Hemoglobin & Myoglobin: 3. Hemoglobin

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. – This is hemoglobin's deoxy form; its oxygen affinity is low.

We redraw the tetramer.

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. – 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.

Hemoglobin & Myoglobin: 3. Hemoglobin

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