Question

Describe the binding change mechanism model with regard to the functional role of the \(\gamma\) sub-unit in mediating the synthesis of \(\sim 3\) ATP for each \(360^{\circ}\) rotation.

Short answer

Question: Explain the functional role of the γ sub-unit in ATP synthesis through the binding change mechanism model and how it facilitates the synthesis of approximately 3 ATP molecules during each 360° rotation. Answer: The γ sub-unit in ATP synthase plays a crucial role in converting potential energy from the proton gradient into mechanical energy, driving the rotation of the catalytic sites on the β subunits. During each 360° rotation, the γ sub-unit sequentially induces conformational changes in each of the three β subunits, promoting ATP synthesis at each catalytic site through the binding change mechanism model. This mechanism involves three steps: binding of ADP and inorganic phosphate, catalysis to form ATP, and release of the synthesized ATP molecule. As a result, approximately 3 ATP molecules are synthesized and released during one complete rotation.

Step-by-step solution

1. Introduction to the binding change mechanism model

The binding change mechanism model is a part of the enzymatic process that explains ATP synthesis in the F1 portion of ATP synthase. This enzyme consists of multiple subunits working together to convert the energy generated during cellular respiration (the flow of protons across the membrane) into chemical energy stored as adenosine triphosphate (ATP). It utilizes the rotation of three catalytic sites on the β subunits to produce ATP for every 360° rotation.

2. The role of the γ sub-unit within ATP synthase

The γ sub-unit is the central axis of the enzyme and is situated inside the catalytic hexameric ring formed by α and β subunits. This subunit plays a crucial role in converting the potential energy from the proton gradient into mechanical energy, which drives the rotation of the catalytic sites on the β subunits. The γ sub-unit is responsible for the conformational changes of the three β subunits during the rotational process, allowing for sequential synthesis of ATP molecules.

3. The three steps of the binding change mechanism model

There are three major steps in the binding change mechanism model, each of which is associated with a different conformation of the β subunits. These conformational changes are powered by the rotation of the γ sub-unit. Step 1: Binding: The empty (open) catalytic site on the β subunit binds an ADP and an inorganic phosphate (\(P_i\)) molecule. Step 2: Catalysis: After binding the substrates, a conformational change, specifically induced by the interaction of the γ sub-unit with the adjacent β subunit, promotes the synthesis of ATP. In this closed (tight) state, ADP and \(P_i\) molecules combine to form the ATP molecule. Step 3: Release: Another conformational change induced by the rotational movement of the γ sub-unit returns the β subunit to the open state and releases the synthesized ATP molecule.

4. The γ sub-unit and the synthesis of ~3 ATP during one complete 360° rotation

As the γ sub-unit rotates 360° within the hexameric ring, it sequentially induces conformational changes in each of the three β subunits, promoting ATP synthesis at each of the catalytic sites. Therefore, during one complete rotation, the γ sub-unit facilitates the synthesis and release of approximately 3 ATP molecules in total (one per catalytic site). In conclusion, the γ sub-unit of ATP synthase plays a crucial role in mediating the synthesis of ~3 ATP molecules during each 360° rotation. It achieves this through the binding change mechanism model, which involves inducing sequential conformational changes in the β subunits to enable ATP synthesis and release.