Question

A) Zn-Ferrocyanide inhibits the enzyme FNR and prevents reduction of NADP+ to NADPH. In the presence of Zn-Ferrocyanide, what would happen to the pH gradient, and the rates of electron transport, oxygen evolution, and ATP synthesis in the thylakoids? Explain.

B) If ferredoxin cannot donate its electrons to the FNR to reduce NADP+ to NADPH, where else can it donate its electrons?

Answer 1

A) If Zn-Ferrocyanide inhibits FNR, then reduced ferredoxin cannot further reduce NADP+ to NADPH by donating its electrons. This will lead to a build up of reduced ferrodoxin within the thylakoid membrane. Eventually all ferredoxin molecules will become reduced with electrons without being able to pass them on, resulting in a blockage of electron transport. Due to inactivity of FNR to pass the electrons to NADH+, rate of electron transport chain will be greatly reduced. Oxygen evolution will not be hampered at first as it is not directly related to ETC, but if the entire ETC comes to a halt because of the blockage, then oxygen evolution will eventually diminish. By activity of FNR, H+ ions were utilized to form NADPH, which resulted in a lower H+ concentration and a higher pH at the stroma side. This helped in the formation of the pH gradient, which is utilized to synthesize ATP. If FNR stops to function, H+ will build up in stroma causing a lower pH. Thus a pH gradient (difference in pH) cannot be established, leading to cessation of ATP production in the thylakoids.

B) Ferredoxin can alternatively donate its electrons to CO₂ or to enzymes like glutamate synthase, nitrite reductase etc.