Cooperative Site Binding (11.8)

1
Cooperative Site Binding (11.8)

• Binding of ligands to a biomolecule can affect
  the ability of other active sites to bind ligands
  and is called cooperative binding
• Full cooperativity occurs when either all sites
  are occupied or unoccupied
• In full cooperativity, the number of free sites
  is related to the number of free biopolymer
  molecules
• Ratio of occupied to unoccupied sites is related
  to the ratio of bound biopolymer to free
  biopolymer
• The Hill equation is used to determine the
  binding constant and the number of active sites
  on a biopolymer that exhibits full cooperativity
• A normalized saturation parameter can be defined
  and is related to the fraction of active sites
  occupied on the biopolymer

2
Real World Cooperativity (11.8-11.9)

• Fully cooperative binding is an idealized
  situation
• Intermediate cooperativity can be described using
  the Hill parameter (a)
• Hill parameter can range from 1 (independent
  binding) to N (fully cooperative), called
  positive cooperativity
• Hill parameter can also be less than 1, called
  negative cooperativity
• Allosterism is the property of a protein where
  ligand binding regulates the activity of the
  protein
• Structural changes in the protein as ligands bind
  are responsible for allosterism
• Allosteric effects are often described by one of
  two models concerted or sequential
• Oxygen transport by hemoglobin and myoglobin are
  examples of cooperative and independent binding
• Myoglobin exhibits independent binding (a1 in a
  Hill plot)
• Hemoglobin exhibits cooperative binding at
  intermediate partial pressures of oxygen
• Hemoglobin exhibits independent binding at high
  and low oxygen concentrations

3
Concerted and Sequential Models (11.9)

• The concerted model describes ligand binding in a
  two-state model
• Binding sites are either in a tense state (T) or
  a relaxed state (R), with the relaxed state
  having a higher binding affinity
• All sites on the protein exist in either the T or
  R state
• An equilibrium exists between the T and R forms,
  with the value of the equilibrium constant
  depending on the number of ligands bound to
  protein
• As the number of ligands bound to the protein
  increases, the equilibrium shifts to the relaxed
  form
• High and low concentration binding of hemoglobin
  is explained well using the concerted model
  (binding becomes independent at high and low
  concentrations of oxygen)
• Sequential model states that the binding of each
  active site is affected by the successive binding
  of ligands
• A mix of tight and relaxed active sites is
  allowed
• This model can be used to explain negative
  cooperativity

4
Independent vs. Cooperative Binding
5
Hemoglobin
6
Myoglobin
7
Binding in Hemoglobin and Myoglobin
8
Hill Plot for Hemoglobin and Myoglobin
9
Concerted Model
10
Sequential Model