The structural chemistry and structural biology of colouration in marine crustacea

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The colouration of the lobster shell, famously known from its colour change on cooking, derives from a complicated mix of astaxanthin carotenoid molecules and several proteins in complex. Crystals of various components have been known for many years but the breakthrough in structure determination came from the use of synchrotron radiation ‘softer X-rays’ of wavelength 2 Å, thereby targeting the increase in the sulphur anomalous scattering and the xenon LI absorption edge. One structure of the gene-group of proteins was thus solved and this could be used for molecular replacement solution of the β-crustacyanin dimer complex in spite of a very high solvent content of ∼80%. The crystal structure of the α-crustacyanin complex of eight β-crustacyanins still eludes us but electron microscopy (EM) structure determination would be a way forward. At present, the molecular tuning parameters causing the 100 nm bathochromic shift of the β-crustacyanin are at least known from our work and have already stimulated considerable further research in theoretical and carotenoid chemistry. The protonation state of two critically placed histidines is of keen interest here, as well as the proximity of the two astaxanthins bound to the β-crustacyanin protein. There are wider biological implications too, including the colours of rare lobsters, where site-specific amino acid changes could be a cause and much molecular biology for colour tuning would be possible. A colour-based heat sensor could also be designed. Nutraceuticals based on the health giving properties of astaxanthin are also of keen interest. The mimicking of the colour change solely with carotenoids is being sought supported by our structural crystallography results of unbound carotenoids. Public interest has been especially strong not least because ‘the’ question: ‘Why does a lobster change colour on cooking?’ is known to nearly everyone. Notably, popular science-summary articles have been published in magazines. We will soon be entering a new era of ultra-bright X-ray Free Electron Lasers (XFELs) and the possibility of single molecule protein structure determination; the α-crustacyanin complex of eight β-crustacyanins would provide a test of the new methods and cross-comparison with EM. The comparisons of amino acid sequences of other crustacea colouration proteins with that of β-crustacyanin provide 3D ‘homology models’ and allow species and evolution comparisons. This topical review provides a summary of recent work and likely future research goals.