Homarus gammarus (European lobster)
The protein which gives red lobster its color is a member of a family of proteins called lipocalins. These are proteins which bind compounds that are mainly hydrophobic in the center of a barrel formed by beta strands. The protein here, crustacyanin, has similar overall structure, but shows some differences.
First of all it is made as a dimer of two different subunits. Secondly, there are two hydrophobic compounds called astaxanthin bound to the dimer. Astaxanthin is the compound responsible for the red color of cooked lobster. These two molecules are shared by both subunits and between the two astaxanthin molecules sits a dodecane molecule, another hydrophobic compound, roughly in the center of the dimer. Although the astaxanthin molecules are symmetrical about their central bond, the two monomers of the protein are different. The two binding sites of each monomer also have different es. Astaxanthin actually is not always red but can change color depending on the chemical circumstances. In live lobster the shell is not red, but of a blueish color. How this color change happens was not well understood for a long time. The structure helps to explain this effect.
Protein Data Bank (PDB)
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-A resolution."; Proc.Natl.Acad.Sci.USA; (2002) 99:9795-9800 PubMed:12119396.
author: Arno Paehler