Coupling of Biochemical Reactions

• Exergonic reactions can drive endergonic ones.
• ATP provides the capacity to serve as a linking agent between energy yielding and energy requiring cellular reactions.
• ATP has a high free energy of hydrolysis.
• The energy of oxidation is conserved as ATP energy in the cell.
• Living organisms cannot create energy from nothing, nor can they destroy energy into nothing. Living organisms use energy to build complex, low-entropy structures. The ultimate source of this energy on earth is the sun.
• Living organisms may transform energy from one form to another.
• The first law of thermodynamics states that the principle of the conservation of energy applies to living organisms.
• The second law of thermodynamics states that the universe always tends toward increasing disorder. In the process of transforming energy, living organisms must increase the entropy of the universe by releasing energy as heat to their surroundings
• Gibbs free energy, G, expresses the amount of an energy capable of doing work during a reaction at constant temperature and pressure.
• When a reaction proceeds with the release of free energy the free-energy change, ∆G, has a negative value.
• Enthalpy, H, is the heat content of the reacting system and reflects the number and kinds of chemical bonds in the reactants and products.
• The energy required to drive the reaction of synthesizing ATP from ADP and Pi is supplied by oxidative processes occurring in metabolism.
• Two independent reactions having no common reactants or products can be coupled through a catalyst: A + B X + Y → C Z.
• If pyruvate is further oxidized where DG0 = -61.8 kcal, does this enhance the formation of glutamine? YES.
• Glutamine synthetase monitors the amounts of nitrogen-rich molecules in the cell.
• Coupling of reactions is a device to drive endergonic processes.
• At 25 o C, the equilibrium constant (Keq) for this reaction is 1.003.
• Transaminases (amino transferases) transfer amino groups from one amino acid to another: α-ketoglutarate + alanine → glutamine + pyruvate.
• The occurrence of an exergonic process, such as oxidative processes, which will convert X into Y,will be coupled with the endergonic processes.
• Entropy, S, is a quantitative expression for the randomness or disorder in a system.
• When the products of a reaction are less complex and more disordered than the reactants, the reaction is said to proceed with a gain in entropy.
• Free Energy, or the Equilibrium Constant, determines the spontaneity of processes.
• Although joules and kilojoules are the standard units of energy and are used throughout this text, biochemists and nutritionists sometimes express D G'˚ values in kilocalories per mole.
• The relationship between equilibrium constants and standard free energy changes of chemical reactions is shown in Table 13-2.
• Many biochemical reactions have a very large (+) value for Delta G at the conditions of the reaction in living organism. These reactions do occur but only when they are paired with a second reaction with a very negative delta G
• The actual free-energy change of a reaction in the cell depends on the standard change in free energy, actual concentrations of products and reactants, and the reaction mechanism.
• These reactions do occur but only when they are tightly coupled to a second reaction having a large NEGATIVE value of D G.
• The immediate removal of the products of a reaction can keep the ratio [products]/[reactants] well below 1, such that the term RT ln ([products]/[reactants] has a large, negative value.
• The criterion for spontaneity of a reaction is the value of ∆G, not ∆G0.
• A reaction with a positive ∆G 0 can go in the forward direction if ∆G is negative.
• The negative proceeds forward, the zero is at equilibrium, and the positive proceeds in reverse.
• The equilibrium constant, also known as the free energy, determines the spontaneity of processes.
• Standard free-energy changes are additive

  1. A -> B = delta G1
  2. B -> C = delta G2

  A -> C = delta G1 + delta G2
• This is possible if the term RT ln([products]/[reactants]) in Equation 13-4 is negative and has a larger absolute value than ∆G 0.
• The free energy change of ATP can give us the amount of work the reaction can perform under isothermal conditions.
• The high phosphoryl transfer potential of ATP is due to three factors: electrostatic repulsion, resonance stabilization and stabilization due to hydration.
• ATP is highly charged at pH 7.0 with each of the three phosphate groups completely ionized, resulting in four negative charges.
• At pH 7.0, the structure of ATP has four negative charges that repel each other very strongly, relieving some of the electrostatic stress upon hydrolysis.
• The two products ADP and Pi undergo stabilization as resonance hybrids, having the lowest possible energy level.
• ATP does not have the highest free energy of hydrolysis among all the other phosphate esters, it has a value that is intermediate, forming the center or midpoint of a thermodynamic scale of phosphorylated compounds.