Gray hair, albininism, melanoma stem from molecular structure
By Ruth SoRelle, M.P.H.
The oxygen-carrying protein complex called hemocyanin confers "blueblood" on arthropods and mollusks.
It is a member of the phenoloxidase family that is involved in skin and hair coloring. Mutated, members of this family can cause albinism – the loss of coloring in skin and hair. If produced in abundance, they are associated with the deadly skin cancer melanoma.
Understanding hair, skin color
In an elegant structural study, a team of Baylor College of Medicine and German biophysicists from Mainz explain how hemocyanin and chemicals such as tyrosinase are activated – a finding that could lead to a better understanding of both ends of the skin and hair color spectrum. A report of their work appears in the journal Structure.
When Dr. Yao Cong, a postdoctoral researcher in the laboratory of Wah Chiu, Ph.D., displays the computer representation of hemocyanin, it glows like a four-part jewel on the computer screen (see Figure 1)."It is very large and composed of 24 molecules," Cong said. In fact, it consists of four hexamers, each with six monomers (Movie 1 and Figure 1).
Just getting this far required the use of a single particle electron cryomicroscope (cryo-EM) to produce three-dimensional density maps of the molecule at sub-nanometer resolution.
Structural tool
"Cryo-EM is becoming a structural tool that can be used for understanding structural mechanisms of large proteins, which has translational and biotechnological application as demonstrated in this study," said Chiu, senior author of the paper and a professor of biochemistry and molecular biology at BCM and director of the National Center for Macromolecular Imaging.
"There are some critical structural features are very well resolved in our maps," said Cong, "which could not be captured using other techniques."
Dr. Heinz Decker and his colleagues, who made up the collaborating team from Johannes Gutenberg-University in Mainz, Germany, used the detergent SDS, which is usually used as denaturant to degrade protein, to activate hemocyanin. At certain high concentrations, instead of destroying the complex, it turns hemocyanin and tyrosinase into an enzymatically active phenoloxidases.
Three domains
Each monomer of the hemocyanin protein has three domains.
"It is very interesting," said Cong. "One domain is more flexible than the other two domains because it has much less interaction with neighboring subunits as compared with the other two domains."
Upon activation, there is an overall conformational change of the complex (Movie 2). The most obvious is formation of two bridges in the previously vacant middle of the protein, which strengthens the interaction between the two halves of the complex.
"Zoom into the active site," said Cong. "The intrinsically flexible domain twists away from the other two domains, dragging away a blocking residue and exposes the entrance to the active site (Movie 3). This movement is then stabilized by enhanced interhexamer interactions."
This molecular mechanism of activating hemocyanins and tyrosinases is consistent with that proposed by Decker and members of his laboratory based on their biochemistry experiments.
"This is all about interaction," said Cong. "A single change in the local domain of a subunit can result in conformation changes in the entire complex and make it work cooperatively. This is really a molecular machine."
Implications
Using hemocyanin as a model system, scientists can learn about the activation mechanism of other phenoloxidase enzymes in the same family, opening the door to new understanding of both melanoma and albinism, she said.
"If you know the mechanism of activating the protein, you could mutate it to enhance the interaction or inhibit it – depending on what you want to accomplish," she said.
Not only does this research have implications for human characteristics and disorders such as vitiligo, gray hair and albinism, it could also play a role in agriculture, where enzymes in this protein family are responsible for fruit and vegetables turning brown as they age.
Others who took part in this work include Qinfen Zhang, David Woolford, Htet Khant, Matthew Dougherty and Steven J Ludtke, all of BCM, and Thorsten Schweikardt and Heinz Decker of Johannes Gutenberg-University in Mainz, Germany. Zhang is now with Sun Yat-Sen University in Guangzhou, China, and Schweikardt is Boehringer Ingelheim Pharma GmbH & Co. in Germany.
Funding for this work came from the National Center for Research Resources, the Roadmap Initiative for Medical Research and the German Research Foundation and Research Center for Immunology in Mainz.
