Toggle between wireframe and spacefilled models of the heme group in deoxymyoglobin. Heme consists of a porphyrin and an iron atom in the 2+ oxidation state (shown in yellow). The Fe(II) is coordinated to the four nitrogens of the porphyrin ring. Rotate the model so that the plane of the porphyrin is perpendicular to the plane of the screen. Observe that the iron atom is situated 0.55 Å out of the heme plane. The iron atom is covalently bonded to the imidazole ring of His F8. This histidine, which occupies the fifth coordination position, is called the proximal histidine. Another histidine residue (His E7), termed the distal histidine, is near the heme on the oxygen-binding side on the heme. However, its imidazole group is too far away from the Fe(II) to coordinate with it. In deoxymyoglobin, the sixth coordination position remains empty; in oxymyoglobin, it is occupied by O2. Close-up view of the O2-heme complex. The alpha carbons are shown in dark gray. The model shows the position of His E7 in the crystal. We will return to this point below. With the exception of the distal histidine, the side chains lining the oxygen-binding pocket are nonpolar. Valine E11 is an invariant amino acid residue. Its side chain is shown in green. Observe that Val E11 forces the O2 molecule to tilt away from a preferred perpendicular alignment with the plane of the heme. Phenylalanine CD1 is invariant in all myoglobins and hemoglobins. Its aromatic ring makes contact with the heme. Leucine F4 is also invariant. Its side chain makes contact with both the proximal histidine and the heme. Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
The iron atom is covalently bonded to the imidazole ring of His F8. This histidine, which occupies the fifth coordination position, is called the proximal histidine. Another histidine residue (His E7), termed the distal histidine, is near the heme on the oxygen-binding side on the heme. However, its imidazole group is too far away from the Fe(II) to coordinate with it. In deoxymyoglobin, the sixth coordination position remains empty; in oxymyoglobin, it is occupied by O2. Close-up view of the O2-heme complex. The alpha carbons are shown in dark gray. The model shows the position of His E7 in the crystal. We will return to this point below. With the exception of the distal histidine, the side chains lining the oxygen-binding pocket are nonpolar. Valine E11 is an invariant amino acid residue. Its side chain is shown in green. Observe that Val E11 forces the O2 molecule to tilt away from a preferred perpendicular alignment with the plane of the heme. Phenylalanine CD1 is invariant in all myoglobins and hemoglobins. Its aromatic ring makes contact with the heme. Leucine F4 is also invariant. Its side chain makes contact with both the proximal histidine and the heme. Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Valine E11 is an invariant amino acid residue. Its side chain is shown in green. Observe that Val E11 forces the O2 molecule to tilt away from a preferred perpendicular alignment with the plane of the heme. Phenylalanine CD1 is invariant in all myoglobins and hemoglobins. Its aromatic ring makes contact with the heme. Leucine F4 is also invariant. Its side chain makes contact with both the proximal histidine and the heme. Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Phenylalanine CD1 is invariant in all myoglobins and hemoglobins. Its aromatic ring makes contact with the heme. Leucine F4 is also invariant. Its side chain makes contact with both the proximal histidine and the heme. Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Leucine F4 is also invariant. Its side chain makes contact with both the proximal histidine and the heme. Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Return to opening image. Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Spacefilling model of oxymyoglobin. But where is the oxygen molecule? Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.
Toggle blow-up. The bound oxygen molecule is behind the (transparent) side chain of the distal histidine. The distal histidine acts as a gatekeeper. Its imidazole group is free to swing in and out of the heme pocket. This allows O2 to enter and leave the pocket.