Chapter 21, Problem 15P

Interpretation Introduction

(a)

Interpretation:

The consequence of c-subunit stoichiometry for the ratio of hydrogen ion and ATP needs to be determined.

Concept Introduction:

The several types of ATP syntheses are A-type, F-typeand V-type. The main transmembrane subunit of these ATP syntheses is ATPase subunit C of ${\text{F}}_{\text{o}}{\text{/V}}_{\text{o}}$ complex.

They are enzyme complexes which are membrane-bound, and they also transport ions like protons across the membrane. The energy can be produced in the form proton gradient making use of flux of the ions through the membrane with the help of ATPase proton network that drives the ATP synthesis.

Some of the ATPases use the energy obtained from ATP hydrolysis and build proton gradient. In this way, they work in reverse direction. The different ATPase types have different functions and the ion type transported by them.

Interpretation Introduction

(b)

Interpretation:

The relationship between c-subunit stoichiometry and the magnitude of the proton-motive force needs to be explained.

Concept Introduction:

For ATP synthesis, the approximate value of the proton motive force is -250 mV. This is denoted as $\Delta p$ and composed of membrane potential, $\Delta \varphi$ and pH gradient $\Delta \text{pH}$ . The proton movie force is ratio of change in Gibbs free energy and Faraday’s constant. Thus, the formula will be as follows:

  $\Delta p=\frac{\Delta G}{\theta }=\Delta \varphi -\left(\frac{2.3RT}{\theta }\right)\Delta \text{pH}$

In the chloroplast the value of $\Delta \varphi$ is -50 to -100 mV. Also, the value of pH gradient is approximately 3 units. The value of photon motive force can be calculated from the above formula. It is approximately -170 mV.

The subunit C in F-ATPases is also known as subunit 9 as well as proteolipid. The F-ATPases contains 10 C subunits in the form of an oligomeric ring that build up the $F_o$ rotor up.

The protons flux drives the C subunit ring rotation that is coupled with the $F_1$ complex gamma subunit rotor. There is permanent bonding between the epsilon and gamma subunits in $F_1$ and C subunit ring of $F_o$ .

The continuous protonation and deprotonation of $\text{Asp}61$ in the subunit C results in the movement of the rotor stepwise.