Question

. 2,4-dinitrophenol (DNP) is a weak acid that can diffuse across lipid membranes. When added to eukaryotic cells, DNP can enter the mitochondria and move protons between the intermembrane space and the mitochondrial matrix by binding protons on one side, diffusing across the inner mitochondrial matrix and releasing protons on the other side, i.e. move protons down their concentration gradient across the inner mitochondrial membrane.

a. How will DNP affect the proton gradient generated by ETC?

b. Will the addition of DNP inhibit the ETC, ATP synthase or both? Briefly explain your answer.

Answer 1

The production of ATP by complex ATP Synthase is driven by the Proton Motive Force,
Chemical Potential due to Concentration Difference of H⁺ Ions between the Intermembrane Space and Mitochondrial Matrix.

The ETC-Electron Transport Chain is responsible for building up this gradient by carrying out oxidation of NADH and FADH₂ and accumulating the Protons in the Intermembrane Space.
When the gradient is set up, H⁺  ions tend to diffuse back to the matrix. The only possible route is the complex ATP Synthase, a rotary complex which produces 1 molecule of ATP for every 2 H⁺  diffused through it.

a. When 2,4-DNP is present which is soluble across lipid membranes, the H⁺ ions have another route to cross the membane. It collapses the Proton Gradient Potential without any production of ATP.  
The gradient which was being used to for the production of energy is now just lost as heat.

b. As the production of energy declines, the cells starve for ATP. They drive the ETC intensely in order to re-establish the collapsing proton gradient.
It however inhibits the ATP synthase by providing an alternate path to the proton ions and decreasing the H⁺  concentration gradient.