Question

Why is an enzyme critical for coupling an exergonic and ednergonic reaction?

Answer 1

Ans: A coupled reaction is one in which energy from an exergonic reaction is used to drive an endergonic reaction. The energy released from an exergonic reaction can be used to drive an energy-requiring endergonic reaction. The breakdown of glucose, producing ATP. This is an example of how cells couple Exergonic reactions - breaking the bonds in glucose- to Endergonic reactions - synthesis of the high-energy phosphate bond in ATP.

Exergonic reactions will occur spontaneously. Reactions that release energy will occur spontaneously those that require energy must wait until energy is provided to occur. If the products of a reaction have more energy than the reactants, then we say this is an endergonic reaction. Endergonic reactions require energy input.

Exergonic reaction proceeds with a net release of free energy. Because the chemical mixture loses free energy (G decreases) delta G is negative for an exergonic reaction. Using delta G as a standard for spontaneity, exergonic reactions are those that occur spontaneously. The word spontaneous implies that it is energetically favorable not that it will occur rapidly. The greater the decrease in free energy, the greater the amount of work that can be done. It is important to realize that the breaking of bonds does not release energy contrary it requires energy. The phrase energy stored in bonds is short hand for the potential energy that can be released when new bonds are formed after the original bonds break, as long as the products are of lower free energy than the reactants.

An endergonic reaction is one that absorbs free energy from its surroundings. Because this kind of reaction stores free energy in molecules (G increases) delta G is positive. Such reactions are nonspontaneous, and the magnitude of delta G is the quantity of energy required to drive the reaction.

Enzymes are essentially protein in their chemical compound. An enzyme is a macromolecule that acts as a catalyst, a chemical agent that speeds up a reaction without being consumed by the reaction. Without regulation by enzymes, chemical pathways of metabolism would become very crowded because many chemical reactions would take such a long time. An enzyme catalyzes a reaction by lowering the activation energy barrier, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures.

An enzyme cannot change the delta G for a reaction; it cannot make an endergonic (nonspontaneous) reaction exergonic (spontaneous). Enzymes can only hasten reactions that would eventually occur anyway, but this function makes it possible for the cell to have a dynamic metabolism, routing chemicals smoothly through the cell's metabolic pathways. And because enzymes are very specific for the reactions they catalyze they determine which chemical processes will be going on in the cell at any particular time. When the enzyme and substrate are joined, the catalytic action of the enzyme converts the substrate to the product of the reaction for example the enzyme sucrase catalyzes the hydrolysis of the disaccharide sucrose into its products, two monosaccharides, glucose and fructose. The reaction catalyzed by each enzyme is very specific an enzyme can recognize its specific substrate even among closely related compounds.