Electron Transport and Oxidative Phosphorylation - Biochemistry

First, let's consider the half reactions involved to determine .

This overall reaction involves the donation of 2 electrons, so

is defined as . The reaction we drew earlier is shown below:

We can see that was oxidized and was reduced. Find .

is Faraday's constant, and is defined as:

Solve for

A positive Gibbs free energy implies that the process in question should be unfavorable under normal conditions. The only process listed that is unfavorable and requires an input of energy is the pumping of hydrogen ions into the intermembrane space. This occurs during the electron transport chain.

The phases of cellular respiration are glycolysis, pyruvate dehydrogenase complex, Krebs cycle, electron transport chain. Glycolysis produces a net total of 2 ATP, the Krebs cycle produces 1 GTP that is converted to ATP in another process, and the electron transport chain is where almost all of the ATP made in cellular respiration is formed. However, during the pyruvate dehydrogenase complex phase of cellular respiration, pyruvate is converted to acetyl-CoA as a preparation for the Krebs cycle, but no ATP is created.

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.

Complex I pumps 4 protons, complex IV pumps 4 protons, and the interaction between complex III and complex II is more complicated.

Complex II pumps no electrons in itself, but releases the fully reduced quinone species, , which interacts with complex III through the Q cycle. Simplified, the net result of the Q cycle is that 4 protons are pumped out into the intermembrane space. complex III pumps 2 protons from the mitochondrial matrix and 2 protons from .

This is a simplification of the 4 complexes, providing only the information necessary to complete the question. But a full understanding of the 4 complexes, and the flow of electrons is nonetheless essential for understanding why each complex pumps the number of protons it does.

first delivers its electrons to complex 2 of the electron transport chain. Subsequently, the electrons are delivered to ubiquinone, and then they move through complex 3, cytochrome C, and complex 4. Complex 2, therefore, is the only protein complex that directly accepts electrons from .

The fourth complex in the electron transport chain is unique in that it has the important responsibility of reducing molecular oxygen to water. Oxygen is the final electron acceptor for cellular respiration, so this is a very important role. Complex 2 is the only one that accepts electrons from and is the smallest of the protein complexes. Complex 2 is also the one that does not have hydrogen pumping ability. Iron is a component of complexes 1, 3, and 4.

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.