Oxygen binding proteins myoglobin and hemoglobin transport and store oxygen in the body. Myoglobin is a monomer that binds one oxygen molecule to carry oxygen to mitochondria in cells. Hemoglobin is a tetramer made of two alpha and two beta chains that binds four oxygen molecules cooperatively to carry oxygen from lungs to tissues. The heme group contains iron that binds oxygen. Hemoglobin exhibits positive cooperativity where binding of oxygen to one subunit increases the affinity of the other subunits. The Monod-Wyman-Changeux and Koshland models describe the transition between tense and relaxed hemoglobin states during cooperativity. 2,3-bisphosphoglycerate binds preferentially to deoxygenated hemoglobin and shifts

1. Madrona, Vivien Alexandra C.
3Bio5
College of Science
University of Santo Tomas
Espana, Manila
Oxygen binding by Myoglobin &
Hemoglobin

2. What is Oxygen binding?
Oxygen binding is the binding of an oxygen
molecule to a specific functional protein for
either transport or storage in vivo.

3. Myoglobin (Mb)
• Monomeric
• Binds 1 oxygen molecule.
• Carries O2 from capillaries to
sites of usage (mitochondria) in
cells.
• Non-cooperative binding of O2.

4. Hemoglobin (Hb)
• Tetrametric, two alpha chains and
two beta chains
• binds a total of 4 oxygen
molecules
• carries O2 from lungs to tissues
• cooperative binding of O2
• required to increase the solubility
of O2 in blood

5. The Heme group
• Example of a prosthetic group
• Heterocylic ring containing 4
pyrrole rings
• Central atom is Fe2+ (usual
oxidation state)
• Proximal Histidine is important
in transducing the binding event
to protein.

6. Ligands
a ligand (from the Latin ligandum,
binding) is a substance (usually a
small molecule), that forms a complex
with a biomolecule to serve a
biological purpose.
In a narrower sense, it is a signal
triggering molecule, binding to a site
on a target protein.
Myoglobin (blue) with its
ligand heme (orange)
bound

7. Mechanism of positive cooperativity in
Hemoglobin
• Binding of O2 to Fe moves proximal Histidine residue and
its attached helix (F)
• Helix F adjusts conformation by movement of a b subunits
(hinge and helix ratchet)
• Alters conformation of Iron (Fe) at un-liganded sites.

8. Allosteric Effects and Cooperativity

9. Allosteric effect
Allosteric effects occur when the binding
properties of a macromolecule change as a
consequence of a second ligand binding to the
macromolecule and altering its affinity towards
the first, or primary, ligand.

10. Types of Allosteric effects
I. If the two ligands are the same (e.g. oxygen) then
this is called a homotropic allosteric effect.
II. If the two ligands are different (e.g. oxygen and
BPG), then this is called a heterotropic allosteric
effect.

11. Allosteric effect
• Macromolecules that have multiple ligand binding sites
(e.g. Hb), allosteric effects can generate cooperative
behavior.
• Allosteric effects are important in the regulation of
enzymatic reactions.
• Both allosteric activators (which enhance activity) and
allosteric inhibitors (which reduce activity) are utilized to
control enzyme reactions.

12. Allosteric effect
Allosteric effects require the presence of two forms of the
macromolecule.
1. One form, T or tense state, binds the primary ligand
(e.g. oxygen) with low affinity.
2. The other form, R or relaxed state, binds ligand with
high affinity.
The T and R states are in equilibrium with each other.
In the case of positive cooperativity the fraction of T states
exceeds that of the R state.

13. Models of Allosteric changes &
cooperativity in Hemoglobin

14. Transition from T to R
Change from T to R states may occur:
• In unison via the Monod-Wyman-Changeux (MWC)
model
• Sequentially via the Koshland (KNF) model

15. Monod-Wyman-Changeux (MWC) model
A concerted model for the basis of cooperativity in multimeric
binding proteins,
In the absence of a ligand (oxygen in the case of hemoglobin)
the subunits exist in a T-form.
The ligand may bind to as many as two monomers in this
conformation, at which point all four monomers convert from
T-form to R-form.

16. Koshland (KNF) model
• The binding of the ligand causes conformational change.
• Although the subunits go through conformational changes
independently (as opposed to in the MWC model).
• The switch of one subunit makes the other subunits more
likely to change, by reducing the energy needed for
subsequent subunits to undergo the same conformational
change.

17. BPG
• 2,3-Bisphosphoglyceric acid is a
three-carbon isomer of the glycolytic
intermediate 1,3-bisphosphoglyceric
acid (1,3-BPG)
• 2,3-BPG is present in human red blood
• It binds with greater affinity to
deoxygenated hemoglobin (e.g. when
the red cell is near respiring tissue) than
it does to oxygenated hemoglobin (e.g.,
in the lungs)

18. BPG
• It interacts with deoxygenated hemoglobin beta subunits by
decreasing their affinity for oxygen
• allosterically promotes the release of the remaining oxygen
molecules bound to the hemoglobin, thus enhancing the ability
of RBCs to release oxygen near tissues that need it most. 2,3-
BPG is thus an allosteric effector.

19. Heterotropic Allosteric effectors in Hemoglobin
• BPG binds preferentially to the tense (lower binding
form) of hemoglobin.
• It shifts the binding curve to the right, higher
concentrations of oxygen are required to fully saturate
hemoglobin.
• Oxygen delivery at low oxygen pressure (high
altitude) is enhanced by increasing the amount of
diphosphoglycerate in the red cell.

20. Oxygen binding curves
The Oxygen binding
curves of Myoglobin and
Hemoglobin with respect
to the percent saturation
of O2 and pressure.

21. References
• Oxygen binding by myoglobin & hemoglobin.
(2004, October 1). Retrieved from
https://www.bio.cmu.edu/courses/03231/LecF04/Lec13/le
c13.html
• Bucci, E., Razynska, A., Kwansa, H., Gryczynski, Z., Colli
ns, J. H., & Fronticelli, C. (1996). Positive and negative
cooperativities at subsequent steps of oxygenation
regulate the allosteric behavior of multistate
sebacylhemoglobin. Biochemistry, 35, 3418 - 3425.