Oxidative Phosphorylation & Electron Transport Flashcards

Question

What does the Complex III (Cytochrome c reductase / cytochrome _bc1) do?

Answer 1

Complex III mediates electron transport from CoQ to cytochrome c. This cycle is known as the Q cycle. Complex III also transports two protons across the inner mitochondrial membrane.

Answer 2

Complex IV transfers electrons from cytochrome c to reduce oxygen in the mitochondrial matrix. Cytochrom c oxidase and oxygen are the final destination for the electrons in the ETC. The combined processes of oxygen reduction and proton transport require eight protons. Four are used to reduce two molecules of oxygen (2 per cycle) and four are transported from the matrix to the intermembrane space (2 per cycle) Proton transport in Complex IV takes place via two channels that are lined with water molecules and polar amino acids.

Answer 3

The electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates a proton gradient. This gradient is used by the ATP synthase complex to make ATP via oxidative phosphorylation. The F_O component of ATP synthase acts as an ton channelthat provides for a proton flux back into the mitochondrial matrix. This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD⁺ and Q). The free energy is used to drive ATP synthesis, catalyzed by the F₁ component of the complex. Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin—present in the inner mitochondrial membrane of brown fat—provides for an alternative flow of protons back to the inner mitochondrial matrix. This alternative flow results in thermogenesis rather than ATP production. Synthetic uncouplers (e.g., [2,4-dinitrophenol](http://en.wikipedia.org/wiki/2,4-Dinitrophenol "2,4-Dinitrophenol")) also exist, and, at high doses, are lethal.

Answer 4

Cytochrome C is a mobile electron carrier. Electrons are passed from Complex III to cytochrome c. It is associated with the inner mitochondrial membrane where it carries electrons in its reduced state to to cytochrome c oxidase.

Answer 5

ATP synthase is a combined enzyme, proton pump, and rotating molecular motor. It is made up of the F₁ subunit and the F₀ subunit.

Answer 6

The F₁ subunit of the ATP synthase is the catalytic subunit of the ATP synthase; it catalyzes ATP production. The F₁ subunit is made up of five modules: Alpha, beta, gamma, delta, and epsilon. The F₁ portion of the ATP synthase is above the membrane, inside the matrix of the mitochondria. The alpha and beta subunits are arranged in alternating patterns in the hexamer and are similar but not identical. There are six ATP-binding sites on this subunit hexamer. Three of these, located on the beta subunit, ar ethe catalytic sites. The other three, located on the alpha subunit, are non-catalytic. The release of newly synthesized ATP is what powers the subunits catalytic activity.

Answer 7

The F₀ subunit serves as the attachment point for the F₁ subunit. It is made up three hydrophobic subunits (A, B, and C) and six additional subunits, [d](http://en.wikipedia.org/wiki/ATP5H "ATP5H"), [e](http://en.wikipedia.org/wiki/ATP5I "ATP5I"), [f](http://en.wikipedia.org/wiki/ATP5J2 "ATP5J2"), [g](http://en.wikipedia.org/wiki/ATP5L "ATP5L"), F6, and [8.](http://en.wikipedia.org/wiki/MT-ATP8 "MT-ATP8") The a and b subunits form part of the stator. This is a stalk-like structore that is anchored into the membrane and connects to the F₁ subunit. There is also a ring composed of 1-15 subunits; this makes up the rotor of the motor. Protons flowing through the a-c complex cause the c-ring to rotate in the membrane. The flow of protons through this subunit drives the conformational changes that result in the binding of substrate on the ATP synthase, ATP synthesis, and release of product (ATP).

Answer 8

The proton-motive force across the inner mitochondrial membrane, generated by the electron transport chain, drives the passage of [protons](http://en.wikipedia.org/wiki/Proton "Proton") through the membrane via the F_O region of ATP synthase. A portion of the F_O (the ring of [c-subunits](http://en.wikipedia.org/wiki/ATP_synthase_subunit_C "ATP synthase subunit C")) [rotates](http://en.wikipedia.org/wiki/Rotating_locomotion_in_living_systems "Rotating locomotion in living systems") as the protons pass through the membrane. The [c-ring](http://en.wikipedia.org/wiki/ATP_synthase_subunit_C "ATP synthase subunit C") is tightly attached to the asymmetric central stalk (consisting primarily of the gamma subunit), which rotates within the alpha₃beta₃ of F₁ causing the 3 catalytic nucleotide binding sites to go through a series of conformational changes that leads to ATP synthesis. The binding change mechanism involves the active site of a β subunit's cycling between three states.In the "open" state, ADP and phosphate enter the active site. The protein then closes up around the molecules and binds them loosely — the "loose" state. The enzyme then undergoes another change in shape and forces these molecules together, with the active site in the resulting "tight" state (shown in pink) binding the newly produced ATP molecule with very high [affinity](http://en.wikipedia.org/wiki/Dissociation_constant "Dissociation constant"). Finally, the active site cycles back to the open state, releasing ATP and binding more ADP and phosphate, ready for the next cycle of ATP production.

Answer 9

Hazardous compounds such as superoxides and free radicals would be produced.

Answer 10

Inhibitors of Complex I, II, and III block electron transport: a) Rotenone inhibits NADH-UQ oxidoreductase in Complex I; this prevents the reduction of CoQ and the oxidation of the Fe-S clusters of NADH-UQ oxidoreductase. b) Blocking the activities of Q cytochrome c oxidoreductase prevents passage of protons on to cytochrome c c) Cyanide, azide, and carbonmonoxide inhibit Complex IV (Cytochrome C oxidase): This inhibits passing protons on to O₂ to produce water. d) ATP synthase inhibitors (Oligomycin and DCCD) block the movement of protons through the F_o subunit. No ATP synthesis occurs. This proves that electron transport and ATP synthysis is tightly coupled.

Answer 11

Uncouplers disrupt the coupling of electron transport and ATP synthesis. They dissapate the proton gradient across the inner mitochondrial membrane that is created by the ETC. Examples include: 2,4-DNP They are hydrophobic and have a dissociable proton. They carry protons across the inner membrane to the matrix. In otherwords, protons leak back in via the uncouplers and so ATP synthasis does not occur. Increased O₂ consumption and NADH oxidation. The energy released in electron transport in this case is released as heat.

Answer 12

Instead of ATP being synthesized by ATP synthase, it is hydrolyzed.

Answer 13

Uncouple oxid. Phos. from ATP synthesis to generate heat. In response to a temp drop, the release of hormones leads to the liberation of free fatty acids from triacylglycerols that in turn activate uncoupling protein 1 (UCP-1). UCP-1 forms a pathway for the flow of H+ from the cytoplasm to the matrix. The tissue with high UCP-1 called brown fat. (combination of greenish cytochromes and red hemoglobin in blood supply)

Answer 14

2.7 protons are required to make each ATP.

Answer 15

ADP enters the mitochondrial matrix only if ATP exits. ATP movement out of the matrix is favored due to the negative charge on the ATP; the cytosol is + relative to the matrix. One ATP out requires the energy from a hydrogen. Thus, making and transporting one ATP requires four hydrogens. ATP-ADP translocase tightly couples the movement of of ATP out of the matrix with the movement of ADP into the matrix.

Answer 16

NADH-Q reductase - 4 Q-cytochrome C oxidoreductase - 2 Cytochrome c oxidase - 4 Thus 2.5 molecules of cytoplasmic ATP are generated as a result of the flow of a pair of electrons from NADH to O2 and those from the oxidation of succinate, the yield is about 1.5 molecules of ATP per electron pair.

Answer 17

a) Glycerophosphate shuttle: Two different glycerophosphate dehydrogenases (one in the cytosol and one on the outer face of the mitochondrial inner membrane) work together to carry electrons into the matrix of the mitochondrial matrix. NADH produced in the cytosol transfers its electrons to dihydroxyacetone phosphate and it is reduced to glycerol-3-phosphate. It is reoxidized FADH₂ and its two protons are passed to UQ to form UQH. This yields 1.5 molecules of ATP. This mode is ireversible and operates when NADH levels are low. b) Malate-Aspartate shuttle: Oxaloacetate is reduced in the cytosol and thus acquires the electrons from NADH (which is oxidized NAD⁺). Malate is transported across the inner membrane where it is reoxidized by malate dehydrogenase converting NAD⁺ to NADH in the matrix. This NADH enters the ETC. The oxaloacetate is transaminated to form aspartate and is transported back to the cytosolic face of the mitochondrial matrix.