Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 23
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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.
Therefore at equilibrium there will be more products thanreactants; in other words, the formation of products from reactants is favored. Conversely, if Gº is positive, the exponent will be negative and Keq will be less than 1.(1) ABXG 5 kcal/mol(2) XY ZG 10 kcal/molBYZGº' 5 kcal/molSum: AIn the absence of the second reaction, there would be muchmore A than B at equilibrium.
However, because the conversion of X to Y Z is such a favorable reaction, it will pullthe first process toward the formation of B and the consumption of A. Energetically unfavorable reactions in cellsoften are coupled to the hydrolysis of ATP, as we discussnext.Hydrolysis of ATP Releases Substantial FreeEnergy and Drives Many Cellular ProcessesIn almost all organisms, adenosine triphosphate, or ATP, isthe most important molecule for capturing, transiently storing, and subsequently transferring energy to perform work(e.g., biosynthesis, mechanical motion).
The useful energyin an ATP molecule is contained in phosphoanhydride bonds,which are covalent bonds formed from the condensation oftwo molecules of phosphate by the loss of water:OOOOHPHOPOOOOOOPOO H2OPOOAn ATP molecule has two key phosphoanhydride bonds(Figure 2-24). Hydrolysis of a phosphoanhydride bond (~) ineach of the following reactions has a highly negative Gºof about 7.3 kcal/mol:NH2CAn Unfavorable Chemical ReactionCan Proceed If It Is Coupled withan Energetically Favorable ReactionMany processes in cells are energetically unfavorable (G 0)and will not proceed spontaneously.
Examples include thesynthesis of DNA from nucleotides and transport of a substance across the plasma membrane from a lower to a higherconcentration. Cells can carry out an energy-requiring reaction (G1 0) by coupling it to an energy-releasing reaction (G2 0) if the sum of the two reactions has a netnegative G.Suppose, for example, that the reaction AB X hasa G of 5 kcal/mol and that the reaction XY Z hasa G of 10 kcal/mol.Phosphoanhydride bondsOOPOOOPONCHCCCHNOONPOCH2HONOHHHHOOHAdenosine triphosphate(ATP)▲ FIGURE 2-24 Adenosine triphosphate (ATP). The twophosphoanhydride bonds (red) in ATP, which link the threephosphate groups, each has a G˚ of 7.3 kcal/mol for hydrolysis.
Hydrolysis of these bonds, especially the terminal one, drivesmany energy-requiring reactions in biological systems.2.4 • Biochemical EnergeticsAp~p~p H2O On Ap~p Pi H(ATP)(ADP)Ap~p~p H2O On Ap PPi H(ATP)(AMP)Ap~p H2O On Ap Pi H(ADP)(AMP)In these reactions, Pi stands for inorganic phosphate (PO43)and PPi for inorganic pyrophosphate, two phosphate groupslinked by a phosphodiester bond. As the top two reactionsshow, the removal of a phosphate or a pyrophosphate groupfrom ATP leaves adenosine diphosphate (ADP) or adenosinemonophosphate (AMP), respectively.A phosphoanhydride bond or other high-energy bond(commonly denoted by ~) is not intrinsically different fromother covalent bonds.
High-energy bonds simply release especially large amounts of energy when broken by addition ofwater (hydrolyzed). For instance, the Gº for hydrolysis ofa phosphoanhydride bond in ATP (7.3 kcal/mol) is morethan three times the Gº for hydrolysis of the phosphoesterbond (red) in glycerol 3-phosphate (2.2 kcal/mol):OHOOHPOOCH2CHCH2OHGlycerol 3-phosphateA principal reason for this difference is that ATP and its hydrolysis products ADP and Pi are highly charged at neutralpH.
During synthesis of ATP, a large input of energy is required to force the negative charges in ADP and Pi together.Conversely, much energy is released when ATP is hydrolyzedto ADP and Pi. In comparison, formation of the phosphoester bond between an uncharged hydroxyl in glycerol and Pirequires less energy, and less energy is released when thisbond is hydrolyzed.Cells have evolved protein-mediated mechanisms fortransferring the free energy released by hydrolysis of phosphoanhydride bonds to other molecules, thereby driving reactions that would otherwise be energetically unfavorable.For example, if the G for the reaction B C On D is positive but less than the G for hydrolysis of ATP, the reactioncan be driven to the right by coupling it to hydrolysis of theterminal phosphoanhydride bond in ATP.
In one commonmechanism of such energy coupling, some of the energystored in this phosphoanhydride bond is transferred to theone of the reactants by breaking the bond in ATP and forming a covalent bond between the released phosphate groupand one of the reactants. The phosphorylated intermediategenerated in this fashion can then react with C to form D Pi in a reaction that has a negative G :B Ap~p~p On B~p Ap~pB~p C On D Pi53The overall reactionB C ATPD ADP Piis energetically favorable (G 0).An alternative mechanism of energy coupling is to use theenergy released by ATP hydrolysis to change the conformation of the molecule to an “energy-rich” stressed state. Inturn, the energy stored as conformational stress can be released as the molecule “relaxes” back into its unstressed conformation. If this relaxation process can be mechanisticallycoupled to another reaction, the released energy can be harnessed to drive important cellular processes.As with many biosynthetic reactions, transport of molecules into or out of the cell often has a positive G and thusrequires an input of energy to proceed.
Such simple transportreactions do not directly involve the making or breaking ofcovalent bonds; thus the Gº is 0. In the case of a substancemoving into a cell, Equation 2-7 becomes[Cin]G RT ln(2-10)[Cout]where [Cin] is the initial concentration of the substance inside the cell and [Cout] is its concentration outside the cell.
Wecan see from Equation 2-10 that G is positive for transportof a substance into a cell against its concentration gradient(when [Cin] [Cout]); the energy to drive such “uphill” transport often is supplied by the hydrolysis of ATP. Conversely,when a substance moves down its concentration gradient([Cout] [Cin]), G is negative. Such “downhill” transportreleases energy that can be coupled to an energy-requiring reaction, say, the movement of another substance uphill acrossa membrane or the synthesis of ATP itself (see Chapter 7).ATP Is Generated During Photosynthesisand RespirationClearly, to continue functioning cells must constantly replenish their ATP supply.
The initial energy source whose energy is ultimately transformed into the phosphoanhydridebonds of ATP and bonds in other compounds in nearly allcells is sunlight. In photosynthesis, plants and certain microorganisms can trap the energy in light and use it to synthesize ATP from ADP and Pi. Much of the ATP producedin photosynthesis is hydrolyzed to provide energy for theconversion of carbon dioxide to six-carbon sugars, a processcalled carbon fixation:ATP ADP Pi6 CO2 6 H2OC6H12O6 6 O2In animals, the free energy in sugars and other molecules derived from food is released in the process of respiration.
Allsynthesis of ATP in animal cells and in nonphotosyntheticmicroorganisms results from the chemical transformation ofenergy-rich compounds in the diet (e.g., glucose, starch). Wediscuss the mechanisms of photosynthesis and cellular respiration in Chapter 8.54CHAPTER 2 • Chemical FoundationsThe complete oxidation of glucose to yield carbon dioxideOCOCOHCHCHHCHCHCOC6H12O6 6 O2 On 6 CO2 6 H2Ohas a Gº of 686 kcal/mol and is the reverse of photosynthetic carbon fixation. Cells employ an elaborate set ofenzyme-catalyzed reactions to couple the metabolism of1 molecule of glucose to the synthesis of as many as30 molecules of ATP from 30 molecules of ADP. Thisoxygen-dependent (aerobic) degradation (catabolism) ofglucose is the major pathway for generating ATP in all animal cells, nonphotosynthetic plant cells, and many bacterial cells.Light energy captured in photosynthesis is not the onlysource of chemical energy for all cells.
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