oxidative phosphorylation

Oxidative Phosphorylation and the Electron Transport System

Overview

This section focuses on oxidative phosphorylation and the electron transport system (ETS). The previous section covered the conversion of pyruvate to acetyl CoA and the reduction of cofactors NADH and FADH2 during the tricarboxylic acid (TCA) cycle. Oxidative phosphorylation serves to reoxidize these cofactors and is vital for ATP production.

Dioxygen Reduction

Molecular oxygen (O2) is ultimately reduced to water (H2O) in this process, generating a proton gradient which is utilized for ATP synthesis. This reduction occurs at the end of the electron transport chain and is critical for the final steps of cellular respiration.

Electron Transport System (ETS)

Location and Function

The ETS occurs within the mitochondria, specifically embedded in the inner mitochondrial membrane. This strategic location facilitates the efficient transfer of electrons from NADH and FADH2 to a series of electron acceptors, which are predominantly proteins or enzyme complexes known as complexes I through IV.

Process Overview

Electrons are transferred from NADH and FADH2 sequentially through various intermediate electron acceptors, culminating in the reduction of dioxygen to water. This electron transport not only recycles NAD+ and FAD but is also essential for generating a proton gradient necessary for ATP synthesis, which is crucial for energizing cellular processes.

ATP Generation

The electron transport system is divided into two main components:

1. Generation of the Proton Gradient: As electrons move through the chain, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, establishing an electrochemical gradient.

2. Utilization of the Proton Gradient to Synthesize ATP: The proton gradient drives protons back into the mitochondrial matrix through ATP synthase, enabling the synthesis of ATP from ADP and inorganic phosphate (Pi). This entire process occurs exclusively during aerobic respiration and significantly contributes to overall ATP production - approximately 26 ATP molecules are generated via oxidative phosphorylation out of a total of 30 ATP produced from the complete oxidation of one glucose molecule.

Key Components of the Electron Transport Chain

• Complex I (NADH dehydrogenase): Accepts electrons from NADH, facilitating the pumping of protons into the intermembrane space.

• Complex II (Succinate dehydrogenase): Accepts electrons from FADH2 yet does not contribute to proton pumping.

• Complex III (Cytochrome bc1 complex): Further transfers electrons to cytochrome c and contributes to proton gradient formation.

• Complex IV (Cytochrome c oxidase): Final acceptor of electrons and catalyzes the reduction of O2 to H2O, completing the chain.

Energy Considerations

A significant potential difference of 1.14 volts is created between NADH and dioxygen, correlating to an energy release of about 220 kJ/mol. This energy is instrumental for ATP synthesis and provides a stark contrast to the 31.4 kJ/mol released during ATP hydrolysis, emphasizing the efficiency of cellular respiration.

Proton Gradient Formation

The pumping of protons into the intermembrane space generates a proton gradient characterized by a pH difference of about 1.14 units, resulting in a concentration difference of 10 to 100 times between the intermembrane space and the matrix. This gradient is the driving force behind ATP synthesis.

ATP Formation Mechanism

Mechanism of ATP Synthesis

The formation of ATP is closely linked to the re-entry of protons into the mitochondrial matrix via ATP synthase, an enzyme that facilitates this process. According to the Mitchell hypothesis (proposed in the early 1960s), the flow of protons back into the matrix powers the synthesis of ATP from ADP and Pi.

Correction to the Hypothesis

Initially, it was believed that the presence of protons was essential for ATP synthesis; however, current understanding posits that ATP formation occurs spontaneously. The proton gradient assists in the substrate binding of ADP and Pi, while protons also help in releasing newly formed ATP from ATP synthase.

Illustrative Explanation

As electrons traverse the transport chain, protons are actively pumped into the intermembrane space, thereby enhancing the proton gradient. ATP synthase utilizes this gradient, rotating and undergoing conformational changes that facilitate the release of bound ATP molecules.

Summary

The complete degradation of glucose yields a substantial quantity of ATP, predominantly produced through the electron transport system, which efficiently reoxidizes NADH and FADH2. In addition to ATP created via oxidative phosphorylation, there are also contributions from intermediate steps in the tricarboxylic acid cycle, such as the conversion of GTP to ATP. The next section will delve into the storage of excess glucose as glycogen, ensuring the availability of energy sources for later cellular needs.