Question

(a) What is the function of cytochrome \(c\) oxidase? (b) Describe the four active metal-containing sites in cytochrome \(c\) oxidase and the proposed way in which they work together to fulfil the role of the metalloprotein.

Short answer

(a) Cytochrome c oxidase reduces oxygen to water, aiding ATP synthesis. (b) Cu_A, Cu_B, heme a, and heme a_3 are metal sites that facilitate electron transfer and oxygen reduction.

Step-by-step solution

Understanding Cytochrome c Oxidase Function

Cytochrome c oxidase is a vital component of the electron transport chain in mitochondria. Its primary function is to drive the transfer of electrons from cytochrome c to oxygen, facilitating the reduction of oxygen to water. This process creates a proton gradient across the mitochondrial membrane, which powers ATP synthesis through oxidative phosphorylation.

Identifying Active Metal Sites

Cytochrome c oxidase contains four active metal-containing sites: two copper sites, Cu_A and Cu_B, and two heme groups, heme a and heme a_3. These sites play a critical role in the enzyme's catalytic activity and electron transfer process.

Role of Cu_A and Heme a

The Cu_A site acts as an initial electron acceptor from cytochrome c and transfers electrons to heme a. Heme a then facilitates electron transfer to heme a_3 and the Cu_B site, positioning them for involvement in the reduction of oxygen.

Role of Heme a_3 and Cu_B

The heme a_3 and Cu_B form a binuclear center where molecular oxygen is bound and reduced. This site is crucial for the conversion of oxygen into water, which releases energy used to contribute to the proton gradient necessary for ATP synthesis.

Key concepts

Electron Transport Chain

The electron transport chain is like an energy highway in cells. It's located in the mitochondria, which is the powerhouse of the cell. This chain involves a series of protein complexes and molecules that transfer electrons from electron donors to electron acceptors through redox reactions. Each time an electron moves through these complexes, it releases energy.

This released energy is used to pump protons across the mitochondrial membrane, creating a gradient. This gradient generates a difference in charge and pH across the membrane.

• The movement of electrons down this chain ultimately leads to the synthesis of ATP, the cell's main energy currency.
• Cytochrome c oxidase functions as the last enzyme in this chain, facilitating the step where electrons are transferred to oxygen.
• The presence of this process ensures that oxygen is converted into water, completing the electron transport chain's role in cellular respiration.

Metal-Containing Sites

Cytochrome c oxidase features four important metal-containing sites that play a pivotal role in its function. These sites are two copper centers, Cu_A and Cu_B, and two heme groups, heme a and heme a_3.

• Cu_A Site: This site is the initial electron acceptor from cytochrome c. It collects the electrons and moves them forward in the process.
• Heme a: After accepting electrons from Cu_A, heme a transfers them to the next important sites.
• Heme a_3 and Cu_B: These form a crucial duo, known as the binuclear center, where oxygen binds and is reduced to water.

These metal sites work collaboratively to ensure a smooth transfer of electrons, facilitating vital processes that maintain cellular energy production.

Oxidative Phosphorylation

Oxidative phosphorylation is essential for producing ATP in cells, occurring after the electron transport chain. It's like the 'pay-off' phase where energy from electrons is turned into usable energy currency.

This process harnesses the proton gradient created by the electron transport chain across the mitochondrial membrane.

• This gradient drives ATP synthase, a molecular machine, to synthesize ATP by adding a phosphate group to ADP.
• The energy needed for this addition comes directly from the flow of protons back across the membrane through ATP synthase.

This entire sequence is critical for cells to fuel various activities and sustain life.

Electron Transfer

Electron transfer is a fundamental process in cellular respiration, including in the electron transport chain. It is the movement of electrons from one molecule to another which releases stored energy.

In cytochrome c oxidase, electron transfer is a sophisticated sequence involving different metal sites that ensure efficiency and precision.

• Electrons are passed from the Cu_A site to heme a and finally to the heme a_3 and Cu_B binuclear center.
• This sequential transfer is designed to ensure minimal loss of energy, allowing for maximum ATP production.

The successful transfer of electrons is crucial as it not only completes the reduction of oxygen to water but also helps maintain the proton gradient needed for ATP synthesis.