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Complex II - Succinate Dehydrogenase Explained

Complex II is an enzyme complex of the mitochondrial respiratory chain that plays a key role in cellular energy metabolism.

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Things worth knowing about "Complex II"

Complex II is an enzyme complex of the mitochondrial respiratory chain that plays a key role in cellular energy metabolism.

What is Complex II?

Complex II, also known as succinate dehydrogenase (SDH) or succinate-ubiquinone oxidoreductase, is one of the five major enzyme complexes of the mitochondrial respiratory chain. It is embedded in the inner mitochondrial membrane and represents the only direct link between the citric acid cycle (Krebs cycle) and oxidative phosphorylation. Unlike the other respiratory chain complexes, Complex II does not pump protons across the membrane and therefore does not directly contribute to the generation of the mitochondrial membrane potential.

Structure and Composition

Complex II consists of four subunits:

SDHA: the catalytic subunit, containing FAD (flavin adenine dinucleotide) as a prosthetic group
SDHB: the iron-sulfur subunit with three iron-sulfur clusters
SDHC: a membrane-anchoring subunit
SDHD: a membrane-anchoring subunit

The SDHA and SDHB subunits form the hydrophilic, catalytically active portion of the complex, while SDHC and SDHD anchor it in the inner mitochondrial membrane and provide the binding site for ubiquinone (Coenzyme Q).

Mechanism of Action

Complex II catalyzes the oxidation of succinate to fumarate as part of the citric acid cycle. During this reaction, two electrons are transferred to FAD, producing FADH2. The electrons are then transferred via the iron-sulfur clusters to ubiquinone (Coenzyme Q), reducing it to ubiquinol (QH2). Ubiquinol subsequently donates its electrons to Complex III of the respiratory chain.

The overall reaction is:

Succinate + Ubiquinone → Fumarate + Ubiquinol

Because no protons are pumped during this reaction, the oxidation of FADH2 via Complex II yields less ATP than the oxidation of NADH via Complex I.

Clinical Significance

Mitochondrial Diseases

Mutations in the genes encoding the subunits of Complex II can lead to serious mitochondrial diseases. Affected individuals often present with symptoms such as:

Muscle wasting and weakness (myopathy)
Neurological impairment
Cardiac problems (cardiomyopathy)
Leigh syndrome (a severe neurodegenerative disorder of childhood)

Cancer

Mutations in the SDH genes, particularly in SDHB, SDHC, and SDHD, are associated with an increased susceptibility to certain tumors:

Pheochromocytoma: a tumor of the adrenal medulla
Paraganglioma: a tumor of the autonomic nervous system
Gastrointestinal stromal tumor (GIST)
Renal cell carcinoma

These tumors are collectively referred to as SDH-deficient tumors. Loss of SDH function leads to the accumulation of succinate, which acts as an oncometabolite, inducing epigenetic changes and stabilizing HIF-1α (hypoxia-inducible factor).

Diagnosis

The diagnosis of a Complex II defect is established by:

Measurement of enzymatic activity in muscle biopsies or other tissues
Molecular genetic analysis of the SDH genes
Immunohistochemistry to detect loss of SDH protein expression in tumor tissue
Measurement of succinate in blood or urine (elevated levels may indicate an SDH defect)

References

Rutter J, Winge DR, Schiffman JD. Succinate dehydrogenase - Assembly, regulation and role in human disease. Mitochondrion. 2010;10(4):393-401.
Baysal BE et al. Mutations in SDHD, a mitochondrial complex II gene, in hereditary paraganglioma. Science. 2000;287(5454):848-851.
Nicholls DG, Ferguson SJ. Bioenergetics 4. Academic Press, 2013.

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