Structural insights into electron transfer in caa3-type cytochrome oxidase

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Cytochrome c oxidase (HCO) enzymes are crucial for cellular respiration, catalyzing oxygen reduction and proton pumping.
• Electron transfer from cytochrome c to the oxidase is essential, typically involving a transient protein association.
• Current models propose electron entry and exit at the same site on cytochrome c.

Purpose of the Study:

• To determine the crystal structure of the caa3-type cytochrome oxidase from Thermus thermophilus.
• To elucidate the mechanism of electron transfer in this enzyme, particularly with a fused cytochrome c domain.
• To identify novel subunits and lipids associated with the enzyme complex.

Main Methods:

• Crystallization of Thermus thermophilus cytochrome c oxidase in a bicontinuous mesophase using a synthetic monoacylglycerol.
• X-ray crystallography at 2.36 Å resolution.
• Analysis of the electron density map to identify structural components and their arrangement.

Main Results:

• The crystal structure revealed a novel integral membrane subunit and an embedded native glycoglycerophospholipid.
• The enzyme features a covalently tethered cytochrome c domain (cupredoxin/cytochrome c).
• The structure indicates distinct sites for electron entry and exit on cytochrome c, differing from soluble cytochrome c mechanisms.

Conclusions:

• The fused cytochrome c domain architecture necessitates alternative electron transfer pathways compared to soluble cytochrome c.
• The findings suggest a departure from the classical proton gate mechanism in this haem copper oxidase.
• This study provides new insights into the structural basis of electron transfer in respiratory enzymes.