Hemoglobin: Sequential and Symmetry Model

Hemoglobin binds in a cooperative fashion because of four different heme groups that are involved.

As a result, hemoglobin is capable of allosteric binding which means that binding of a ligand at one site affects the binding of another site.

Two models have been proposed to account for the cooperativity ogf ligand binding.

The first is the Symmetry Model:


Source: https://www3.nd.edu/~aseriann/CHAP10B.html/sld039.htm

Here are some guidelines for the symmetry model:

  1. An allosteric protein is an oligomer of symmetrically related subunits
  2. Each oligomer can exist in two conformational states, designated as R and T state
  3. The ligand can bind to a subunit in either conformation
  4. Molecular symmetry is conserved during conformation. In other words, R and T states do not simultaneously contain both conformations

The second is the Sequential Model:


The major difference here is that the ligand binding induces a conformational change in the subunit (no symmetry) thus enabling cooperativity to occur.

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