Practice: Oxidative Phosphorylation 2 - Online Tutor, Practice Problems & Exam Prep

Electron transport in mitochondria releases protons into the intermembrane space, creating a proton motive force essential for ATP synthesis via ATP synthase. Mitochondria contain about 9,100 proteins encoded by nuclear DNA, while their genes are maternally inherited. Uncoupling agents like DNP and FCCP inhibit ATP production by dissipating the proton gradient, leading to a decreased phosphate-oxygen (PO) ratio, as oxygen consumption continues without effective ATP synthesis. This highlights the importance of the chemiosmotic theory in understanding mitochondrial function.

Practice

Of the following statements, only statement a is true: electron transport through the inner mitochondrial membrane results in the release of protons on the outside into the intermembrane space. The proton motive force causes a conformational change in ATPase, and that is what allows it to synthesize ATP from ADP. The F₀ portion of the protein spins, causing the gamma subunit of the F₁ portion to spin around and cause conformational changes in the beta, alpha, and beta subunits, which leads to the formation and release of ATP.

Now, which of the following about human mitochondria is true? About 9,100 mitochondrial proteins are encoded in the nucleus, not the mitochondria. Mitochondrial genes actually come exclusively from the mother; it is one of the forms of non-Mendelian inheritance. Additionally, mitochondria do provide their own materials for protein synthesis, so they make their own ribosomes and their own transfer RNA. Now, mitochondrial genomes, as we just learned, do not encode all the mitochondrial proteins, and of course, it's a normal set of genes, so it's obviously subject to mutation—that is without question.

The addition of 2,4-Dinitrophenyl or DNP, as it's called, or FCCP to mitochondria carrying out oxidative phosphorylation inhibits ATP production, and these are uncoupling agents. So, they inhibit ATP production by dissipating the proton gradient and transporting protons across the membrane to dissipate the gradient. FCCP is an ionophore, actually. You might recall we learned about those in an earlier review. Now, what will happen to the P/O or phosphate/oxygen ratio of the mitochondria after the addition of these agents? The answer is that the ratio will decrease. Basically, what that means is that the amount of phosphate consumed to the amount of oxygen consumed is going to go down. The reason for this is that these uncoupling agents dissipate the proton gradient but they don't stop electron transport. Electron transport is going to continue, so oxygen is going to continue to be consumed normally. However, ATP synthase won't be able to function properly because it won't have the proton motive force necessary to carry out ATP synthesis. So, phosphate consumption is going to decrease. So, this P/O ratio is going to go down.

Here’s what students ask on this topic:

What is the role of the proton motive force in oxidative phosphorylation?

The proton motive force (PMF) is crucial in oxidative phosphorylation as it drives the synthesis of ATP. During electron transport, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient generates an electrochemical potential difference across the inner mitochondrial membrane. ATP synthase, an enzyme complex, utilizes this PMF to catalyze the conversion of ADP and inorganic phosphate (P_i) into ATP. The flow of protons back into the matrix through ATP synthase causes conformational changes in the enzyme, enabling the phosphorylation of ADP to ATP. Without the PMF, ATP synthesis would not occur efficiently.

How do uncoupling agents like DNP and FCCP affect oxidative phosphorylation?

Uncoupling agents such as DNP (2,4-Dinitrophenol) and FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone) disrupt oxidative phosphorylation by dissipating the proton gradient across the inner mitochondrial membrane. These agents transport protons back into the mitochondrial matrix without passing through ATP synthase, thereby reducing the proton motive force. As a result, ATP synthesis is inhibited because the necessary proton gradient is not maintained. However, electron transport and oxygen consumption continue, leading to a decreased phosphate-oxygen (P/O) ratio. This means that less ATP is produced per oxygen molecule consumed, highlighting the importance of the proton gradient in ATP production.

Why is the phosphate-oxygen (P/O) ratio important in oxidative phosphorylation?

The phosphate-oxygen (P/O) ratio is a measure of the efficiency of oxidative phosphorylation. It represents the number of ATP molecules synthesized per oxygen atom reduced in the electron transport chain. A high P/O ratio indicates efficient ATP production, while a low ratio suggests inefficiency. Uncoupling agents, which dissipate the proton gradient, lower the P/O ratio because they allow electron transport and oxygen consumption to continue without effective ATP synthesis. Understanding the P/O ratio helps in assessing the functionality of the mitochondrial electron transport chain and ATP synthase in energy metabolism.

How is mitochondrial DNA inherited and what is its significance?

Mitochondrial DNA (mtDNA) is maternally inherited, meaning it is passed down from the mother to her offspring. This form of inheritance is non-Mendelian, as it does not follow the typical patterns of inheritance seen with nuclear DNA. The significance of mtDNA lies in its role in encoding essential components of the mitochondrial electron transport chain and ATP synthesis machinery. Although most mitochondrial proteins are encoded by nuclear DNA, mtDNA provides critical genes for mitochondrial function. Mutations in mtDNA can lead to mitochondrial diseases, highlighting its importance in cellular energy metabolism and overall health.

What is the chemiosmotic theory and why is it important in understanding mitochondrial function?

The chemiosmotic theory, proposed by Peter Mitchell, explains how ATP is generated in mitochondria. According to this theory, the electron transport chain creates a proton gradient across the inner mitochondrial membrane by pumping protons from the matrix to the intermembrane space. This gradient generates a proton motive force, which drives ATP synthesis as protons flow back into the matrix through ATP synthase. The chemiosmotic theory is crucial for understanding mitochondrial function because it links the processes of electron transport and ATP synthesis, providing a comprehensive explanation of how cells produce energy efficiently.