The Conversion of Chemical Energy Part 2. Biochemical Examples Donald J. Wink New York University, New York, NY 10003 All chemical reactions that involve a change in the Gibbs free energy of a system also involve a transfer of energy to (if AG < 0) or from (of AG > 0) the surroundings. The other forms of energy can be thermal, mechanical, electrical,nuclear, or electromagnetic (light). In the previous article, technological examples of such coupling were discussed. Here examples are taken from the field of bioenergetics. Biological systems regulate energy transfer reactions through enzymes that permit a spontaneous reaction to go faster through a mechanism that also accomplishes work instead of, or in addition to, releasing energy as heat (1). Work is usually defined in the context of the creation of mechanical energy, such as in the acceleration of a mass. But a more proper and broader definition is that work is the production of energy in any form except thermal energy. Biological work includes the synthesis of new substances or the active transport of substances (involving the production of chemical energy), the accurate processing of information (involving the production of localized order in a system), light emission (light energy production), electrical signaling (electrical energy production) as well as the mechanical energy associated with muscles and similar organs (2). Energy Coupling in Biological Systems Most organisms have some way of storing energy for lengthy periods in a specific chemical form, for example in carbohydrates or lipids. But all organisms must also have energy available in a more general form that can be used or produced in a wide variety of reactions. There are just two dominant mechanisms of energy conversion for rapid coupling to biological work. One is the reaction involving the cleavage of a P-0 bond in ATP (adenosine triphosphate), depicted with water in eq 1.The chemical energy
the case where [M?] = 0.001 M, ionic strength = 0.25, and pH = 7)are large. Work is extracted by enzymes that capture the energy released from the net hydrolysis of ATP in nonthermal energy. It is easy to think that an organism uses ATP as a fuel per se, but this is a misleading statement because there is little if any net consumption of ATP in a healthy organism. A more accurate picture is that other fuels are used to maintain a high ATPADP ratio so that the net hydrolysis of ATP can be used productively over and over again. The cell is maintained in a state of high energy, with a high ATP:ADP ratio coupled to many otherwise unfavorable processes. ATP is not a fuel so much as it is a convenient link between genuine energy sources such as food or light and the processes needed for life. Rapid and versatile conversion of energy is also aceomplished by coupling process to a concentration difference established across a membrane such as the one around a cell or the covering of a mitochondrion or other organelle. These membranes do not permit the free flow of many substances, especially thosethat are charged. Therefore, they can be used to maintain a given substance at different concentrations. in different chemical states (I). con. . Asvstem " taining a cbncentration difference is at a higher energy than a system where concentrations are equal (as shown by spontaneous mixing when solutions are poured together or the spontaneous flow of gas into a vacuum). There are two terms in the expression of the energy difference for membrane gradients. For any substance present on two sides of a membrane at concentrations C1(inside) and Cz (outside) the free energy differencebetween the two states is
This represents the energy released (or consumed)when a quantity of the substanci is transported from the side with the higher to the lower concentration (or lower to higher). There is another energetic term for charged substances. It arises because a concentration gradient can also be aceompanied by a charge gradient. This is analogous to an electronic capacitor. The total electrical potential is a result of the net sum over all possible species; it is denoted by the symbol AY and it is usually on the order of 150 mV, though this can vary dramatically in certain environments or tissues. Transport of protons through such a potential corresponds, according to Faraday's Law, to AG = FAY = 98,500x 0.15 = 15 kJ mol-'
of this bond would be converted to simple thermal energy in a direct reaction with water, for the driving force and equilibrium constant under these conditions(presented for
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The sum of the two potentials, chemical and electrical, gives rise to a condition where a process that allows protons to flow may yield, or cost, energy according to a combined expression (eq 2).
the evolution of all of the 62 kJ mol-'of chemical energy as heat. However, an enzyme called pyruvate kinase catalyzes the reaction to direct it specifically through a mechanism to couple it to ATP synthesis (63. How much energy does this membrane concentration
-eradient reoresent? That is verv de~endenton conditions. A simple example is that a concentration difference of a "
co;
&
factor of 10 (1pH unit for proton) and an electrical potential of 150 mV corresponds to -20 kJ mol-'. The chemical and electrical energy stored in a proton gradient is often referred to as a "proton-motive force" to indicate its ability to be coupled to other processes so that, in the process of moving protons, other events are made to proceed. Since both the ATPIADP couple and membrane poteutials are used in storing chemical energy in biology it is not surprising that these two energy systems are themselves coupled. Release of a proton gradient can be used to synthesize ATP and the hydrolysis of ATP can accomplish "proton pumping." This is done in either direction by enzymes known as a class as H+-ATPases(3).The thermodynamics are quite simple, as shown in eq 3 for the case where a pH gradient of 1unit exists along with a membrane electrical potential of 150 mV.
H++ ADP
+
AG = -31 kJImol (6)
Pyruvate kinase acts as a site for ADP and PEP to bind in a manner that makes the transfer of a phosphate group from PEP to ATP very easy to accomplish. The consumption of PEP is directly linked to ATP synthesis because the energy for attaching the third phosphate to ATP comes from the energy released fmm the loss of the phosphate from PEP. The path from glucose to lactic acid in glywlysis yields 64 kJ mol-' of work in the form of two equivalents of ATP per glucose. This represents roughly a 30% efficiency from the 198 kJ mol-' available in principle. But there is a lot of energy "left over" in the lactate that can be recovered in processes that require oxygen. This corresponds to the controlled combustion of the two equivalents of lactate to carbon dioxide and water (eq 7). 2 C3%0
