Biological Oxidation and Electron Transport Chain

Comprehensive vocabulary flashcards covering the components, mechanisms, regulation, and clinical pathologies of the electron transport chain and oxidative phosphorylation.

Electron Transport Chain (ETC)

A series of protein complexes in the inner mitochondrial membrane that catalyze oxidation-reduction reactions to generate a proton gradient between the matrix and the intermembrane space.

ATP Synthase ($F_oF_1$)

A protein complex on the inner mitochondrial membrane that harnesses the energy of the proton gradient to phosphorylate ADP into ATP.

Inner Mitochondrial Membrane

A membrane impermeable to most small molecules and ions, including $H^+$, containing respiratory electron carriers (Complexes I-IV), ATP synthase, and ADP-ATP translocase.

Mitochondrial Matrix

The internal space of the mitochondria containing the pyruvate dehydrogenase complex, citric acid cycle enzymes, fatty acid $\beta$-oxidation enzymes, and mitochondrial DNA.

Complex I (NADH:CoQ oxidoreductase)

A 42-subunit complex that contains a binding site for NADH, several FMN and Fe–S center-binding proteins, and binding sites for CoQ.

Complex II (Succinate Dehydrogenase)

A component of both the TCA cycle and the ETC that passes electrons from succinate to CoQ without pumping protons across the membrane.

Coenzyme Q (Ubiquinone)

A lipid-soluble component of the ETC that can diffuse through the inner mitochondrial membrane to shuttle electrons between complexes.

Cytochromes

Proteins containing a bound heme with iron ($Fe^{2+}/Fe^{3+}$) that transfer electrons along the ETC; examples include cytochrome complex $b-c_1$ and cytochrome oxidase ($aa_3$).

Cytochrome oxidase (Complex IV)

The final cytochrome complex that passes electrons to $O_2$; it contains cytochromes $aa_3$ and bound copper ($Cu^{2+}$) ions to facilitate the reduction of $O_2$ to $H_2O$.

Chemiosmotic Theory

Postulated by Peter Mitchell in 1961, it states that the electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane.

Adenine nucleotide translocase

A transporter responsible for the unidirectional antiport exchange of ATP moving out of the matrix for ADP moving in.

P/O ratio

The ratio of ATP formed per oxygen atom reduced; it is $2.5$ for NADH and $1.5$ for FADH2.

Uncouplers

Agents like 2,4-dinitrophenol (2,4-DNP) or aspirin that decrease the proton gradient, resulting in decreased ATP synthesis, increased oxygen consumption, and heat release.

Thermogenin (UCP)

A natural uncoupling protein in brown adipose tissue that generates heat to maintain basal temperature in newborns.

Cyanide

A poison that binds irreversibly to cytochrome $aa_3$, preventing electron transfer to oxygen and inducing effects similar to tissue hypoxia.

Reactive Oxygen Species (ROS)

Byproducts of incomplete oxygen reduction including superoxide ($O_2\bullet$), hydrogen peroxide ($H_2O_2$), and hydroxyl radicals ($OH\bullet$) that damage DNA, proteins, and lipids.

Caspases

A family of proteolytic enzymes activated by cytochrome c when it enters the cytosol, leading to the changes characteristic of apoptosis.

MELAS

Mitochondrial encephalopathy, lactic acidosis, and stroke; an inherited condition due to Complex I or Complex IV deficiency caused by mutations in mitochondrial DNA.

Leber’s hereditary optic neuropathy

A condition caused by mtDNA point mutations disrupting Complex I, leading to bilateral vision loss in young adults.

MERRF

Myoclonic epilepsy with ragged red fibers; a mitochondrial disorder characterized by epilepsy, short stature, and hearing loss, with muscles showing “ragged-red fibers” on biopsy.

Glycerol-3-phosphate shuttle

A substrate shuttle present in skeletal muscle and brain that transfers reducing equivalents from cytosolic NADH to mitochondrial FAD.

Malate-aspartate shuttle

A substrate shuttle present in the liver and kidney that transfers reducing equivalents from cytosolic NADH to mitochondrial NAD.