Central Metabolism: Electron Transport Chain and Chemiosmosis

Vocabulary practice flashcards covering cellular respiration topics from the lecture, including glycolysis regulation, Krebs cycle intermediates, electron transport chain complexes, chemiosmosis, and oxidative phosphorylation energetics.

Hexokinase

The enzyme that phosphorylates glucose upon its entry into the cell to produce glucose-6-phosphate during the first step of glycolysis.

Phosphofructokinase (PFK)

A major regulatory glycolytic enzyme controlling the step from six carbons to three carbons; serves as a site of negative feedback inhibited by high levels of ATP and citrate, and stimulated by ADP and AMP.

Substrate-level phosphorylation

A mechanism of ATP or GTP synthesis where a phosphate group is transferred directly from a phosphorylated metabolic intermediate to ADP or GDP, as observed in glycolysis and the Krebs cycle.

Acetyl CoA

A central metabolic hinge-point molecule formed in the mitochondrial matrix by coupling an acetyl group to coenzyme A; acts as the primary fuel for the Krebs cycle and the product of fatty acid breakdown.

Oxaloacetate

A four-carbon compound in the mitochondrial matrix that condenses with acetyl CoA to form the six-carbon molecule citrate in the first step of the Krebs cycle.

Alpha-ketoglutarate

A five-carbon intermediate in the Krebs cycle formed from isocitrate through the loss of a carbon as $CO_2$ and the reduction of $NAD^+$ to $NADH + H^+$.

Cristae

The infoldings or invaginations of the inner mitochondrial membrane that provide extensive surface area to accommodate high concentrations of electron transport chain complexes and ATP synthases.

Complex I (NADH dehydrogenase complex)

The initial multiprotein complex of the electron transport chain that extracts high-energy electrons from NADH, regenerates $NAD^+$, and uses the released exergonic energy to pump protons from the matrix into the intermembrane space.

Iron-sulfur clusters

Inorganic prosthetic groups embedded in electron transport chain complexes that utilize specialized inorganic chemistry to carry and pass high-energy electrons between proteins.

Ubiquinone (Coenzyme Q)

A mobile electron carrier that shuttles electrons through the inner mitochondrial membrane from Complex I and Complex II to Complex III.

Complex III (Cytochrome $bc_1$ complex)

The electron transport chain complex that accepts electrons from ubiquinone, pumps protons into the intermembrane space, and transfers the electrons to cytochrome c.

Cytochrome c

A mobile intermediate electron carrier located on the intermembrane side that transfers electrons from Complex III to Complex IV.

Complex IV (Cytochrome c oxidase / Cytochrome $aa_3$ complex)

The terminal electron transport chain complex that oxidizes cytochrome c, pumps protons across the inner mitochondrial membrane, and passes the electrons to oxygen.

Complex II (Succinate dehydrogenase)

A membrane-bound Krebs cycle enzyme that oxidizes succinate to fumarate and directly transfers electrons from $FADH_2$ to ubiquinone without pumping protons into the intermembrane space.

Metabolic water

Water ($H_2O$) produced at the end of the electron transport chain when molecular oxygen acts as the final electron acceptor and combines with protons.

Chemiosmosis

The process by which the energy stored in an electrochemical proton gradient across the inner mitochondrial membrane is utilized by ATP synthase to drive ATP phosphorylation.

Peter Mitchell

The Nobel laureate who was awarded the Nobel Prize in the late 1970s for discovering the chemiosmotic hypothesis coupling proton gradients to ATP synthesis.

Proton-motive force

The combined electrochemical driving force pushing protons back into the matrix, composed of a minor chemical concentration gradient (roughly $1$ pH unit) and a major electrostatic potential.

$F_0$ domain

The hydrophobic, membrane-embedded rotor domain of ATP synthase containing c subunits that rotate like horses on a carousel as protons bind and traverse the inner membrane.

$F_1$ catalytic domain

The matrix-facing portion of ATP synthase that harnesses mechanical rotational energy delivered by the rotor domain to overcome the energy barrier and chemically synthesize ATP from ADP and inorganic phosphate.

ATP/ADP antiporter

A dedicated inner mitochondrial membrane transport protein that exports newly synthesized ATP from the matrix to the cytoplasm while importing ADP into the matrix.

Oxidative phosphorylation

The predominant metabolic pathway for ATP production in aerobic cells, consisting of the coupled actions of the electron transport chain and chemiosmosis.

Secondary active transport (mitochondrial)

The process of using the potential energy of the preexisting proton gradient to drive the transport of solutes, such as pyruvate and inorganic phosphate, into the mitochondrial matrix against their gradients.

Beta-oxidation

The metabolic pathway in the mitochondrial matrix that breaks down fatty acids into two-carbon units of acetyl CoA, which subsequently enter the Krebs cycle.