Complex II is succinate dehydrogenase, the electron transport chain enzyme that oxidizes succinate to fumarate and transfers electrons to ubiquinone. In Biological Chemistry I, it links the Krebs cycle to oxidative phosphorylation.
Complex II is succinate dehydrogenase, the one electron transport chain complex that is also a Krebs cycle enzyme in Biological Chemistry I. It sits in the inner mitochondrial membrane and takes electrons from succinate as part of the succinate to fumarate step.
Here is the basic move: succinate is oxidized to fumarate, and the electrons removed from succinate do not disappear. Complex II passes them through its internal redox centers, including iron sulfur clusters, and hands them to ubiquinone. Ubiquinone then becomes ubiquinol and carries those electrons onward in the membrane.
That double identity matters. In the Krebs cycle, the enzyme is helping convert a carbon skeleton into the next metabolic intermediate. In the electron transport chain, the same protein is feeding electrons into the membrane carrier pool, where they can continue toward Complex III and Complex IV.
Complex II is different from Complex I, III, and IV because it does not pump protons across the inner mitochondrial membrane. That means it contributes electrons to the chain, but it does not directly build the proton gradient. If a pathway starts at Complex II, the cell gets less proton pumping than it would from electrons that enter through NADH at Complex I.
The enzyme is built to move electrons efficiently rather than to act as a proton pump. Its iron sulfur clusters help relay the electrons step by step from the succinate oxidation site to ubiquinone. If any part of that relay is disrupted, electron flow slows and mitochondrial ATP production can drop.
A helpful way to picture Complex II is as a junction point. It connects the carbon metabolism of the Krebs cycle to the membrane based electron traffic of oxidative phosphorylation, but it does so without adding to the proton pumping work itself.
Complex II shows how metabolism is wired together instead of isolated into separate chapters. In Biological Chemistry I, this enzyme is one of the clearest examples of a single protein doing two jobs in two linked pathways: it belongs to the Krebs cycle and it also feeds electrons into the electron transport chain.
That makes it a great checkpoint for tracing cause and effect. If succinate oxidation slows, you are not just blocking one Krebs cycle step. You are also reducing electron input to ubiquinone, which can change downstream ATP production. That kind of chain reaction is exactly what metabolism questions often test.
It also helps you compare electron entry points. Electrons that enter through Complex II bypass Complex I, so they do not drive the same proton pumping total. That is why the source of electrons matters when you think about oxidative phosphorylation efficiency.
In disease context, Complex II comes up again because defects can disrupt mitochondrial energy output and contribute to mitochondrial disease. So this term is useful for pathway tracing, enzyme mechanism questions, and case-based problems about low ATP production, tissue energy demand, or inherited mitochondrial disorders.
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view galleryKrebs Cycle
Complex II is one of the few electron transport chain proteins that is also a Krebs cycle enzyme. In the cycle, it catalyzes succinate to fumarate, so it sits right at the point where carbon oxidation starts feeding electrons into the membrane carrier system. If you miss that connection, the pathway can feel like two separate processes instead of one linked flow.
Ubiquinone
Ubiquinone is the electron carrier that receives electrons from Complex II. Once reduced to ubiquinol, it moves within the inner mitochondrial membrane and delivers those electrons to later complexes. A lot of students mix up which carrier accepts electrons from which complex, so it helps to remember Complex II hands off to ubiquinone, not directly to oxygen.
Oxidative Phosphorylation
Complex II contributes to oxidative phosphorylation by supplying electrons to the chain, but it does not pump protons. That means it supports ATP production indirectly, through electron flow into the rest of the chain. When you compare ATP yield, electrons entering at Complex II usually support less proton gradient building than electrons entering at Complex I.
Complex III
Complex III is one of the next stops after ubiquinone carries electrons away from Complex II. If Complex II is working, electrons can still continue through Complex III and onward to the rest of the chain. This makes Complex III a useful reference point when tracing the electron path beyond the succinate dehydrogenase step.
A quiz item or problem set question may ask you to identify which complex oxidizes succinate, which carrier receives the electrons, or which complexes do not pump protons. You might also see a pathway diagram and need to trace electrons from succinate through ubiquinone into the rest of the electron transport chain. In a lab or case question about mitochondrial disease, you may need to predict what happens to ATP production when Complex II is defective. The safest move is to track the substrate, the electron carrier, and the membrane effect separately. If you can say, "succinate becomes fumarate, electrons go to ubiquinone, and no protons are pumped at this step," you usually have the core answer.
Complex I and Complex II both feed electrons into the electron transport chain, but they start with different donors and do different work. Complex I oxidizes NADH and pumps protons, while Complex II oxidizes succinate and does not pump protons. If a question asks which one is also a Krebs cycle enzyme, the answer is Complex II.
Complex II is succinate dehydrogenase, the enzyme that links the Krebs cycle to the electron transport chain.
It oxidizes succinate to fumarate and transfers the electrons to ubiquinone, which becomes ubiquinol.
Unlike Complex I, III, and IV, Complex II does not pump protons across the inner mitochondrial membrane.
Its iron sulfur clusters move electrons through the protein in a stepwise redox relay.
If Complex II is defective, electron flow and mitochondrial ATP production can drop, which shows up in energy metabolism problems.
Complex II is succinate dehydrogenase, the membrane enzyme that connects the Krebs cycle to the electron transport chain. It oxidizes succinate to fumarate and passes the electrons to ubiquinone. In this course, it is one of the cleanest examples of one protein serving both metabolism and oxidative phosphorylation.
No, Complex II does not pump protons across the inner mitochondrial membrane. That is one of the easiest ways to distinguish it from Complex I, III, and IV. It still matters a lot because it feeds electrons into the chain, even though it does not directly build the proton gradient.
Complex II uses succinate as its electron donor. During the reaction, succinate is converted into fumarate, and the electrons move through iron sulfur clusters before reaching ubiquinone. That is why the enzyme is counted in both the Krebs cycle and the electron transport chain.
Complex I oxidizes NADH and pumps protons, while Complex II oxidizes succinate and does not pump protons. Both add electrons to the electron transport chain, but they enter at different points and affect ATP yield differently. If you are tracing energy flow, that difference matters a lot.