A novel cryoprotection scheme for enhancing the diffraction of crystals of recombinant cytochrome ba₃ oxidase from Thermus thermophilus

Cytochrome ba₃ oxidase is an integral membrane protein identified in the thermophilic bacterium Thermus thermophilus. The enzyme has now been expressed recombinantly and purified with a histidine tag. As such, it crystallizes under similar conditions and in the same space group (P4₃2₁2) as the native protein. A novel cryoprotection scheme is described here to obtain high-resolution diffraction from these crystals, which involves soaking in a mixture of glycerol and ethylene glycol under a layer of oil. The unit-cell parameters for these crystals are larger than the native protein, apparently deriving from increased ordering of the N-terminus and an internal loop (residues 495-500) in subunit I. Hence, compared with native cytochrome ba₃ oxidase, the recombinant His-tagged protein is accommodated in an expanded but equally well ordered lattice via an alternate set of specific intermolecular contacts. The structure was refined against data to 2.3 Å resolution to an R factor of 21.7% and an R_free of 23.7%.