Complex 1 and its Inhibitors 2021 PDF

Complex 1 and its inhibitors Image credit: wikipedia Meet complex 1 Image credit: wikipedia  Complex 1 is the first enzyme in the electron transfer chain and thus it acts as the rate limiting step for ATP synthesis. It is also called NADH-coenzyme Q oxoreductase or NADH de...

Complex 1 and its inhibitors Image credit: wikipedia Meet complex 1 Image credit: wikipedia  Complex 1 is the first enzyme in the electron transfer chain and thus it acts as the rate limiting step for ATP synthesis. It is also called NADH-coenzyme Q oxoreductase or NADH dehydrogenase. It is the least well understood of all the electron transfer chain complexes and is the most difficult to obtain a crystal structure for. It is made of 46 different subunits and has a huge molecular mass of 1 giga Dalton. Some of those complexes are encoded in the mitochondrial genome and the rest are encoded by the nuclear genome. You can see from this diagram that some of the complex, the hydrophobic part, is embedded in the inner mitochondrial membrane and some of it, the hydrophilic part, sticks out into the lumen of the mitochondria. We don’t actually know what all the different subunits do, but we have been able to work out that 14 of them are most important for its catalytic and redox activity and the other may modulate its activity. The enzyme complex catalyses the oxidation of NADH, which comes from the Krebs cycle reactions occurring in the mitochondrial lumen, by ubiquinone, which is lipid soluble and sits in the inner membrane. The oxidation takes two electrons per NADH and pumps 4 protons. We’ll see what happens to ubiquinone in a later slide set. The redox energy released allows the pumping of protons from the lumen of the mitochondria across the inner membrane against their concentration gradient. What do all the bits do? Image credit: wikipedia  Complex I has three modules: the electron input module, or dehydrogenase module (N module), which accepts electrons from NADH; the electron output module, or hydrogenase module (Q module), which delivers electrons to ubiquinone; and the proton translocation module (P module), which pumps protons across the inner membrane. N and Q modules are parts of the matrix arm of the complex, whereas the P module lies within the portion embedded in the membrane. Step by step Image credit: wikipedia  NADH binds to complex I, and transfers two electrons to the flavin mononucleotide or FMN prosthetic group of the enzyme. You can see this at the bottom of the diagram. This recycles NAD+ and produces a proton and, within the complex, creates FMNH2. The electrons are then transferred through the FMN via a series of iron-sulfur (Fe-S) clusters, shown travelling up the hydrophilic arm of the complex here. The altered redox state of the protein induces a conformational change that alters the dissociation constant of the side chains, and causes four hydrogen ions to be pumped from the mitochondrial matrix across into the intermembrane space. The electrons are ultimately are handed to ubiquinone, also called co enzyme Q and indicated by a Q in the hydrophobic part of the enzyme in this diagram. Ubiquinone accepts 2 electrons and is reduced to ubiquinol (CoQH2). As we know, the hydrogen ions are trapped in the intermembrane space until they can move back, either through UCP1, by leaking across naturally (which is slow) or through ATP synthase  Complex 1 and ROS Image credit: Biotek instruments  Complex 1 is one of the two main sites of production of free radicals in respiration. Free radicals are highly reactive compounds because they have unpaired electrons. In small quantities they are useful cell signals. A build up of them can cause cell dysfunction because they damage macromolecules with double bonds, and this leads to inflammation, calcium influx and cell death if not controlled.. If oxygen delivery is blocked, so the final electron acceptor is in short supply, ROS levels can build up very quickly to damaging levels. Complex one is a site of production of ROS because electrons can ‘leak’ out at this point and interact with oxygen, especially if later complexes are working more slowly. This role of complex 1 in ROS generation is important in cell signalling and apoptosis , the process of programmed cell death.  Complex 1 inhibitors Image credit: Wikipedia Image credit: Hygeia Analytics  There are at least 60 complex 1 inhibitors. Some have been developed as agents to use in research to explore mitochondrial function. Others are used in medicine. More commonly they are used as pesticides. They fall into 3 groups: depending on how they work: quinone antagonists, such as the acetogenins, act at the entry of the hydrophobic site; semiquinone antagonists, such as rotenone, act in the intermediate steps by disrupting the electron transfer between the terminal FeS cluster and ubiquinone, but the specific site of action isn’t known. Finally, quinol antagonists, such as myxothiazol, prevent formation and/or release of the product. Inhibitors that prevent NADH interaction with the enzyme are not specific for complex 1 and will act on all other enzymes that rely on NADH. Rotenone is the most commonly used complex 1 inhibitor but piericidin and myxobacteial antibiotics, acetogenins that may be useful anti cancer drugs, the tranquilliser barbiturate amytal, some neuroleptic drugs and neurotoxins, and capsaicin from hot chilli peppers all act on complex 1. Even the antidiabetic drug metformin appears to inhibit complex 1 transiently and this effect is important for its function. Bullatacin is the strongest inhibitor of complex 1. Insect and fish mitochondria are particularly sensitive to complex I inhibition which is why rotenone and similar compounds are used as pesticides. Consequences of inhibition Image credit: Ohio state university  If complex one is inhibited, NADH is not oxidised and, because there is no supply of NAD+, oxygen consumption slows because Krebs cycle activity is reduced. The pumping of hydrogen slows down and so the membrane gradient is reduced and the production of ATP is slowed down because the protonmotive force is weaker. The reduction in ATP can have global effects on the cell because all enzymes will be affected. The most notable effect is reduced ion pumping so that potassium leaves the cell and Diseases of complex 1 insufficiency Image credit: pcori.org  Complex 1 is inactivated during ischaemia reperfusion and this is often a cause of tissue damage from stroke or heart attacks. In the absence of oxygen the enzyme loses the FMN co factor and becomes inactive. Complex 1 dysfunction is also implicated in Parkinson’s disease. Complex 1 inhibitors can cause cell death and induce changes that resemble Parkinsons disease in neurones in cell culture. Several forms of neurological disease, type 2 diabetes, cardiac disease and various cancers are associated with complex 1 dysfunction. Neurons and pancreatic islet beta cells rely heavily on mitochondrial ATP and NAD-linked pathways, which makes them very vulnerable. Cancer cells in culture have low mitochondrial density which can make them more vulnerable to complex 1 inhibition than other cells.