Small-angle neutron scattering study of bicelles and proteobicelles with incorporated mitochondrial cytochrome c oxidase

The structural investigations of a model membrane system, bicelles, and the aggregation state of isolated and purified bovine heart cytochrome c oxidase (CcO) in bicelles have been per-formed using small-angle neutron scattering (SANS), SANS contrast variation, and com-plemented by various biophysical and biochemical techniques. The average size of bicelles prepared from long-chain 1,2-dimyristoyl-sn-glycero-3-phosphocholine and short-chain 1,2-dihexanoyl-sn-glycero-3-phosphocholine was found to be about 22 nm with a thickness of about 4 nm. Enzyme in bicelles was remained active and structurally unaltered. The estimated vol-ume of protein in bicelles of 240 nm³ corresponded well to the monomeric form of CcO. The ab initio modeling supports the experimental data and suggests that CcO in bicelles is a ho-mogeneous monomeric complex incorporated into bicelles.