Hemoglobin
Comprises four polypeptides instead of just one
- Dimer in which each subunit contains a heme group.
– Each subunit closely resembles myoglobin's tertiary structure.
- Subunits are labeled alpha and beta.
– Strong hydrophobic interactions stabilize the dimer.
- We show an identical dimer below this one.
Weak ionic and hydrogen bonds stabilize this dimer's interaction with the one above it. Thus, hemoglobin is a tetramer with four subunits. Its actual three-dimensional structure is more complex, but we will not worry about that, here.*
– This tetramer is labeled the T-form for "taut" or "tense," in which the polypeptide chains are restricted in their movement.
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This is hemoglobin's deoxy form; its oxygen affinity is low.
We show that the heme groups are bound to oxygen. We leave out the stabilizing histidines for clarity.*
- We show representative weak ionic and hydrogen bond between the two dimers.
– Oxygen disrupts these bonds.
– This is the R-form for "relaxed."
– The weaker ionic and hydrogen bonds allow the subunits to move slightly in this state.
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This is hemoglobin's oxy form; its oxygen affinity is high.
- We show reversible arrows to represent the conformational change between the T-form and R-form hemoglobin molecules.
cooperative binding
The conformational change: cooperative binding
- Step 1: One oxygen binds a hemoglobin subunit.
- Step 2: this binding disrupts inter-dimer bonds, which causes a conformational change.
- Step 3: this change in hemoglobin's three-dimensional structure increases the remaining subunits' affinities for oxygen.
Why doesn't myoglobin exhibit cooperative binding?
- Because it only has one heme group, and thus only one oxygen binding site.
– Myoglobin is fully saturated when bound to one oxygen; its affinity cannot increase when there are no additional oxygen binding sites.
