Cytochrome oxidase: structural insights from electron microscopy and from secondary structure prediction

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Proteins

Background:

• Cytochrome oxidase is a crucial enzyme complex involved in cellular respiration.
• Understanding its structure is key to elucidating its function in energy production.
• Previous studies have determined monomer structures, but dimer organization requires further investigation.

Purpose of the Study:

• To interpret electron microscopic images of cytochrome oxidase dimer crystals.
• To elucidate the structural arrangement of monomers within the dimer.
• To localize subunit IV and predict transmembrane helices.

Main Methods:

• Electron microscopy of selectively contrasted cytochrome oxidase dimer crystals.
• Interpretation of images based on known monomer structures.
• Antibody fragment decoration to identify subunit location.
• Membrane propensity algorithm for predicting transmembrane alpha-helices.

Main Results:

• Monomer arms are perpendicular to the lipid bilayer, protruding 25 A on the matrix side.
• Cytoplasmic-side tails of monomers form a cleft within the dimer.
• Subunit IV is located along the a-crystal axis, approximately 20 A from the dimer's center.
• A total of 19 transmembrane alpha-helices are predicted per monomer.

Conclusions:

• The study provides a structural model for cytochrome oxidase dimers consistent with monomeric structures.
• Subunit IV's position and the predicted transmembrane helices offer insights into the enzyme's membrane integration and function.
• This work contributes to a deeper understanding of mitochondrial respiration machinery.