All reversible reactions tend to proceed in the direction that decreases the energy of the participants. However, even in this direction the reaction may not proceed very quickly because very few molecules overcome the energy potential barrier and make the transition toward the lower energy state.
In a population of molecules there is a distribution of energies, because individual molecules are subject to collisions with other molecules, atomic vibrations, and external energy sources (such as light). Most molecules have energy near the average energy, not enough to jump over the potential barrier. Only a small percentage of the molecules have enough energy to make the transition over the barrier.
Consider a reaction proceeding in the opposite direction, where the end products are at a higher energy level than the starting materials. As an example, suppose you want to convert a mixture of amino acids into a protein. Such a reaction requires energy. It is called an endothermic reaction and will not go spontaneously, even with enzymes to catalyze it.
In general, all series of reactions involved in the synthesis
of large molecules are endothermic. For example, the conversion of
sugars to sugar-phosphates used to make polysaccharide requires an input
of energy. The simple reaction cannot proceed:
glucose +Pi --/--> glucose-6-phosphate
How is energy made available in cells? In what form is it stored? In what form and by what reactions is it used?
When a cell needs to make a reaction go in an energetically unfavorable (endergonic) direction, it uses the mechanism called coupling: it converts the substrate to something with higher chemical potential by using another reaction that is energetically favorable, that is, exergonic. The chemical potential of the modified substrate is higher than that of the product so that the reaction will now tend to proceed spontaneously in the desired direction. For example, the conversion glucose + Pi --> glucose-6-P + H20 requires energy. The reaction ATP + H20 --> ADP + H3P04 releases more energy. They are coupled (on an enzyme surface) into the overall reaction: glucose + ATP --> glucose-6-P + ADP, which is still exergonic. Here, to carry out the reaction the cell uses a phosphate group that is trapped into the ATP molecule, which acts a donor of phosphate to glucose.
Cells use similar mechanisms to make all sorts of energetically unfavorable reactions proceed. In protein synthesis, for example, the first step, aa + ATP --> AMP-aa + PPi (inorganic pyrophosphate), is exergonic and converts the amino acid (aa) into a new form, which still retains some of the high chemical potential and serves as a donor of amino acid for making protein. Note that ATP was used in both the above examples of coupled reactions. In the first case it donated one of its phosphate groups to glucose and thereby increased the reactivity of the glucose so that it could be used for other reactions. In the second case, ATP donated another part of its molecule to an amino acid.
Since the chemical machinery of the cell is driven by substances with high group donor potential, the next question is, How can these substances be made? Cells use only two kinds of energy:
Delta E = T * Delta S + Delta G
where Delta E = overall change in energy, T = absolute temperature, Delta S = change in entropy (which measures changes in the order of the system), and Delta G = change in free energy or chemical potential. The chemical potential (or Gibbs free energy) is the amount of energy that is available to do work. The change in free energy for a reversible reaction, A + B = C + D is given by
[C] [D] Delta G = Delta Go + RT ln ------- [A] [B]where R = gas constant, T = absolute temperature, and Delta Go is a constant characteristic of the substances involved. Suppose we let th e reaction go to equilibrium. Then the ratio [C][D]/[A][B] equals Keq. By definition at equilibrium there are no changes in concentration and Delta G = 0. Therefore
Delta G = Delta Go + RT ln Keq
Delta Go = -RT ln Keq
This relationship makes it possible to measure the change in free energy for a given chemical reaction if one can measure the equilibrium concentration of products and substrates. Rule: A chemical reaction always goes in the direction that tends to decrease the free energy (the direction for which Delta G < 0).
A reaction will not go in the opposite direction unless it is pushed or pulled. How can one push or pull? By varying the concentrations of reactants and/or products. One way, as I mentioned earlier, is to remove one product. For example, if the reversible reaction is A + B = C + D another reaction may remove D as soon as it is made. Alternatively, another reaction can produce more and more A or B. Increased production of C or D would push the reaction backward.
It is very important to remember that the two factors that determine the change in free energy in a reaction are the intrinsic properties of the substances and the concentrations of the reactants and products, because it helps us to understand why certain reactions go. For example, in the conversion of glucose to lactic acid, which serves to produce ATP for muscle contraction and many other processes, there is a series of reactions, some of which are uphill (energy-requiring, Delta G > 0) and some which are downhill (energy-releasing, Delta G < 0). The uphill reactions can proceed only because the products are continuously removed for use in later downhill reactions. If you have to go over a series of hills the only thing that matters is that the initial level is higher than the ultimate level so that the process as a whole represents a descent. [Water in a completely filled tube can go up and downhill provided the initial pressure is higher than the final pressure.] The Delta G for the entire series of reactions is the algebraic sum of the Delta G's of the individual reactions.
The relation of the concentration of a substance to the free energy becomes clearer when we consider active transport across a membrane, that is, the transfer of a substance from a region of lower concentration to one of higher concentration. Energy is needed to overcome the gradient of concentrations (since the substance would tend to flow down the gradient till the concentrations are equal):
If conc(in) is higher than conc(out) the DeltaG for transport inward is positive and energy is required. In living cells this energy comes from ATP or other substances with high group donor potential, but how it is actually used for transport remains obscure. (That energy is most likely used to distort or rotate molecules of specific transporter proteins. See Membrane transport proteins for more info.)
Substances like ATP, with high group donor potentials, have Delta Go for hydrolysis lower than -7 kcal/mole. The transfer of one phosphate or of two phosphates (P-P) or of the AMP group to water or to some other substance releases more than 7 kcal/mole. For example, in the hydrolysis of ATP in water,
ATP + H20 --> ADP + H3PO4 ,
8,000 calories are released as heat (measurable in a calorimeter) for every mole of ATP that disappears. Sometimes, ATP donates a phosphate to another substance to generate a compound that itself has a high group donor potential. For example, acetyl phosphate, formed in the reaction
ATP + acetic acid --> ADP + acetyl phosphate
is itself capable of donating phosphate to other substances. In general, any phosphate group that is attached to an acid through an anhydride bond -(CO)-O-P is very reactive. [An anhydride bond is formed by the joining of two acids face to face with removal of a molecule of water. It has a high tendency to be split by water to reform the two acids or to donate one of the acid groups.] When ATP donates a phosphate to an alcohol (-C-OH) as in the already familiar reaction
glucose + ATP --> glucose-6-phosphate + ADP
the substance that is formed does not have a high group donor potential. For example, if glucose-6-P were to hydrolyze in water it would release only about 3 kcal/mole; in fact, it has little tendency to do so.
last modified 23 January 1996 17:12