Question

Show why coupling the reaction for the hydrolysis of 1,3 -bisphosphoglycerate to the phosphorylation of \(\mathrm{ADP}\) is energetically favorable. Combine the equations and calculate \(\Delta G\) for the coupled process. You need only give names or abbreviations in your equations not chemical structures.

Short answer

ΔG for the coupled process is -18.8 kJ/mol, indicating it's energetically favorable.

Step-by-step solution

Identify the Reactions

To determine the energetic favorability of coupling, we must first identify the individual reactions involved in the process. 1. The hydrolysis of 1,3-bisphosphoglycerate (1,3-BPG) can be represented as: 1,3-BPG + H2O → 3-phosphoglycerate (3-PG) + Pi 2. The phosphorylation of ADP to form ATP via condensation would be: ADP + Pi → ATP + H2O.

Determine the ΔG for Each Reaction

Next, we need to determine the Gibbs free energy change (ΔG) for each reaction: 1. The hydrolysis of 1,3-BPG is a highly exergonic reaction with a ΔG of approximately -49.3 kJ/mol. 2. The phosphorylation of ADP to produce ATP is an endergonic reaction with a ΔG of approximately +30.5 kJ/mol.

Combine the Reactions

To find the overall coupled reaction, add the two equations. The inorganic phosphate (Pi) produced in the first reaction is used in the second, cancelling it out on both sides of the equation: 1,3-BPG + ADP → 3-PG + ATP

Calculate the Net ΔG for the Coupled Process

The net ΔG for the coupled reactions is the sum of the ΔG values of the individual reactions:\[ \Delta G_{net} = \Delta G_{hydrolysis} + \Delta G_{phosphorylation} = -49.3 \, \text{kJ/mol} + 30.5 \, \text{kJ/mol} = -18.8 \, \text{kJ/mol} \]This negative ΔG indicates that the overall coupled process is energetically favorable.

Key concepts

Gibbs free energy

In bioenergetics, Gibbs free energy helps us understand if a chemical reaction occurs spontaneously. It is represented by the symbol \( \Delta G \). The change in Gibbs free energy during a reaction reflects how much energy is available to do work.
When \( \Delta G \) is negative, the reaction releases energy and proceeds spontaneously. This is called exergonic. If \( \Delta G \) is positive, the reaction requires energy input to occur, known as endergonic.
In the exercise you looked at, the hydrolysis of 1,3-bisphosphoglycerate (1,3-BPG) has a \( \Delta G \) of -49.3 kJ/mol, making it a strong exergonic process. On the other hand, ADP phosphorylation to ATP needs energy, as shown by its \( \Delta G \) of +30.5 kJ/mol. The calculation for the net \( \Delta G \) when these reactions are coupled is -18.8 kJ/mol. This means the combined reaction is exergonic and can proceed spontaneously.

coupled reactions

Coupled reactions are when two reactions occur together because one provides the energy needed for the other. This is common in biological systems where energy transfer is crucial for many processes.
An exergonic reaction, which releases energy, is paired with an endergonic reaction, which requires energy. This coupling helps the overall process occur smoothly and efficiently.
In the textbook exercise, the reaction of 1,3-bisphosphoglycerate breaking down to 3-phosphoglycerate releases energy. This energy is "captured" and used to power the phosphorylation of ADP to make ATP. Through this coupling, a usually non-spontaneous ATP formation becomes possible.

• Exergonic reactions drive endergonic processes.
• Energy in the form of ATP forms an energy "currency" in cells.

adenosine triphosphate (ATP)

Adenosine triphosphate (ATP) is the energy "currency" of cells. It stores and supplies energy for many cellular functions. ATP can be broken down into ADP and inorganic phosphate (Pi), releasing energy that cells use to perform activity like moving muscles or synthesizing macromolecules.
The structure of ATP includes three phosphates, one ribose sugar, and adenine. When it loses a phosphate (forming ADP), this process releases approximately 30.5 kJ/mol of energy. This energy is essential for many cellular processes.
In your exercise, bacteria and our cells harness ATP to store energy released from breaking down 1,3-bisphosphoglycerate. This transformation to ATP acts as a way to safely transport and later utilize the captured energy, proving vital for life functions.

• ATP functions as a rechargeable energy source.
• It links energy-yielding processes with energy-requiring ones.
• Many enzymes act to facilitate phosphorylations that make and use ATP.