Endergonic

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°... 

Equation 1 has AG° = +14 kJ and is endergonic The mam reason for this is that one of the very stable carboxylate groups of glutamic acid is converted to a less stable amide function... 

Nevertheless the biosynthesis of glutamine proceeds from glutamic acid The dif ference is that the endergonic process m Equation 1 is coupled with the strongly exer gome hydrolysis of ATP... 

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

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. 

If free energy is stored and transfened by way of ATP, where does the ATP come from It comes from ADP by the endergonic reaction... 

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. 

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 5.13 Sketch an energy diagram for a two-step reaction with an endergonic first step and an exergonic second step. Label the parts of the diagram corresponding to reactant, product, and intermediate. 

Figure 6.14 Energy diagrams for endergonic and exergonic steps, [a) In an endergonic step, the energy levels of transition state and product are closer.

Hammond postulate The structure of a transition state resembles the structure of the nearest stable species. Transition states for endergonic steps structurally resemble products, and transition states for exergonic steps structurally resemble reactants. 

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