Which of the following statements about enzyme mechanism reaction steps are true

Question

Which of the following statements about enzyme mechanism reaction steps are true (select all that apply)?

On a reaction coordinate diagram, a reversible step is indicated by reactant and product at similar energy

Irreversible steps are important in enzyme mechanisms to help ensure enzymes catalyze reactions in the desired direction

Substrate binding is usually an irreversible step

Irreversible steps of a reaction mechanism are always very slow

To determine if a step is reversible or irreversible, it is necessary to know the activation energy of the step

On a reaction coordinate diagram, a reversible step is indicated by reactant and product at similar energy

Irreversible steps are important in enzyme mechanisms to help ensure enzymes catalyze reactions in the desired direction

Substrate binding is usually an irreversible step

Irreversible steps of a reaction mechanism are always very slow

To determine if a step is reversible or irreversible, it is necessary to know the activation energy of the step

Explanation / Answer

Ans. Option A. False. The energy of product is generally lower than that of the reactant.

Option B. True. The irreversible reaction ensures the unidirectional conversion of reactant into product, but never in backward direction.

For example, phosphorylation of glycose into G-6-P in first step of glycolysis ensures that this step and other reactions of the pathways proceed in specified desired direction.

Option C. False. Substrate binding is always reversible.

Option D. False. The rate of a reaction depending on relative concertation of products, reactants, temperature, etc. Irreversibility of a reaction is NOT a parameter for its rate. For example, hydrolysis of phosphoenol pyruvate into pyruvate and ATP is irreversible and fast.

Option F. False. To know if a reaction is reversible or irreversible, the standard free energy change (dG0’) and experimental free energy change (dG) are required.

            If, dG < dG0’ - the reaction is irreversible.     

Also, for simplicity, a reaction with –ve value of dG0’ is usually considered irreversible.

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