III. Crossing of the B and E Helices

Backbone trace of the B and E helices in myoglobin. The close approach of the two helices is possible because two invariant glycine residues (B6 and E8) are present at the crossing. Any side chain larger than a hydrogen atom pushes the helices apart, thereby disrupting the 3° structure.

  Toggle spacefilling of Gly residues. The alpha carbons are shown in green.
  Toggle spacefilling of the E helix. Imagine where the methyl group would be if Gly B6 (cyan helix) were replaced with Ala.  Tight packing of the B and E helices is important because they cradle the oxygen-binding pocket.

  Select the E helix and heme, then toggle the following buttons to locate the two invariant residues on the distal side of the heme.
His E7
Val E11

  Now the B helix. Leu B10 and Phe B14 are highly conserved in myoglobin and hemoglobin a and b chains.
Leu B10
Leu B13
Phe B14

We will explore the binding of oxygen in the next section.

Previous exampleBack to indexNext example