Similarities and differences between human cyclophilin A and other beta-barrel structures. Structural refinement at...

Area of Science:

• Structural biology
• Biochemistry
• Protein science

Background:

• Cyclophilin A (CyP A) is an enzyme involved in protein folding and cellular processes.
• Understanding CyP A's structure is crucial for elucidating its function and interactions.
• Previous studies have characterized various eight-stranded beta-barrel proteins.

Purpose of the Study:

• To determine the high-resolution structure of unligated recombinant human cyclophilin A (CyP A).
• To compare the CyP A structure with other eight-stranded beta-barrel proteins.
• To investigate potential mechanisms of CyP A activity and ligand binding.

Main Methods:

• X-ray crystallography was used to refine the structure of unligated recombinant human CyP A.
• The refined structure was analyzed for geometric parameters and compared to known beta-barrel structures.
• Interactions between solvent molecules and active site residues were examined.

Main Results:

• The structure of unligated CyP A was refined to 1.63 Å resolution, revealing a right-handed, eight-antiparallel beta-strand structure.
• CyP A exhibits a unique topology and a closed beta-barrel structure, unlike most other beta-barrels.
• The closed nature of the barrel prevents binding of cyclosporin A (CsA) and proline-containing substrates to the hydrophobic core.

Conclusions:

• The unique, closed beta-barrel structure of CyP A suggests it is functionally and evolutionarily distinct from other beta-barrel proteins.
• The refined structure provides insights into potential water-cooperated mechanisms for cis<-->trans isomerization.
• The absence of disulfide bridges and steric hindrance at Cys115 rule out certain proposed mechanisms for CsA binding and isomerization.