Electron Transport and Oxidative Phosphorylation - Biochemistry

The electron transport chain moves high energy electrons through its complexes in order to create a proton gradient across the mitochondrial inner membrane. The ATP synthase then uses this gradient to pass hydrogen atoms through it. Because this is a favorable movement, it can be coupled to unfavorable processes such as conversion of ADP to ATP.

ATP synthase catalyzes the reaction that shows ADP and the phosphate group forming ATP. The hydrogen in the reactant side is the one involved in the proton gradient, and water is a byproduct of the reaction.

ATP synthase is located in the inner mitochondrial membrane. It has an F0 portion within the membrane and an F1 portion in the matrix. The F1 portion has a hexameric ring structure and is responsible for the creation of ATP from mechanical energy. The alpha, beta, and gamma subunits are all parts of the F1 portion of ATP synthase, however it is only the alpha and beta subunits that form the ring. Further, the beta subunit is the part of the ring that is considered to be catalytic.

Reduction potential refers to the spontaneity of the reduction half reaction. Remember that reduction refers to a gain of electrons. Thus, reduction potential is similar to the property of electronegativity. It can also be thought of a molecule's tendency to gain electrons or as a measure of its unwillingness to give up electrons.

Since oxygen is the final electron acceptor in the electron transport chain, we know that the reduction of oxygen is highly spontaneous (highly positive E, and highly negative G). It is this reason that the electrons from NADH and FADH2 must be passed step-wise to oxygen. Otherwise, there is such a large release of energy that too much would be lost to heat and become unavailable to do work for the cell.

In this question, we're asked to determine which scenario would cause a reduction in the amount of produced by mitochondria.

First, let's start with and . Both of these cofactors serve as high-energy electron carriers, which donate their electrons into the mitochondrial electron transport chain to ultimately produce . Therefore, high levels of these cofactors would not be expected to reduce production.

Next, let's consider the effect of a higher pH in the matrix than in the intermembrane space. When the above mentioned cofactors donate their electrons into the electron transport chain, protons are actively pumped from the matrix into the intermembrane space. The result of this is that the intermembrane space becomes significantly more acidic than the matrix. This is needed, because the protons are then able to spontaneously flow down their proton gradient to produce . Therefore, we would expect that a higher pH (more basic) in the matrix is the equivalent to saying that the intermembrane space has a lower pH (more acidic). Consequently, this lower pH in the intermembrane space would be expected to produce rather than inhibit its production.

Finally, lets consider how the concentration of affects production. In order to produce via the electron transport chain, needs to be phosphorylated. Therefore, if there is not much around to phosphorylate, then we would expect that most of the cell's adenosine is already in the form of . Thus, we would expect low concentrations to reduce production.

The combustion of glucose to yield carbon dioxide and water refers to aerobic metabolism (oxidative phosphorylation). This process releases energy, so Gibbs free energy is negative. A negative Gibbs free energy indicates that the products are at a lower energy than the reactants, meaning that the reaction is thermodynamically favorable (spontaneous). Lastly, aerobic metabolism is called so because it requires oxygen. Thus, all of the answers are correct.

The complexes that function as a part of the electron transport chain accept electrons from and . When they accept the high energy electrons, they also pump hydrogens across the mitochondrial inner membrane to ultimately be used at the ATP synthase. Creation of ATP is the end goal, but it is not made directly by the electron transport chain complexes.

Oxygen is required for the electron transport chain to function because it is the final electron acceptor for oxidative respiration. Once the high energy electron carriers, and , have delivered electrons to the chain, the electrons run through the protein complexes. When they finish moving through all of the complexes, something must be available for them to attach to. Oxygen is the molecule responsible for this. It accepts the electrons and in turn is converted into .

Complex I (NADH-CoQ reductase) contains iron-sulfur proteins, and complex II (succinate-CoQ reductase) contains both heme and iron-sulfur proteins. Thus, iron deficiency would compromise the function of complex I and II. The other enzyme complexes do not have iron-containing proteins, thus, they would not be impaired by an iron deficiency.

Complex IV (cytochrome oxidase) contains two copper centers, and , thus a copper deficiency would result in loss of function of enzyme complex IV. The other enzyme complexes do not contain copper, thus, they would not be impaired by a copper deficiency.

Complex II of the electron transport chain catalyzes the following reaction:

It uses the enzyme succinate dehydrogenase_._ The immediate result of this complex's loss of function would be a buildup of succinate, since that molecule can no longer be oxidized to fumarate. The multitude of problems that can arise come from this crucial step of the citric acid cycle not being able to move forward.

The conversion of succinate to fumarate is the only reaction that occurs outside of the normal Krebs cycle. Complex II of the electron transport chain has an enzyme known as succinate dehydrogenase. This enzyme is responsible for the conversion of succinate to fumarate. Fumarate is return to the cycle where it is then oxidized to malate continuing the cycle. Each of the other reactions of the Krebs cycle listed all occur in the inner mitochondrial matrix; whereas the conversion of succinate to fumarate occurs at the inner mitochondrial membrane.

ATP synthase uses the proton gradient across the inner membrane to generate ATP. The ATP synthase is essentially like a rotary motor. The proton gradient serves as the priming of the ATP synthase. As proton are moved from the outer mitochondrial matrix back into the mitochondrial matrix they are providing mechanical energy to turn the pump. As the pump is being turned ATP synthase utilizes a unit of ADP and inorganic phosphate to generate one molecule of ATP. This is done for every three turns of the ATP synthase.

Complex I is also called NADH-Coenzyme Q (CoQ) reductase because it transfers 2 electrons from NADH to CoQ. Complex I was formerly known as NADH dehydrogenase. This complex binds NADH and takes up two electrons.The last step of this complex is the transfer of two electrons one at a time to CoQ. The process of transferring electrons from NADH to CoQ by complex I results in the overall transport of protons from the matrix side of the inner mitochondrial membrane to the inter membrane space where the hydrogen ion concentration increases generating a proton motive force which is utilized by ATP synthase.

Complex II of the electron transport chain is generally apart of both the electron transport chain as well as the Krebs cycle. It is the the succinate dehydrogenase that carried out the conversion of succinate to fumarate in the Krebs cycle. The only enzyme of the citric acid cycle that is an integral membrane protein. The conversion of succinate to fumarate generates an . then transfers its electrons one at a time through complex II. The final step of this complex is the transfer of two electrons one at a time to coenzyme Q.

Complex IV is also known as cytochrome c oxidase because it accepts the electrons from cytochrome c and directs them towards the four electron reduction of oxygen to form two molecules of water. ATP synthase is directly responsible for the generation of ATP by utilizing one unit of ADP and one unit of inorganic phosphate along with the proton motive force (PMF). Complex II is also known as succinate dehydrogenase which is responsible for one of the reaction of the Krebs cycle: succinate to fumarate. This reaction generates one molecule of . Complex I is also known as dehydrogenase in that it oxidizes the coenzyme .

Complex II of the electron transport chain is generally apart of both the electron transport chain as well as the Krebs cycle. It is the the succinate dehydrogenase that carried out the conversion of succinate to fumarate in the Krebs cycle. The only enzyme of the citric acid cycle that is an integral membrane protein. The conversion of succinate to fumarate generates an . then transfers its electrons one at a time through complex II. The final step of this complex is the transfer of two electrons one at a time to coenzyme Q.

Oxidative phosphorylation takes place in the mitochondria in a eukaryote. The process is made possible by the double membrane within the mitochondria.

Ubiquinone functions to carry electrons in oxidative phosphorylation from the first enzyme complex to the second enzyme complex. It does not receive electrons from nor directly.