LAGUNA DESIGN / SCIENCE PHOTO LIBRARY LAGUNA DESIGN / SCIENCE PHOTO LIBRARY
Haemagglutinin from bird flu virus, molecular model. This protein, H5, is found on the surface of the bird flu virus H5N1. It is responsible for binding the virus to the cell membrane before it enters the cell. All influenza viruses contain haemagglutinin, but the shape of the protein determines which species the virus will infect. H5N1 cannot infect humans because H5 cannot bind to human cells; normally, only flu viruses with H1, H2 or H3 haemagglutinins can infect humans. In this model, solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices; arrows are beta sheets), and the faded tubular structure behind represents the individual atoms and bonds.
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