ratio of the product to reactant is small, and RT ln([fructose 6-phosphate]/[glucose 6-phosphate]) is large and negative, causing ?G to be negative despite ?G° being positive. Thus, the reaction can proceed in the forward direction.
Figure 6.3. ?G of a reaction depends on the concentration of reactant (A) and product (B).
For the conversion of glucose 6-P to fructose 6-P, ?G is negative when the ratio of reactant (A) to product (B) is large (top, panel A); is positive under standard conditions (middle, panel B); and is zero at equilibrium (bottom, panel C).
D. Standard free energy change, ?G°
The standard free energy change, ?G°, is so called because it is equal to the free energy change, ?G, under standard conditions—that is, when reactants and products are at 1 mol/L concentrations (see Figure 6.3B). Under these conditions, the natural logarithm of the ratio of products to reactants is zero (ln1 = 0) and, therefore, the equation shown at the top of this page becomes:
1. ?G° is predictive only under standard conditions
Under standard conditions, ?G° can be used to predict the direction a reaction proceeds because, under these conditions, ?G° is equal to ?G. However, ?G° cannot predict the direction of a reaction under physiologic conditions, because it is composed solely of constants (R, T, and Keq) and is, therefore, not altered by changes in product or substrate concentrations.
2. Relationship between ?G° and Keq
In a reaction A ? B, a point of equilibrium is reached at which no further net chemical change takes place—that is, when A is being converted to B as fast as B is being converted to A. In this state, the ratio of [B] to [A] is constant, regardless of the actual concentrations of the two compounds:
where Keq is the equilibrium constant, and [A]eq and [B]eq are the concentrations of A and B at equilibrium. If the reaction A ? B is allowed to go to equilibrium at constant temperature and pressure, then at equilibrium the overall free energy change (?G) is zero. Therefore,
where the actual concentrations of A and B are equal to the equilibrium concentrations of reactant and product [A]eq and [B]eq, and their ratio as shown above is equal to the Keq. Thus,
This equation allows some simple predictions:
3. ?G° of two consecutive reactions are additive
The standard free energy changes (?G°) are additive in any sequence of consecutive reactions, as are the free energy changes (?G). For example:
4. ?Gs of a pathway are additive
This additive property of free energy changes is very important in biochemical pathways through which substrates must pass in a particular direction (for example, A ? B ? C ? D ? …). As long as the sum of the ?Gs of the individual reactions is negative, the pathway can potentially proceed as written, even if some of the individual reactions of the pathway have a positive ?G. The actual rate of the reactions does, of course, depend on the lowering of activation energies by the enzymes that catalyze the reactions (see Energy changes occurring during the reaction).
ATP as an Energy Carrier
Reactions or processes that have a large positive ?G, such as moving ions against a concentration gradient across a cell membrane, are made possible by coupling the endergonic movement of ions with a second, spontaneous process with a large negative ?G, such as the exergonic hydrolysis of adenosine triphosphate (ATP). [Note: In the absence of enzymes, ATP is a stable molecule because its hydrolysis has a high activation energy.] Figure 6.4 shows a mechanical model of energy coupling. A gear with an attached weight spontaneously turns in the direction that achieves the lowest energy state, in this case the weight seeks its lowest position (see Figure 6.4A).
The reverse motion (see Figure 6.4B) is energetically unfavored and does not occur spontaneously. Figure 6.4C shows that the energetically favored movement of one gear can be used to turn a second gear in a direction that it would not move spontaneously. The simplest example of energy coupling in biologic reactions occurs when the energy-requiring and the energy-yielding reactions share a common intermediate.
Figure 6.4. Mechanical model of coupling of favorable and unfavorable processes.
A. Reactions are coupled through common intermediates
Two chemical reactions have a common intermediate when they occur sequentially so that the product of the first reaction is a substrate for the second. For example, given the reactions
D is the common intermediate and can serve as a carrier of chemical energy between the two reactions. Many coupled reactions use ATP to generate a common intermediate. These reactions may involve the transfer of a phosphate group from ATP to another molecule. Other reactions involve the transfer of phosphate from an energy-rich intermediate to adenosine diphosphate (ADP), forming ATP
B. Energy carried by ATP
ATP consists of a molecule of adenosine (adenine + ribose) to which three phosphate groups are attached (Figure 6.5). If one phosphate is removed, ADP is produced; if two phosphates are removed, adenosine monophosphate (AMP) results. The standard free energy of hydrolysis of ATP, ?G°, is approximately ?7.3 kcal/mol for each of the two terminal phosphate groups. Because of this large negative ?G°, ATP is called a high-energy phosphate compound.
Figure 6.5. Adenosine triphosphate.
Electron Transport Chain
Energy-rich molecules, such as glucose, are metabolized by a series of oxidation reactions ultimately yielding CO2 and water (Figure 6.6). The metabolic intermediates of these reactions donate electrons to specific coenzymes—nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD)—to form the energy-rich reduced coenzymes, NADH and FADH2. These reduced coenzymes can, in turn, each donate a pair of electrons to a specialized set of electron carriers, collectively called the electron transport chain, described in this section.
As electrons are passed down the electron transport chain, they lose much of their free energy. Part of this energy can be captured and stored by the production of ATP from ADP and inorganic phosphate (Pi). This process is called oxidative phosphorylation and is described on Oxidative Phosphorylation. The remainder of the free energy not trapped as ATP is used to drive ancillary reactions such as Ca2+ transport into mitochondria, and to generate heat.
Figure 6.6.The metabolic breakdown of energy-yielding molecules.
The electron transport chain is present in the inner mitochondrial membrane and is the final common pathway by which electrons derived from different fuels of the body flow to oxygen. Electron transport and ATP synthesis by oxidative phosphorylation proceed continuously in all tissues that contain mitochondria.
1. Membranes of the mitochondrion
The components of the electron transport chain are located in the inner membrane. Although the outer membrane contains special pores, making it freely permeable to most ions and small molecules, the inner mitochondrial membrane is a specialized structure that is impermeable to most small ions, including H+, Na+, and K+, and small molecules such as ATP, ADP, pyruvate, and other metabolites important to mitochondrial function (Figure 6.7).
Specialized carriers or transport systems are required to move ions or molecules across this membrane. The inner mitochondrial membrane is unusually rich in protein, half of which is directly involved in electron transport and oxidative phosphorylation. The inner mitochondrial membrane is highly convoluted. The convolutions, called cristae, serve to greatly increase the surface area of the membrane.
Figure 6.7.Structure of a mitochondrion showing schematic representation of the electron transport chain and ATP synthesizing structures on the inner membrane.
mtDNA = mitochondrial DNA; mtRNA = mitochondrial RNA. [Note: In contrast to the inner membrane, the outer membrane is highly permeable and the milieu of the intermembrane space is like that of the cytosol.]
2. Matrix of the mitochondrion
This gel-like solution in the interior of mitochondria is 50% protein. These molecules include the enzymes responsible for the oxidation of pyruvate, amino acids, fatty acids (by ?-oxidation), and those of the tricarboxylic acid (TCA) cycle. The synthesis of glucose, urea, and heme occur partially in the matrix of mitochondria. In addition, the matrix contains NAD+ and FAD (the oxidized forms of the two coenzymes that are required as hydrogen acceptors) and ADP and Pi, which are used to produce ATP. [Note: The matrix also contains mitochondrial RNA and DNA (mtRNA and mtDNA) and mitochondrial ribosomes.]
B. Organization of the electron transport chain
The inner mitochondrial membrane can be disrupted into five separate protein complexes, called Complexes I, II, III, IV, and V. Complexes I–IV each contain part of the electron transport chain (Figure 6.8). Each complex accepts or donates electrons to relatively mobile electron carriers, such as coenzyme Q and cytochrome c. Each carrier in the electron transport chain can receive electrons from an electron donor, and can subsequently donate electrons to the next carrier in the chain.
The electrons ultimately combine with oxygen and protons to form water. This requirement for oxygen makes the electron transport process the respiratory chain, which accounts for the greatest portion of the body’s use of oxygen. Complex V is a protein complex that contains a domain (Fo) that spans the inner mitochondrial membrane, and a domain (F1) that appears as a sphere that protrudes into the mitochondrial matrix (see ATP synthase). Complex V catalyzes ATP synthesis and so is referred to as ATP synthase.
Figure 6.8.Electron transport chain.
[Note: NADH, produced from a variety of oxidative (catabolic) processes, is the substrate for Complex I. Succinate, an intermediate of the TCA cycle, is the substrate for Complex II.]