Question

Given the roles of NAD+ NADH in oxidation reactions and NADPH- NADP+ reduction reactions, discuss which cofactor is more commonly used in catabolic pathways and which one is in anabolic pathways? What do you expect the intracellular ratios of NAD+/NADH and NADP+/NADPH to be high or low under homeostasis? Explain your answers. Explain the benefits of the Q cycle comparing to direct transfer of electrons from QH2 to two cytochrome c Distinguish substrate-level phosphorylation and oxidative phosphorylation. Provide specific examples and explain their functions.

Answer 1

1A. The role of Nicotinamide adenine dinucleotide (NADH) is significantly seen in catabolic reactions and function as oxidizing agent (NAD+), while, the role of Nicotinamide adenine dinucleotide phosphate (NADPH) is mostly seen in anabolic reactions and function as a reducing agent (NADPH). Generally under homeostasis conditions, the ratio of NAD+: NADH inside the cell is high, while the ratio of NADP+: NADPH is retained low. This offers adequate amount of NAD+ to act as an oxidizing agent whereas, high amount of NADPH act as a reducing agent which is required for their distinctive roles in catabolism and anabolism, respectively.

1B. The Q cycle also known as quinol cycle involves sequential oxidation and reduction of coenzyme Q electron carrier between the ubiquinol and ubiquinone. The Q cycle is commonly seen in complex III of photosynthesis and generates pH gradient across lipid bilayer, which is used for the generation of ATP. Q cycle in Complex III involves reduction of Cytochrome c followed by oxidation of ubiquinol to ubiquinone, as a result four protons are entered into the intermembrane space, which accounts two from the matrix and two from the reduction of two molecules of cytochrome c. The function of Q cycle mainly involves that coenzyme Q receives reducing equivalents from protein Complex I and as well protein Complex II and shuttles these electrons to protein Complex III and then proceeds to get more reducing equivalents.

1C. Substrate level phosphorylation is a direct type of phosphorylation in which a phosphate group is transferred from high energy compound to ADP. It occurs in glycolysis and in Krebs cycle. In glycolysis, Phosphoglycerate kinase and pyruvate kinase are the two enzymes which are involved in the substrate level phosphorylation, as result 4 ATPs are generated.In Krebs cycle, Phosphoenolpyruvate carboxykinase and succinate CoA ligase are the two enzymes involved in the substrate level phosphorylation as a result 2 ATPs are produced.

Oxidative phosphorylation is an indirect type of phosphorylation in which the energy released from the electron transport chain is used for the generation of ATP. Oxidative phosphorylation is most commonly seen in the aerobic respiration, where NADH and FADH that are formed in the glycolysis, Krebs cycle are oxidized back in the electron transport chain. The energy released by those molecules are used in the formation of ATP, as a result 34 ATPs are produced.