Unravelling the structural chemistry of the colouration mechanism in lobster shell

Biochemistry, biological crystallography, spectroscopy, solution X-ray scattering and microscopy have been applied to study the molecular basis of the colouration in lobster shell. This article presents a review of progress concentrating on recent results but set in the context of more than 50 years of work. The blue colouration of the carapace of the lobster Homarus gammarus is provided by a multimolecular caroteno­protein, α-crustacyanin. The complex is a 16-mer of five different subunits each binding the carotenoid, asta­xanthin (AXT). A breakthrough in the structural studies came from the determination of the structure of β-crustacyanin (protein subunits A1 with A3 with two shared bound astaxanthins). This was solved by molecular replacement using apocrustacyanin A1 as the search motif. A molecular movie has now been calculated by linear interpolation based on these two `end-point' protein structures, i.e. apocrustacyanin A1 and A1 associated with the two astaxanthins in β-crustacyanin, and is presented with this paper. This movie highlights the structural changes forced upon the carotenoid on complexation. In contrast, the protein-binding site remains relatively unchanged in the binding region, but there is a large conformational change occurring in a more remote surface-loop region. It is suggested here that this loop could be important in complexation of AXT and contributes to the spectral properties. Also presented here is the first observation of single-crystal diffraction of the full `α-­crustacyanin' complex comprising 16 protein subunits and 16 bound AXT molecules (i.e eight β-crustacyanins) at 5 Å resolution. Optimization of crystallization conditions is still necessary as these patterns show multiple crystallite character, however, 10 Å resolution single-crystal diffraction has now been achieved. Provision of the new SRS MPW 10 and SRS MPW 14 beamline robotic systems will greatly assist in the surveying of the many α-crustacyanin crystallization trials that are being made. New solution X-ray scattering (SXS) measurements of β- and α-crustacyanin are also presented. The β-crustacyanin SXS data serve to show how the holo complex fits the SXS curve, whereas the apocrustacyanin A1 homodimer from the crystal data naturally does not. Reconstructions of α-crustacyanin were accomplished from its scattering-profile shape. The most plausible ultrastructure, based on a fourfold symmetry constraint, was found to be a stool with four legs. The latter is compared with published electron micrographs. A detailed crystal structure of α-crustacyanin is now sought in order to relate the full 150 nm bathochromic shift of AXT to that complete molecular structure, compared with the 100 nm achieved by the β-­crustacyanin protein dimer alone. Rare lobster colourations have been brought to attention as a result of this work and are discussed in an appendix.