Exergonicity

Reactions are classified as exergonic or endergonic according fo fhe sign of AG° An exergomc reacfion is one m which AG° is negafive an endergonic reacfion has a posi five value of AG°... 

Reactions are classified as exergonic or endergonic according to the sign of AG°. An exergonic reaction is one in which AG° is negative, an endergonic reaction has a positive value of AG°. 

The next point takes the standard-state idea and makes it more suitable for biological processes by defining a new AG°, called AG°. This new standard state is one with a pH of 7. This is the standard state used most of the time for biochemical reactions and is the one we will use. Not only does it make a big difference in reactions in which is consumed or produced, it also requir es us to be aware of the form in which various species exist at a pH of 7. A reaction that is endergonic at [H" ] = 1 M can easily become exergonic at [H" ] = 10 M (pH = 7) and vice versa. 

Many fonnally endergonic biochemical processes become exergonic when they are coupled mechanistically to the hydrolysis of ATP. 

If AG is equal to 0, the process is at equilibrium, and there is no net flow either in the forward or reverse direction. When AG = 0, A.S = H/T, and the enthalpic and entropic changes are exactly balanced. Any process with a nonzero AG proceeds spontaneously to a final state of lower free energy. If AG is negative, the process proceeds spontaneously in the direction written. If AG is positive, the reaction or process proceeds spontaneously in the reverse direction. (The sign and value of AG do not allow us to determine how fast the process will go.) If the process has a negative AG, it is said to be exergonic, whereas processes with positive AG values are endergonic. 

This reaction is strongly exergonic and AG° at 37°C is —42.8 kj/mol. Physiological concentrations of phosphocreatine, creatine, and inorganic phosphate are normally between 1 mMand 10 mM. Assuming 1 mMconcentrations and using Equation (3.12), the AG for the hydrolysis of phosphocreatine is... 

When coupled (by phosphofructokinase) with the hydrolysis of ATP, the overall reaction (Figure 19.7) is strongly exergonic ... 

The net reaction accomplished by the TCA cycle, as follows, shows two molecules of COg, one ATP, and four reduced coenzymes produced per acetate group oxidized. The cycle is exergonic, with a net AG° for one pass around the cycle of approximately —40 kj/mol. Table 20.1 compares the AG° values for the individual reactions with the overall AG° for the net reaction. 

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate (Eigure 23.7), like all phosphate ester hydrolyses, is a thermodynamically favorable (exergonic) reaction under standard-state conditions (AG° = —16.7 kj/mol). Under physiological conditions in the liver, the reaction is also exergonic (AG = —8.6 kJ/mol). Fructose-1,6-bisphosphatase is an allosterically regulated enzyme. Citrate stimulates bisphosphatase activity, hut fructose-2,6-bisphosphate is a potent allosteric inhibitor. / MP also inhibits the bisphosphatase the inhibition by / MP is enhanced by fructose-2,6-bisphosphate. 

Suggest an explanation for the exergonic nature of the glycogen synthase reaction (AG° = - 13.3 kj/mol). Consult Chapter 3 to review the energetics of high-energy phosphate compounds if necessary. 

AGr Gibbs free-cnergy change The energy difference between reactants and products. When AG° is negative, the reaction is exergonic, has a favorable equilibrium constant, and can occur spontaneously. When AGC is positive, the reaction is endergonic, has an unfavorable equilibrium constant, and cannot occur spontaneously. 

Problem 5.11 Which reaction is likely to be more exergonic, one with Ktq = 1000 or one with Keq - 0.001 ... 

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