Myoglobin

Myoglobin is a globular protein whose function is to store molecular oxygen. The fold of the protein is conserved but the sequence is more .

The globin consists mostly of alpha helices shown in ; it has no beta sheets and its nonhelical segments mostly serve as links that connect the helices. Look down the barrel of some of the longer helices. Are they all straight? The eight structurally conserved alpha helices are labelled . The protein is colored as a N-->C rainbow in this view; the N terminus is blue, while the C terminus is red.

The heme is shown in spacefilling form with its N, O, and Fe atoms displayed as blue, red, and orange balls. Note how the heme is almost completely enclosed by the globin. Which few chemical groups of the Heme are exposed to the solvent? (holding the mouse over the atoms reveals their identity.) Can you rationalize this exposure?

View1 is the standard view of Mb. Rotate the protein around to convince yourself that the globin is approximately disc-shaped with a diameter that is about twice its thickness. Turn on the "Main Chain" button to display the polypeptide backbone in white with its N and O atoms represented by blue and red balls. How closely does the ribbon follow the main chain? Click "Animate" until the Mb ribbon is gray. Turn off the "Mb Ribbon" button. What are the orientations of the main chain carbonyl groups and the amide N atoms relative to each other which allows formation of the H-bonds of the alpha helices (not drawn)? View2 is a closeup of the heme from the same direction as View1. Turn on the "HemeLigand" button to display, in cyan, the sidechain of His 93, the proximal His, liganding the heme's Fe(II) ion (white bond). The Fe(II) is also liganded in a square-planar array by the heme's four pyrrole N atoms and hence has a total of 5 pyramidally arranged ligands. In oxyMb, the reversibly bound O2 molecule ligands the Fe from the opposite side of the heme as does the proximal His so that the Fe(II) becomes octahedrally coordinated. The Fe atom is not oxidized by its O2 ligand; it remains in the Fe(II) oxidation state. Rotate the image about the vertical axis until you see heme edge-on. Is the Heme planar? Note that the Fe atom is displaced towards the proximal His by 0.55 Å from the best plane though the porphyrin ring atoms. In oxyMb, the Fe is only 0.22 Å out of the heme plane and still on the side of the proximal His (for details see Oxymyoglobin).

See also Molecular Playground/Myoglobin

Myoglobin page in Spanish Myoglobin-Physeter-catodon-structure.