Myoglobin

"This plasticine model, made by Sir John Kendrew, was the first ever model to be made of a protein molecule. Myoglobin is a protein containing iron that carries and stores oxygen in muscle cells. In 1957 John Kendrew (1917-1977) and Max Perutz (1914-2002) were able to produce a model of its 3-dimensional structure, for which they were awarded the Nobel Prize for chemistry in 1962."

Found at Science Museum, via A London Salamagundi.