Squeeze me

by Vivienne Baillie Gerritsen

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Boil a lobster and its colour will go from blue to red. It is a known fact. And a fact that has been known for ages. What has not, however, is how a lobster's outside - which is blue - can turn red. Astaxanthin is the chromophore which lends a lobster its boiled orangey-red hue, as it does to many sea animals such as the pink flesh of salmon, the exoskeletons of crayfish - from where it was first identified - and shrimps, for instance. Surprisingly, astaxanthin is also the chromophore which gives lobsters their initial 'pre-boiled' blue-green colour. Why lobsters turn red in boiling water in the first place is a question which the American biologist George Wald (1906-1997) raised over half a century ago, and to which we now have an answer. What is more, as research progresses, it appears that astaxanthin could have beneficial effects on our health and that the structure of crustacyanin itself could be particularly interesting in the field of drug design.

Astaxanthin is a carotenoid and, in the Northern European lobster Homarus gammarus, it is carried by a protein known as crustacyanin. Lobsters - like all other sea animals - cannot make their own astaxanthin but rely on plant and microalgae for its provision.

The discovery of astaxanthin, and consequently crustacyanin, is an intricate part of the development of modern chromatography. The Russian botanist Mikhail Tsvet (1872-1920) was the first to separate plant pigments by pouring a mixture through a glass column of adsorptive material. Different pigments travel at different rates and, as Tsvet poured, coloured bands appeared down the column. In a 1906 publication, he baptised his method 'chromatography'. The method was largely ignored, however, by his fellow scientists who believed the technique was not good enough for refined analysis.

[ 'Lobster Fest', © Will Rafuse]

'Lobster Fest', © Will Rafuse

Courtesy of Encore Art Group, CAP & Winn Devon

Tsvet's publication was brought to the attention of the Austrian biochemist Richard Kuhn (1900-1967) in the 1930s. Kuhn showed it to Edgar Lederer (1908-1988) - one of his assistants - and asked him to adapt the method and refine it for the separation of carotenoids. Lederer's adaptation of Tsvet's achievement was particularly successful and they managed to isolate and purify a large number of carotenoids, amongst which was astaxanthin.

In Homarus gammarus a large protein complex carries the chromophore: alpha-crustacyanin. Alpha-crustacyanin is an aggregate of sixteen crustacyanin monomers, about 180 amino acids long, each of which supports a molecule of astaxanthin. The sixteen monomers are in fact arranged into eight dimers, each of which is a combination of two crustacyanins and their respective chromophores. Each dimer has been termed beta-crustacyanin and it is this structure which has been scrutinised.

The secret of the change of colour in lobsters - boiled or indeed dehydrated - is to be found within each dimer. A crustacyanin monomer acts as a pocket into which is deeply inserted one chromophore. While one end of the chromophore is hidden in the seat of the pocket, the other protrudes from it, and the second monomer caps the protruding chromophore. In this way, both astaxanthin molecules are protected from the outside world but they are also squeezed out of their original molecular structure. And instead of giving off their natural orangey-red hue, they give off a blue-green one.

To date, it is not clear whether this structural squeeze is the sole cause for the change of colour; the chemical environment of each chromophore is such that it must also have something to do with the bathochromic shift. When a lobster is boiled, the structure of alpha-crustacyanin is denatured. As a result, the molecules of astaxanthin are given enough room to stretch out and adopt their original conformation. And the net result is an orangey-red lobster. Why would a lobster want to be blue in the first place? Camouflage. It is better to be blue-green in a blue-green sea than red. And with the help of natural selection, retain the astaxanthin twist in your crustacyanin pocket and you become less conspicuous than your red relatives.

Adding to the advantages of camouflage, astaxanthin also seems to be involved in an organism's development. Research is also revealing that the chromophore is a plus not only to lobsters but perhaps even to humans. It is a powerful antioxidant and could act as a crutch for our immune and cardiovascular systems, as it could inhibit the development of certain types of cancer. What is more, the crustacyanin dimer could be a choice means of delivering drugs - let alone astaxanthin - into the body.

Now that the structure of the beta-crustacyanin dimer is understood, scientists can move onto the structure of the alpha-crustacyanin complex, which still remains a mystery and will no doubt turn out to be a powerful drug delivery system too. In the meantime, let the astaxanthin squeeze become a conversation piece as we nonchalantly boil our lobsters.

References
1. Cianci M., Rizkallah P.J., Olczak A., Raftery J., Chayen N.E., Zagalsky P.F., Helliwell J.R.
The molecular basis of the coloration mechanism in lobster shell: beta-crustacyanin at 3.2 Å resolution
Proc. Natl. Acad. Sci. USA, 99:9795-9800(2002).
PMID: 12119396

2. A history of the Max Planck Institute for medical research
Richard Kuhn and the chemical department: double bonds and biological mechanisms (1929-1939)
<http://sun0.mpimf-heidelberg.mpg.de/History/Kuhn1.html>

3. Astaxanthin information site
<http://www.astaxanthin.org>
Swiss-Prot cross references
Crustacyanin A1 subunit, Homarus gammarus (European lobster): P58989
Crustacyanin A2 subunit, Homarus gammarus (European lobster): P80007
Crustacyanin C1 subunit, Homarus gammarus (European lobster): P80029
SwissProt
Protein Spotlight (ISSN 1424-4721) is a monthly review written by the Swiss-Prot team of the SIB Swiss Institute of Bioinformatics. Spotlight articles describe a specific protein or family of proteins on an informal tone. Follow us: Subscribe · Twitter · Facebook