Oxidative Phosphorylation

Introduction

Oxidative phosphorylation is a vital metabolic process that occurs within the mitochondria. It involves two closely connected components: the electron transport chain (ETC) and chemiosmosis. The ETC, located in the inner mitochondrial membrane, consists of proteins and organic molecules. Electrons from molecules like NADH and FADH₂ move along the chain through redox reactions, releasing energy. This energy is used to pump protons across the membrane, creating a proton gradient. ATP synthase, an enzyme, then converts ADP and Pi into adenosine triphosphate (ATP) via chemiosmosis. Oxygen, at the end of the ETC, accepts electrons and forms water. In summary, oxidative phosphorylation provides most of the cellular energy (ATP) needed for various life processes. In this article we will see oxidative phosphorylation & its mechanism and substrate phosphorylation.

Table of Contents

Oxidative phosphorylation occurs within the mitochondria, specifically in the inner mitochondrial membrane. It’s composed of two closely connected components as follows.

Electron Transport Chain (ETC)

The ETC consists of a series of proteins and organic molecules embedded in the inner membrane.

Here’s how it works:

Electrons are passed from one molecule to another through a series of redox reactions.
As electrons move along the chain, they release energy.
This energy is harnessed to create an electrochemical proton gradient across the membrane.

Chemiosmosis

The proton gradient generated during the ETC drives a remarkable process called chemiosmosis.

Here’s how it unfolds:

ATP synthase, a protein complex, utilizes the proton gradient.
ATP synthase acts like a tiny turbine, converting the proton flow back into the mitochondrial matrix into the synthesis of adenosine triphosphate (ATP).
ATP is the cell’s primary energy currency.

Why Oxygen Matters

Oxygen plays a pivotal role in oxidative phosphorylation:
It sits at the end of the electron transport chain.
Oxygen accepts electrons and combines with protons to form water (H₂O).

Without oxygen

The ETC cannot function properly.
ATP production via chemiosmosis comes to a halt.
Cells suffer from an energy crisis.
Prolonged oxygen deprivation can lead to cell dysfunction and even death.

Oxidative Phosphorylation source: wikimedia

Substrate level phosphorylation

Substrate-level phosphorylation is a metabolic reaction that results in the production of adenosine triphosphate (ATP) or guanosine triphosphate (GTP). Definition: During substrate-level phosphorylation, a phosphate group (PO₄³⁻) is directly transferred from a substrate to ADP (adenosine diphosphate) or GDP (guanosine diphosphate). This transfer occurs without the involvement of an electron transport chain or an external electron acceptor.

Energy Release and Utilization

The process is exergonic, meaning it releases some free energy.
As the phosphate group is removed from the substrate, this energy is harnessed to phosphorylate ADP or GDP, forming ATP or GTP.

Where It Occurs

Substrate-level phosphorylation takes place in two main contexts:

Glycolysis: In the cytoplasm of cells, during glycolysis, specific reactions lead to the production of ATP through substrate-level phosphorylation.
Mitochondria: In the mitochondria, either during the Krebs cycle or by an enzyme called MTHFD1L, which interconverts ADP, phosphate, and 10-formyltetrahydrofolate to ATP, formate, and tetrahydrofolate (under both aerobic and anaerobic conditions).

Importance

While most ATP is generated by oxidative phosphorylation (in aerobic or anaerobic respiration), substrate-level phosphorylation provides a quicker but less efficient source of ATP.

It’s essential in scenarios where external electron acceptors (like oxygen) are not available, such as in human erythrocytes (which lack mitochondria) or oxygen-depleted muscle.

Summary

Oxidative phosphorylation occurs within mitochondria, involving the electron transport chain (ETC) and chemiosmosis. Electrons from molecules like NADH move along the ETC, creating a proton gradient. ATP synthase harnesses this gradient to convert ADP and Pi into ATP. In contrast, substrate-level phosphorylation directly transfers a phosphate group from a substrate to ADP or GDP, producing ATP without the ETC. While oxidative phosphorylation is highly efficient, substrate-level phosphorylation provides a quick but less efficient source of ATP.

Introduction