H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 22
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Indeed, most of the biochemical reactions described in this book involve the making or breaking of atleast one covalent chemical bond. We recognize this energywhen chemicals undergo energy-releasing reactions. For example, the high potential energy in the covalent bonds of glucose can be released by controlled enzymatic combustion incells (see later discussion). This energy is harnessed by thecell to do many kinds of work.A second biologically important form of potential energyis the energy in a concentration gradient. When the concentration of a substance on one side of a barrier, such as amembrane, is different from that on the other side, a concentration gradient exists. All cells form concentration gradients between their interior and the external fluids byselectively exchanging nutrients, waste products, and ionswith their surroundings.
Also, organelles within cells (e.g.,mitochondria, lysosomes) frequently contain different concentrations of ions and other molecules; the concentration ofprotons within a lysosome, as we saw in the last section, isabout 500 times that of the cytoplasm.A third form of potential energy in cells is an electricpotential—the energy of charge separation. For instance,there is a gradient of electric charge of ≈200,000 volts per cmacross the plasma membrane of virtually all cells. We discusshow concentration gradients and the potential differenceacross cell membranes are generated and maintained inChapter 7.Cells Can Transform One Typeof Energy into AnotherAccording to the first law of thermodynamics, energy is neither created nor destroyed, but can be converted from oneform to another.
(In nuclear reactions mass is converted toenergy, but this is irrelevant to biological systems.) In photosynthesis, for example, the radiant energy of light is transformed into the chemical potential energy of the covalentbonds between the atoms in a sucrose or starch molecule. Inmuscles and nerves, chemical potential energy stored in covalent bonds is transformed, respectively, into the kineticenergy of muscle contraction and the electric energy of nervetransmission.
In all cells, potential energy, released by breaking certain chemical bonds, is used to generate potential energy in the form of concentration and electric potentialgradients. Similarly, energy stored in chemical concentrationgradients or electric potential gradients is used to synthesize2.4 • Biochemical Energeticschemical bonds or to transport molecules from one side of amembrane to another to generate a concentration gradient.This latter process occurs during the transport of nutrientssuch as glucose into certain cells and transport of manywaste products out of cells.Because all forms of energy are interconvertible, they canbe expressed in the same units of measurement. Although thestandard unit of energy is the joule, biochemists have traditionally used an alternative unit, the calorie (1 joule 0.239calories).
Throughout this book, we use the kilocalorie tomeasure energy changes (1 kcal = 1000 cal).The Change in Free Energy Determinesthe Direction of a Chemical ReactionBecause biological systems are generally held at constanttemperature and pressure, it is possible to predict the direction of a chemical reaction from the change in the free energyG, named after J. W.
Gibbs, who showed that “all systemschange in such a way that free energy [G] is minimized.” Inthe case of a chemical reaction, reactantsproducts, thechange in free energy G is given byG Gproducts GreactantsThe relation of G to the direction of any chemical reactioncan be summarized in three statements:If G is negative, the forward reaction (from left toright as written) will tend to occur spontaneously.■51more bond energy than the reactants, heat is absorbed, andH is positive.
The combined effects of the changes in the enthalpy and entropy determine if the G for a reaction is positive or negative. An exothermic reaction (H 0) in whichentropy increases (S 0) occurs spontaneously (G 0).An endothermic reaction (H 0) will occur spontaneouslyif S increases enough so that the T S term can overcomethe positive H.Many biological reactions lead to an increase in order,and thus a decrease in entropy (S 0). An obvious example is the reaction that links amino acids together to form aprotein.
A solution of protein molecules has a lower entropythan does a solution of the same amino acids unlinked, because the free movement of any amino acid in a protein isrestricted when it is bound into a long chain. Often cellscompensate for decreases in entropy by “coupling” such synthetic reactions with independent reactions that have a veryhighly negative G (see below). In this fashion cells can convert sources of energy in their environment into the buildingof highly organized structures and metabolic pathways thatare essential for life.The actual change in free energy G during a reactionis influenced by temperature, pressure, and the initial concentrations of reactants and products and usually differsfrom Gº.
Most biological reactions—like others that takeplace in aqueous solutions—also are affected by the pH ofthe solution. We can estimate free-energy changes for different temperatures and initial concentrations, using theequation[products][reactants]■If G is positive, the reverse reaction (from right to leftas written) will tend to occur.G Gº RT ln Q Gº' RT lnIf G is zero, both forward and reverse reactions occurat equal rates; the reaction is at equilibrium.where R is the gas constant of 1.987 cal/(degree·mol), T isthe temperature (in degrees Kelvin), and Q is the initial ratioof products to reactants.
For a reaction A BC, inwhich two molecules combine to form a third, Q in Equation2-7 equals [C]/[A][B]. In this case, an increase in the initialconcentration of either [A] or [B] will result in a large negative value for G and thus drive the reaction toward moreformation of C.Regardless of the Gº for a particular biochemicalreaction, it will proceed spontaneously within cells only ifG is negative, given the usual intracellular concentrationsof reactants and products. For example, the conversion ofglyceraldehyde 3-phosphate (G3P) to dihydroxyacetonephosphate (DHAP), two intermediates in the breakdown ofglucose,■The standard free-energy change of a reaction Gº is thevalue of the change in free energy under the conditions of298 K (25 ºC), 1 atm pressure, pH 7.0 (as in pure water), andinitial concentrations of 1 M for all reactants and productsexcept protons, which are kept at 107 M (pH 7.0).
Most biological reactions differ from standard conditions, particularly in the concentrations of reactants, which are normallyless than 1 M.The free energy of a chemical system can be defined asG H TS, where H is the bond energy, or enthalpy, ofthe system; T is its temperature in degrees Kelvin (K); and Sis the entropy, a measure of its randomness or disorder. Iftemperature remains constant, a reaction proceeds spontaneously only if the free-energy change G in the followingequation is negative:G H T S(2-6)In an exothermic reaction, the products contain less bond energy than the reactants, the liberated energy is usually converted to heat (the energy of molecular motion), and H isnegative.
In an endothermic reaction, the products containG3P(2-7)DHAPhas a Gº of 1840 cal/mol. If the initial concentrations ofG3P and DHAP are equal, then G Gº, because RT ln 1 0; in this situation, the reversible reaction G3PDHAPwill proceed in the direction of DHAP formation until equilibrium is reached. However, if the initial [DHAP] is 0.1 Mand the initial [G3P] is 0.001 M, with other conditions beingstandard, then Q in Equation 2-7 equals 0.1/0.001 100,52CHAPTER 2 • Chemical Foundationsgiving a G of 887 cal/mole.
Under these conditions,the reaction will proceed in the direction of formation ofG3P.The G for a reaction is independent of the reaction rate.Indeed, under usual physiological conditions, few, if any, ofthe biochemical reactions needed to sustain life would occurwithout some mechanism for increasing reaction rates. As wedescribe in Chapter 3, the rates of reactions in biological systems are usually determined by the activity of enzymes, theprotein catalysts that accelerate the formation of productsfrom reactants without altering the value of G.The Gº of a Reaction Can BeCalculated from Its KeqA chemical mixture at equilibrium is already in a state ofminimal free energy; that is, no free energy is being generatedor released. Thus, for a system at equilibrium (G 0,Q Keq), we can writeGº 2.3RT log Keq 1362 log Keq(2-8)under standard conditions (note the change to base 10 logarithms).
Thus, if the concentrations of reactants and products at equilibrium (i.e., the Keq) are determined, the value ofGº can be calculated. For example, Keq for the interconversion of glyceraldehyde 3-phosphate to dihydroxyacetonephosphate (G3PDHAP) is 22.2 under standard conditions. Substituting this value into Equation 2-8, we can easily calculate the Gº for this reaction as 1840 cal/mol.By rearranging Equation 2-8 and taking the antilogarithm, we obtainKeq 10(Gº/2.3RT)(2-9)From this expression, it is clear that if Gº is negative, theexponent will be positive and hence Keq will be greater than1.
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