Crustacyanin is a protein found in the exoskeletons of crustaceans like lobsters and crabs. It is responsible for their distinct coloration, which can range from blues and greens to browns.
Understanding Crustacyanin’s Structure
Crustacyanin is a complex protein that tightly binds to astaxanthin, a carotenoid pigment. Astaxanthin is naturally reddish-orange when unbound, but its color changes when incorporated into the crustacyanin structure.
The full crustacyanin complex, alpha-crustacyanin, is a large assembly of eight beta-crustacyanin dimers. Each beta-crustacyanin dimer is formed from two different protein subunits that create a binding site for two astaxanthin molecules.
The astaxanthin molecules are nestled within the protein’s structure. The interaction between the protein and astaxanthin is non-covalent, meaning they are held together by weaker forces rather than strong chemical bonds. This molecular arrangement alters astaxanthin’s light-absorbing properties, leading to the colors seen in live crustaceans.
The Mechanism Behind Crustacean Coloration
The blue, green, or brown colors of live crustaceans arise from how astaxanthin interacts with the crustacyanin protein. When astaxanthin is bound within the protein complex, its light absorption properties are altered. Unbound astaxanthin absorbs light around 472 nanometers, appearing orange-red. When bound within beta-crustacyanin, the absorption maximum shifts to around 580-590 nanometers, causing the complex to appear blue.
This shift in light absorption, known as a bathochromic shift, means the complex absorbs more yellow and red light and reflects more blue light. In the larger alpha-crustacyanin complex, absorption can shift further, to approximately 632 nanometers, resulting in a deeper blue. This precise color depends on the specific arrangement and interactions between astaxanthin molecules and protein subunits.
The protein’s three-dimensional structure modifies the astaxanthin molecule’s electron cloud. This modification changes the energy levels required for electrons to jump between orbitals, affecting which wavelengths of light are absorbed or reflected. The intricate structural mechanism evolved by lobsters, using astaxanthin and the protein, accounts for the coloration of their shells.
The Cooking Color Transformation Explained
When crustaceans are cooked, they change from blue or green to red. This is a direct consequence of what happens to the crustacyanin protein. Heat causes the crustacyanin protein to denature, which is the unraveling of its three-dimensional structure.
As the protein denatures, the binding sites holding astaxanthin molecules are disrupted. This releases the astaxanthin pigment. Once unbound, astaxanthin reverts to its natural, free state, absorbing light around 472 nanometers, which makes it appear orange-red.
This irreversible denaturation and subsequent release of astaxanthin explains why cooked lobsters and crabs turn red. The red color is the inherent color of the astaxanthin pigment when it is no longer influenced by the protein’s binding environment.