Question

Cytochrome oxidase and succinate-CoQ oxidoreductase are isolated from mitochondria and are incubated in the presence of oxygen, along with cytochrome \(c,\) coenzyme \(Q\) and succinate. What is the overall oxidation- reduction reaction that can be expected to take place?

Short answer

Succinate + 2 Cytochrome c (oxidized) + 1/2 O2 -> Fumarate + 2 Cytochrome c (reduced) + H2O

Step-by-step solution

Identify the enzymes involved

Cytochrome oxidase and succinate-CoQ oxidoreductase are the enzymes isolated from mitochondria. Cytochrome oxidase is responsible for the final step of the electron transport chain, whereas succinate-CoQ oxidoreductase (Complex II) facilitates the oxidation of succinate to fumarate and transfers electrons to coenzyme Q (CoQ).

Understand the role of substrates and cofactors

Cytochrome c, coenzyme Q and succinate are the substrates involved. Succinate is oxidized to fumarate by succinate-CoQ oxidoreductase, which transfers electrons to coenzyme Q. Cytochrome c acts as an electron carrier, and cytochrome oxidase (Complex IV) transfers electrons from cytochrome c to oxygen.

Write the reactions catalyzed by each enzyme

1. Succinate-CoQ oxidoreductase reaction:Succinate + CoQ -> Fumarate + CoQH22. Cytochrome oxidase reaction:4 Cytochrome c (reduced) + 4H+ + O2 -> 4 Cytochrome c (oxidized) + 2H2O

Combine the reactions to get the overall reaction

Combine the reactions catalyzed by each enzyme to get the overall oxidation-reduction reaction. The flow of electrons can be traced from succinate to CoQ, then from CoQH2 to cytochrome c, and finally from cytochrome c to oxygen.Overall reaction:Succinate + 2 Cytochrome c (oxidized) + 1/2 O2 -> Fumarate + 2 Cytochrome c (reduced) + H2O

Key concepts

Cytochrome Oxidase

Cytochrome oxidase, also known as Complex IV, is a critical enzyme in the electron transport chain (ETC). It is located in the inner membrane of mitochondria.
Cytochrome oxidase is responsible for the final step of the ETC. It transfers electrons from cytochrome c to oxygen. This is important because oxygen acts as the final electron acceptor.
This process is a key part of cellular respiration. It helps produce water from oxygen and hydrogen ions.
Here are some important aspects of cytochrome oxidase:

• It binds to reduced cytochrome c.
• Transfers four electrons to molecular oxygen.
• Helps form two molecules of water.

Catalyzing this reaction helps maintain the proton gradient across the mitochondrial membrane. This gradient is essential for ATP synthesis.

Succinate-CoQ Oxidoreductase

Succinate-CoQ oxidoreductase, or Complex II, is another enzyme in the electron transport chain. It facilitates the oxidation of succinate to fumarate.
This enzyme is unique because it interacts directly with the citric acid cycle. Here are its main roles:

• Oxidizes succinate to fumarate.
• Transfers electrons to coenzyme Q (CoQ), forming CoQH₂.
• It does not pump protons like other complexes in the ETC.

Despite not pumping protons, it plays a key role in transferring electrons. These electrons will eventually help produce ATP. The overall reaction catalyzed is:
Succinate + CoQ → Fumarate + CoQH₂.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are at the core of the electron transport chain.
These reactions involve the transfer of electrons between molecules. Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons.
In the ETC, several redox reactions occur sequentially. Electrons are transferred from donors to acceptors through various complexes, including cytochrome oxidase and succinate-CoQ oxidoreductase.
Here's a simplified breakdown:

• Succinate is oxidized to fumarate, with electrons transferred to CoQ.
• CoQH₂ transfers electrons to cytochrome c via other complexes.
• Cytochrome c transfers electrons to cytochrome oxidase.
• Finally, cytochrome oxidase transfers electrons to oxygen, reducing it to water.

Each step is essential for efficient ATP production. The energy from these reactions helps maintain the proton gradient used by ATP synthase.

Mitochondria

Mitochondria are the powerhouses of the cell, essential for energy production. They are unique double-membraned organelles.
The inner membrane of mitochondria is where the electron transport chain resides. This membrane is highly folded into structures called cristae, increasing surface area for reactions.
Mitochondria have their own DNA and replicate independently of the cell.
Key functions include:

• Generating ATP through oxidative phosphorylation.
• Regulating cellular metabolism.
• Playing roles in apoptosis (programmed cell death).

Mitochondria are crucial for both aerobic respiration and energy production. They encompass multiple complexes, enzymes, and coenzymes that all work together in the ETC.