What is the primary location of mitochondrial respiratory complexes? These complexes are embedded in the inner mitochondrial membrane.

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Roy Rice
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1 Interactive Questions Question 1: What is the primary location of mitochondrial respiratory complexes? Outer mitochondrial membrane Mitochondrial inter-membrane space Inner mitochondrial membrane These complexes are embedded in the inner mitochondrial membrane. Mitochondrial matrix Neither of the above options Question 2: Which of the following polypeptides is not a subunit of complex II? SDHAF1 This is an assembly factor in complex II biogenesis. SDHB SDHA SDHC Question 3: - 1 -

2 How many protein subunits contains the mature complex II? 4 Complex II contains in total 4 protein subunits called SDHA, SDHB, SDHC and SDHD Question 4: What are the main substrates involved in the normal (forward) enzymatic activity of complex II? Succinate and 2-oxalacetate Succinate and fumarate Succinate and ubiquinone Complex II functioning in the normal forward mode removes 2 electrons from succinate in the TCA cycle (this converts succinate to fumarate)and transfers them to ubiquinone, yielding ubiquinol. Succinate and ubiquinol Fumarate and ubiquinone Question 5: What happens when succinate dehydrogenase activity of complex II is blocked? - 2 -

3 Accumulation of fumarate Enhanced production of fumarate that is immediately metabolized to malonate Enhanced accumulation of succinate Succinate is the substrate for the SDH activity of complex II. Rapid increase in the concentration of acetyl-coa Accelerated increase in the level of citrate Question 6: What cancer types are associated with complex II deficiency? Paraganglioma (PGL) and pheochromocytoma (PHEO), Gastrointestinal stromal tumour (GIST) and Renal cell carcinoma (RCC) Complex II deficiency is significantly associated with all these tree cancer types, but in PGL/PHEO the association is the strongest. Gastrointestinal stromal tumour (GIST) Renal cell carcinoma (RCC) Breast cancer Lung cancer Question 7: What are the post-translational modifications of complex II? - 3 -

4 Myristoylation Phosphorylation and Myristoylation Acetylation Succinylation Phosphorylation, acetylation and succinylation. These three modifications are best defined in complex II. Additional modifications such as malonylation are also possible, but are less well characterized. Question 8: Does complex II contribute to ROS generation in the OXPHOS system? Complex II never contributes, only complexes I and III can produce ROS. Complex II contributes to ROS generation both directly and indirectly via complex I. Complex II can directly produce ROS, principally from the FAD redox centre in the SDHA subunit. Complex II also has an important indirect role in ROS generation by supplying electrons from succcinate via ubiquinol to complex I, where ROS are formed by the reverse electron transfer (RET). Complex II can contribute to ROS production both diretly and indirectly, but only in isolated mitochondrial and not in cells and tissues, and this is therefore not relevant. Complex II is the only site in the OXPHOS system that can generate ROS. No ROS can be genearated in the OXPHOS system, ROS comes from other mitochondrial sites

5 Question 9: What is the role of succinate in ROS generation by complex II? The only role for succinate is that it is a complex II substrate and is therefore needed for complex II function and ensuing ROS formation. Its concentration does not matter. Intracellular succinate concentration is very important. At lower concentrations, complex II can produce ROS directly and at higher concentratios indirectly. Succinate is the electron donor. However, low succinate allows ROS production at the FAD site, favoring direct ROS production from complex II. High succinate, on the other hand, does not allow ROS production at FAD (probably by blocking oxygen access to the site), but leads to the higher degree of reduction of the ubiquinol pool. This pushes electrons backwards onto complex I (so called reversed electron transfer - RET), and leads to ROS generation from complex I. Normally it has no role, but during ROS production by the reverese electron transfer at complex I it serves as an electron acceptor. Complex II generates ROS only when it functions in reverse, so succinate becomes an electron acceptor in such a situation. Question 10: Is there a potential for complex II to be a therapeutic target? No, complex II is undrugable, succinate and ubiquinone affinities are too high for succesful intervention. Yes, complex II is a potential target. Strategies are being developed to target complex II in cancer, inflamation and to prevent ischemia-reperfusion injury

6 In cancer, complex II inhibitors that compete with ubiquinone are being developed to induce cell death in a ROS-dependent manner in cancer cells. For ischemia-reperfusion and inflammation, succinate competitors are being developed to reduce ROS formation during reverse electron transfer. All these agents are experimental. Yes, complex II inhibtors are succesfully used in the clinic for several years already. No, all complex II inhibitors are extremely toxic and cannot be used in vivo at all

CELL BIOLOGY - CLUTCH CH AEROBIC RESPIRATION.

CELL BIOLOGY - CLUTCH CH AEROBIC RESPIRATION. !! www.clutchprep.com CONCEPT: OVERVIEW OF AEROBIC RESPIRATION Cellular respiration is a series of reactions involving electron transfers to breakdown molecules for (ATP) 1. Glycolytic pathway: Glycolysis