Bond between the heme cofactor and the hemoglobin

[icakeov]

I understand that hemes bind oxygen through ionic bonds, but what is the bond between the heme and the rest of the hemoglobin molecule? Ionic, covalent or something in between?

 

[Ygggdrasil]

Heme is a coordination complex in which the iron ion is bound to the porphyrin ring via coordinate covalent bonds. A coordinate covalent bond is a covalent bond where the shared electron pair comes solely from one of the two atoms in the bond (rather than one or more electron coming from each atom involved in the bond). Such coordinate bonds are common when metal ions bind to organic substances, so they are often seen in bioinorganic chemistry.

 

[icakeov]

Thanks ygggdrasil!
And what about the heme group molecule itself? How does it bind to the bigger hemoglobin structure? Similar covalent bonds?
Many thanks again!

 

[Ygggdrasil]

Iron can form six coordinate covalent bonds, in an octahedral geometry. Four of these bonds are to the porphyrin ring. One bond is left empty to bind oxygen when present. The last bond, which is perpendicular to the plane of the porphyrin ring, is to the nitrogen atom of a specific histidine in the globin protein. This bond links the heme group to the protein.

 

[icakeov]

Thank you!