Question

QUESTION 4 You are studying an electron transport chain from a newly identified life form that has four electron carriers, a, b, c, and d. They function in the following order: substrate → → → → → 02 Three compounds are identified which inhibit this newly discovered electron transport chain in the presence of substrate and oxygen, yielding the patterns of oxidation states shown below. O and R indicate fully oxidized and fully reduced, respectively. Inhibitor ROOO Z o substrate + c = b = a + d + 02 1f21 31 41 + , arrow 2, Identify the inhibitor which acts at site indicated by arrow 1, 4, arrow 3, 4 and arrow neither inhibitor X, Y, or Z QUESTION 5

Answer 1

The inhibitor Y inhibits the electron transport chain having a, b, c and d as electron carriers. The first complex in electron transport chain is NADH-coenzyme Q reductase. It catalyses transfer of electrons from NADH to coenzyme Q10 and translocates protons across the inner mitochondrial membrane. NADH initially binds to complex I and transfers two electrons to the flavin mononucleotide (FMN) prosthetic group of the enzyme, creating FMNH2. The electrons are then transferred through the FMN via a series of iron-sulfur (Fe-S) clusters and finally to coenzyme Q10 (ubiquinone). This brings about conformational change in the protein causes four hydrogen ions to be pumped out of the mitochondrial matrix. Ubiquinone (CoQ) accepts two electrons to be reduced to ubiquinol (CoQH2). The inhibitor blocks the transfer of electrons and thus the substrate remains in reduced state. Complex II of electron transport chain is succinate-coenzyme Q reductase. It oxidises succinate via FAD and the subsequent electrons are transfered to ubiquinone molecule. The inhibitor blocks oxidation of succinate, thus the substrate remains in reduced state. Complex III is coenzyme Q-cytochrome reductase. It pumps proton through the membrane and passes electrons to cytochrome c. Thus the inhibitor blocks the transfer of electrons and the complex remains in the reduced state. The fourth and last complex of electron transport chain is cytochrome c oxidase. It transfers electrons to oxygen and in this process water molecule is formed. The inhibitor blocks the transfer of electrons and thus the substrate remains in oxidised form.