Reaction of myoglobin with phenylhydrazine: a molecular doorstop.

X-ray crystallographic studies of myoglobin do not show an entrance or exit path for potential ligands from the surface to the heme cavity. Efforts to locate such a path have so far centered around dynamic calculations. A structure has now been determined that has a clear opening. Phenylhydrazine reacts with myoglobin in such a way that a phenyl group remains bound to the iron atom. The structure of this complex shows that the side chains of His-64(E7), Arg-45-(CD3), and Val-68(E11) have been forced aside to form an open channel to the surface. Although this may not be the only channel to the iron atom, it seems likely that it is an important one.