Characterizing the proton loading site in cytochrome c oxidase

Area of Science:

• Biochemistry
• Molecular Biology
• Bioenergetics

Background:

• Cytochrome c oxidase (CcO) is a crucial enzyme in cellular respiration, responsible for reducing oxygen to water and pumping protons.
• Proton pumping by CcO generates an electrochemical gradient essential for ATP synthesis.
• Understanding the precise mechanism of proton translocation is key to comprehending energy conversion in biological systems.

Purpose of the Study:

• To elucidate the proton loading site (PLS) mechanism in Cytochrome c oxidase (CcO).
• To determine how residues within CcO facilitate proton loading and release during its catalytic cycle.
• To investigate the conservation and function of the PLS across different CcO species.

Main Methods:

• MultiConformational Continuum Electrostatics (MCCE) simulations were employed to determine residue protonation states.
• The catalytic cycle of CcO was defined using the protonation and redox states of key components: heme a, heme a3, Cu(B), Y288, and E286.
• Structural data from crystallography and molecular dynamics of various CcO species (Rhodobacter sphaeroides, Paracoccus denitrificans, bovine CcO) were analyzed.

Main Results:

• A conserved proton loading site (PLS) was identified on the P-side of the membrane, responsible for loading and releasing one proton per catalytic cycle.
• The PLS functions similarly across different CcO species, indicating a conserved proton pumping mechanism.
• The PLS is a cluster of 1-4 residues, and its proton loading efficiency is influenced by the proton affinity of heme a3 propionic acids.

Conclusions:

• The same proton pumping mechanism, involving the PLS, is utilized in each reduction step of CcO.
• The PLS is a dynamic cluster of residues that adapts to facilitate proton translocation.
• Heme a3 propionic acid proton affinity is a critical factor regulating the number of protons loaded into the PLS, impacting overall proton pumping efficiency.