Animated view of hemoglobin subunit rotation

When hemoglobin switches from T-state to R state, a1 and a2 globins move closer in R state than T state. Likewise b1 and b2 globins move closer in R state.

Is this a contradiction?

How can the N-terminus of a1 and the C-terminus of a2 globins move apart in R state if everything else is moving closer?

The answer is that globin subunits are not single points in space.

If we look at two corresponding points x on a1 and a2 globins, these points will move closer in R state.

However, if we take two non-corresponding locations, point n in a1 and point c in a2 that happen to be close together in T-state, then watch as the subunits rotate into the R state position, point n will move away from point c.

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