Question

In muscle, creatine phosphate is used to store high-energy phosphates. Creatine phosphate is made by the reaction

creatine + ATP « creatine phosphate + ADP.

The standard free energy change for this reaction, ?G?, = +11.7 kJ/mol. How is it possible for the muscle to produce creatine phosphate if the standard free energy change is so unfavorable?

      A. The reaction is catalyzed by an enzyme.

   B. The ATP concentration in the cell is high compared to the concentration of ADP.

C. The ADP concentration in the cell is high compared to the concentration of ATP.

      D. The reaction is kinetically favored even though it is thermodynamically unfavored.

      E.None of the above.

Answer 1

Creatine phosphate, also known as creatine phosphate, phosphocreatine or PCr, is a molecule of phosphorylated creatine which is very important, since its function is to store energy in skeletal muscle. This molecule is used to generate, anaerobically, ATP from ADP, forming creatine for the next 15 seconds of intense effort. It does that by donating a phosphate group and this reaction is catalyzed by the enzyme creatine kinase - the presence of creatine kinase in the plasma is an indicator of damaged muscle tissue and is used among other things for the diagnosis of a myocardial infarction. This reaction is reversible and therefore acts as a temporary buffer of ATP concentration. In other words, phosphocreatine is part of a couple of reactions; the energy that is released in a reaction is used to regenerate another compound, ATP. Phosphocreatine plays a particularly important role in tissues that have a high and fluctuating energy demand such as brain or muscle, acting as an energy transport element from mitochondria to the area of ??cells where ATP is needed and temporary storage of energy (Buffer) for intense and short uses. A process will only happen spontaneously, without adding energy, if it increases the entropy of the universe as a whole (or, in the limit of a reversible process, leaves it unchanged), this is the Second Law of thermodynamics. Basically, some kind of measure is needed that represents the effect of a reaction on the entropy of the universe and that includes both the reaction system and its environment. Conveniently, both factors come together in a single value called Gibbs free energy. The Gibbs free energy (G) of a system is a measure of the amount of usable energy (energy that can perform a job) in that system. The change in Gibbs free energy during a reaction provides useful information about the energy and spontaneity of the reaction (if it can be carried out without adding energy). ?G = Gfinal-Ginicial.

?G is the change in free energy of a system that goes from an initial state, such as reagents, to a final state, like all products. This value indicates the maximum usable energy released (or absorbed) when going from the initial state to the final state. In addition, its sign (positive or negative) tells us if a reaction will occur spontaneously, that is, without additional energy. Reactions with a negative ?G release energy, which means that they can proceed without adding energy (they are spontaneous). In contrast, the reactions with ?G positive need a contribution of energy to be carried out (they are not spontaneous). Enthalpy as the change in entropy contributes to the total sign and value of ?G. When a reaction releases heat (negative ?H) or increases the entropy of the system, these factors make the ?G more negative. On the contrary, when a reaction absorbs energy or decreases the entropy of the system, these factors make the ?G more positive. We can know if a reaction will be spontaneous, if it will not be spontaneous, or it will only be at certain temperatures if we analyze the ?H and the ?S. If a reaction releases heat and increases the entropy, it will always be spontaneous (it will have a negative ?G) regardless of the temperature. Likewise, a reaction that absorbs heat and decreases the entropy will not be spontaneous (?G positive) at any temperature. However, some reactions have a mixture of favorable and unfavorable characteristics (they release heat but decrease entropy or absorb heat but increase entropy). The ?G and the spontaneity of these reactions depends on the temperature. Reactions with a positive ?G (?G> 0), on the other hand, require a supply of energy and are called endergonic reactions. In this case, the products or the final state, have more free energy than the reactants or initial state. Endergonic reactions are not spontaneous, which means that energy must be added before they can proceed. This is an endergonic reaction, with a ?G = +11.7 kJ / mol., Which, under standard conditions of pressure, temperature; the cells of the body, the energy required to synthesize will come from the rupture of combustible molecules such as glucose or other reactions that release energy (exergonics).