Oxidative Phosphorylation 4 - Online Tutor, Practice Problems & Exam Prep

The electron transport chain in mitochondria creates a proton motive force, an electrochemical gradient essential for ATP synthesis via oxidative phosphorylation. This process differs from substrate-level phosphorylation in glycolysis and the citric acid cycle. ADP and inorganic phosphate enter the mitochondrial matrix through specific transporters, utilizing the proton gradient. ATP synthase, a molecular motor, converts this energy into ATP by undergoing conformational changes in its subunits, driven by the rotation of its F0 portion. The release of ATP requires energy from the proton motive force, culminating in efficient ATP production.

Oxidative Phosphorylation 4

So the whole point of performing electron transport, right, where we are moving electrons through the inner mitochondrial membrane through these complexes and pumping all of these protons, the whole point of this is to build a proton gradient, called the proton motive force. And this is an electrochemical gradient meaning that it has both an electrical component and a concentration component. This gradient is actually going to power the enzyme ATP synthase which drives the synthesis of ATP from ADP and inorganic phosphate. This is different from the substrate level in the glycolytic and citric acid cycle reactions. This is what is known as the proverbial oxidative phosphorylation. You have substrate level phosphorylation and this right here is oxidative phosphorylation. Before we get to how ATP synthase works, it's important to think about where the ingredients come from. Now we know how the proton motive force is built. We just talked about that extensively, but how does all the ADP and inorganic phosphate get into the mitochondrial matrix? Well, ADP comes in through an antiporter called adenine nucleotide translocase. It moves ADP in while it takes ATP out. Brilliant antiporter that moves these molecules in and out of the mitochondria for essentially zero energy cost. Now, phosphate translocase brings in the inorganic phosphate that you need. It does that by using the proton gradient. It's taking advantage of the proton motive force that ATP synthase uses to bring inorganic phosphate in. It is a symporter. It pulls in a proton as it pulls in inorganic phosphate. Super eloquent. Super beautiful. I love it when things work out so simply in biology.

ATP synthase. This is probably one of the craziest enzymes you'll ever look at. That's because it is essentially a molecular motor more or less. You have two portions of it; you have the F₀ portion which is going to be embedded in that inner mitochondrial membrane. That's right here. And then you have the F₁ portion. The F₀ portion spins as you can see right here; it spins around. The F₀ portion spins around and it's connected to one of the subunits of the F₁ portion. it's connected to the gamma subunit. And I don't know if you know anything about cars but the gamma subunit is basically a driveshaft. It gets spun around by F₀ and it causes conformational changes in these alpha and beta subunits. And it's within these beta subunits that the magic happens. That's where the ATP is synthesized. The F₁ portion, as I said, experiences conformational changes due to the rotation of the driveshaft, or the rotation of the gamma subunit. Basically, at any one time, each of the 3 beta subunits is in a particular conformation. One is always open, one is always loose, and one is always tight. And what that means is, when it's open, it can release ATP and it can take in ADP and inorganic phosphate. Then when it's in the loose conformation, the enzyme is going to hold those two substrates together in such a way that they form ATP. And that is when it becomes or enters the tight position where it is tightly bound to ATP. This is what requires the energy of the proton motive force. It's not actually this part; the energy is mostly needed to release this tightly bound ATP. It's that shift from the tight state to the open state that is most difficult. You can see that the gamma moves around counter-clockwise. So it spins around counter-clockwise and it causes these conformational shifts in the subunits that cause them to synthesize the ATP. Again, it's actually the release of the ATP that really needs the energy of the proton motive force. This whole process leads to this moment of oxidative phosphorylation. It's been a heck of a journey. We're not quite over yet. So, let's turn the page and finish up talking about photophosphorylation, which is a really cool related process to this.

Here’s what students ask on this topic:

What is the proton motive force and how is it generated in oxidative phosphorylation?

The proton motive force (PMF) is an electrochemical gradient created across the inner mitochondrial membrane during oxidative phosphorylation. It consists of two components: a chemical gradient (difference in proton concentration) and an electrical gradient (difference in charge). The PMF is generated by the electron transport chain (ETC), which transfers electrons through a series of complexes (I, II, III, and IV). As electrons move through these complexes, protons (H⁺) are pumped from the mitochondrial matrix to the intermembrane space, creating the gradient. This gradient is essential for ATP synthesis, as it powers ATP synthase to convert ADP and inorganic phosphate into ATP.

How does ATP synthase utilize the proton motive force to produce ATP?

ATP synthase is a molecular motor embedded in the inner mitochondrial membrane. It consists of two main parts: the F₀ portion, which spans the membrane, and the F₁ portion, which protrudes into the mitochondrial matrix. The proton motive force drives protons through the F₀ portion, causing it to rotate. This rotation is transferred to the F₁ portion via the gamma subunit, which acts like a driveshaft. The conformational changes in the F₁ portion's alpha and beta subunits facilitate the binding of ADP and inorganic phosphate, their conversion to ATP, and the release of ATP. The energy from the proton motive force is primarily used to release the tightly bound ATP from the enzyme.

What roles do adenine nucleotide translocase and phosphate translocase play in oxidative phosphorylation?

Adenine nucleotide translocase and phosphate translocase are crucial for supplying the substrates needed for ATP synthesis in oxidative phosphorylation. Adenine nucleotide translocase is an antiporter that exchanges ADP from the cytosol with ATP from the mitochondrial matrix, ensuring a continuous supply of ADP for ATP synthesis. Phosphate translocase is a symporter that uses the proton motive force to import inorganic phosphate (P_i) into the mitochondrial matrix along with a proton (H⁺). These transporters work together to maintain the necessary levels of ADP and P_i in the matrix for efficient ATP production.

What are the conformational states of the beta subunits in ATP synthase, and how do they contribute to ATP synthesis?

The beta subunits of ATP synthase cycle through three conformational states: open, loose, and tight. In the open state, the subunit can release ATP and bind ADP and inorganic phosphate. In the loose state, the subunit holds ADP and inorganic phosphate in close proximity. In the tight state, the subunit catalyzes the formation of ATP from ADP and inorganic phosphate. The energy from the proton motive force is used to transition the beta subunit from the tight state, where ATP is tightly bound, to the open state, where ATP is released. This conformational cycling is driven by the rotation of the gamma subunit, which is powered by the flow of protons through the F₀ portion of ATP synthase.

How does oxidative phosphorylation differ from substrate-level phosphorylation?

Oxidative phosphorylation and substrate-level phosphorylation are two distinct mechanisms for ATP production. Oxidative phosphorylation occurs in the mitochondria and involves the electron transport chain and ATP synthase. It relies on the proton motive force generated by the electron transport chain to drive ATP synthesis. In contrast, substrate-level phosphorylation occurs in the cytoplasm during glycolysis and in the mitochondrial matrix during the citric acid cycle. It involves the direct transfer of a phosphate group from a high-energy substrate molecule to ADP, forming ATP. Unlike oxidative phosphorylation, substrate-level phosphorylation does not require a proton gradient or the electron transport chain.