A homologous expression system for obtaining engineered cytochrome ba3 from Thermus thermophilus HB8.

Cytochrome ba3 is an integral membrane protein that serves as a terminal oxidase of the respiratory chain in some prokaryotes. We have cloned the complete cba operon of Thermus thermophilus HB8 in an Escherichia coli/T. thermophilus shuttle vector. The ba3-encoding operon, cba, was eliminated from the chromosome of T. thermophilus strain MT111 using the pyrE system of Yamagishi and co-workers. Expression of functional cytochrome ba3 occurred in cells grown at reduced dioxygen levels. A hepta-histidine tag was placed at the N-terminus of subunit I, and a purification method for this form of the enzyme was developed. Growth conditions were investigated for moderate sized cultures (2L) with typical yields of approximately 2 mg of highly pure enzyme per liter of culture medium. The physical properties and enzymatic activities of these recombinant enzymes were compared with those of native enzyme. Recombinant enzyme lacking the histidine tag is spectrally identical to wild-type enzyme. Histidine-tagged cytochrome ba3 shows minor differences from wild-type, and it appears be somewhat less active as a cytochrome c552 oxidase. Exemplary mutants were also produced and compared to native protein. Tyrosine I-237, previously found to be covalently bonded to I-His-233, was changed to phenylalanine (I-Y237F) and to histidine (I-Y237H) in the hepta-histidine tagged cytochrome ba3. The Y to F mutant is devoid of enzyme activity whereas the Y to H mutant possesses approximately 5% wild-type oxidase activity; their properties are compared with those of wild-type enzyme. The above versions of the histidine-tagged enzyme have been crystallized, and our analysis of a 2.3 angstrom resolution electron-density map will be discussed elsewhere.

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

Cytochrome ba(3) 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 (P4(3)2(1)2) 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 ba(3) 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 angstroms resolution to an R factor of 21.7% and an R(free) of 23.7%.