Hemoglobin: Binding O2 — Cooperation Makes It Easier
I’m pretty sure I’m the only one in my biochemistry class that is so excited about how incredible the binding of oxygen to hemoglobin is that I’m losing sleep over it. Anyway, the gorgeous and colourful animination above, from the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB), illustrates the conformational change upon oxygen binding to hemoglobin.
What is hemoglobin? Hemoglobin is a remarkable metalloprotein (a protein that contains a metal ion), found in red blood cells, that plays an important role in oxygen transport. Hemoglobin is a heterotetramer consisting of 2 α subunits (light pink and the other one can’t be seen) and 2 β subunits (light purple and light blue), arranged in 2 αβ subunits (2 sets of dimers). Each subunit contains one heme group (red) — a protoporphyrin ring with Fe2+ in the ring centre — resulting in four heme groups in total.2
How does hemoglobin bind O2? Interestingly, hemoglobin binds O2 (teal) cooperatively: when one heme group binds to O2, it increases the other heme groups’ affinity (ability to bind) for O2. This is a type of allosteric interaction — the change in shape of a protein that results from binding of a molecule at the allosteric site (a site other than the active site).2
Why does this happen? When O2 is not bound (deoxy), Fe2+ lies a little outside of the protoporphyrin ring. When O2 is bound (oxy), Fe2+ ”pops” into the ring, pulling with it a histidine (yellow), His, residue. Also attached to His is an α-helix (orange), which also shifts. All of this movement disrupts and forms new interactions between the α1β1-α2β2 interface.3 It is this conformational change that increases the other hemes’ ability to bind to O2. Noticeably, as more O2 binds to hemoglobin, the α1β1 dimer will rotate 15° relative to the α2β2 dimer, which can be observed in the animation.2
Sources used: 1. Hemoglobin animation: RCSB PDB 2. Krisinger, M. BIOC 202 Lecture on Protein Function. Presented at the University of British Columbia. May 27, 2013. 3. RCSB PDB. Hemoglobin. http://www.pdb.org/pdb/101/motm.do?momID=41 (accessed May 27, 2013)
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