Question

Describe the pH changes that occur in the two aqueous compartments of the mammalian mitochondria, the matrix and the inter membrane space, when the ETC has high flux. Describe the biochemical basis for these changes.

Answer 1

When the ETC has high flux, the inter membrane space is more acidic relative to the matrix due to the ETC activities and related proton pumping. Higher H+ outside constitutes lower pH in comparison to the more basic matrix.

The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that transfer electrons from electron donors (NADH and FADH2) to electron acceptors (molecular oxygen) via redox reactions. In this process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water. It is the electrochemical gradient created by proton pumping that drives the synthesis of ATP via coupling with oxidative phosphorylation by ATP synthase.

The process of electron transport begins when the hydride ion is removed from NADH (to form NAD+). The hydride ion will make a proton and two electrons (H-→ H+ + 2e -). The high energy electrons lose energy as they pass along the chain. Most importantly, the transfer of electrons is coupled to H+ uptake and release. This result in the pumping of H+ across the inner membrane, from the matrix to the intermembrane space. This movement of H+ result in generation of a pH gradient (higher pH in the matrix than outside). This leads to and a voltage gradient (membrane potential, the inside negative and the outside positive) across the inner mitochondrial membrane.

The pH gradient (ΔpH) drives H+ back into the matrix and OH- out of the matrix. The resulting membrane potential (ΔV), attract any positive ion into the matrix and to push any negative ion out. Together, the ΔpH and the ΔV constitute an electrochemical proton gradient. The hydrogen ions in the inter membrane space can only pass through a membrane protein called ATP synthase. The activation of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP.