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Entries in carotenoids (1)


Simple questions with complex answers: why is a cooked lobster red?

ResearchBlogging.orgSome really simple questions have surprisingly complex answers.  “Why is the sky blue?” ends up being all about differential absorbance of varying wavelengths of electromagnetic radiat… see, there, I’ve already wandered off into jargon land.

And so it is with the question “Why is a cooked lobster red, when a live lobster is not?”.  An odd question, but its exactly that kind of “I wonder why…” moment that has led to some of the greatest discoveries.  Anyway, you can argue that it is not a trivial question; indeed, the name of an entire restaurant franchise depends on the correct color change occurring when you drop a Homarus americanus into a pot of boiling Old Bay.  So what’s going on?

Well, its all about the astaxanthin, (lets call it AXT from now on).  AXT is a carotenoid, which means it’s a fat-soluble pigment that – generally speaking - is red or orange in colour.  Carotenoids give tomatoes their red (lycopene), egg yolks their yellow (lutein), carrots their orange (beta carotene), salmon their pink (canthaxanthin) and televangelists their freakish alien fake tans (but they do offset the glowing white dental veneers ever so nicely, don’t they?).  Lobsters don’t make AXT, they get it from eating their veggies like a good little lobster, because ultimately it’s a plant pigment (plants use it as a sunscreen – but that’s another post for another day!).  In its basic form, AXT is really vivid orange, almost vermilion.  But in lobster shells it doesn’t occur in its basic form; instead it’s mostly bound to a protein, called crustacyanin, which we’ll call CR for short.  AXT binds to CR in much the same way as oxygen binds to the haemoglobin in our blood, except for one big difference.  Unlike oxygen, which fits neatly in a haemoglobin molecule, AXT has to bend to fit into the CR molecule, like one of those freakshow contortionists who fold themselves up in a box.  In bending the AXT molecule to make it fit, the natural colour of astaxanthin changes – it shifts – from red to blue or blue-green.  Historically, this shift has been an interesting mystery to chemists and physicists interested in properties of pigments, because its unusual for the same pigment molecule to have both red and blue forms, as most avid flower gardeners can tell you.  On the right is a picture of the rare all-blue form of the American lobster (read more at the University of Maine website)

Enter Michele Cianci and colleagues from the University of Manchester in England.  These clever folks showed in 2002 that the colour change – technically called the bathochromic shift – is a result of the structure of the CR molecule and the way it flexes the AXT molecule like a loaded spring.  This is where the simple question yields the really complex answer.  Get a load of this phrase from their abstract:  “Recently, the innovative use of softer x-rays and xenon derivatization yielded the three dimensional structure of the A1 apoprotein subunit of CR, confirming it as a member of the lipocalin superfamily. That work provided the molecular replacement search model for a crystal form of the beta-CR holo complex, that is an A1 with A3 subunit assembly including two bound AXT molecules. We have thereby determined the structure of the A3 molecule de novo”.  Ex-squeeze me baking powder?

Yes, well, that's all well and good, but it doesn’t answer the simple question of why they go red when you cook them, does it?  Bear with me…  When next you are at the grocery store, take a look in the live lobster tank and you’ll see that they don't look like the handsome all-blue fellow above; they tend to be a mosaic of colours like orange, yellow, cream, green, blue and brown.  This patchwork arises from varying amounts of free and bound AXT in different layers of the shell, and some other factors like how thick the shell is, and whether the AXT is at the surface or in a deeper layer.  If you go ahead and buy one of these lobsters and drop it into a pot of boiling water, little happens to the AXT because it’s heat stable.  But the protein CR, on the other hand, is not.  Like most proteins, it loses its structure when you apply intense heat, unfolding like a jack-in-the-box, and flinging off the AXT in the process.  Liberated from its oppressive bathochromic bonds, the AXT reverts to its normal colour – intense orange-red.  Et puis, vous voila! – blue/green lobsters turn red when you cook them.

Much the same process happens in shrimp and crabs when you cook them too, but it was worked out for lobsters first because they only have one carotenoid – AXT – whereas other crustaceans had other carotenoids that complicated the picture even further.

PS - some genetic rarities give us all sorts of lobster colour patterns like the all-blue one shown above, but my favourite is the half-and-half.  The first time I saw one of these, I thought it was someone having a joke at my expense, but they're the real deal!  How it happens is still a mystery, but there's probably something wrong with the way they express CR on one side of the body.  Picture from National Geographic.

Tip of the Mackintosh hat to @AboutMarineLife on Twitter, for inspiration.

Cianci M, Rizkallah PJ, Olczak A, Raftery J, Chayen NE, Zagalsky PF, &; Helliwell JR (2002). The molecular basis of the coloration mechanism in lobster shell: beta-crustacyanin at 3.2-A resolution. Proceedings of the National Academy of Sciences of the United States of America, 99 (15), 9795-800 PMID: 12119396