Oxygen Binding by Myoglobin and Hemoglobin

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 O 2 from capillaries to sites of usage ( mitochondria ) in cells. Non-cooperative binding of O 2 .

4. Hemoglobin ( Hb ) Tetrametric , two alpha chains and two beta chains binds a total of 4 oxygen molecules carries O 2 from lungs to tissues cooperative binding of O 2 required to increase the solubility of O 2 in blood

5. The Heme group Example of a prosthetic group Heterocylic ring containing 4 pyrrole rings Central atom is Fe 2+ (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 O 2 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 . One form, T or tense state , binds the primary ligand ( e.g. oxygen) with low affinity. 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). T he 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/lec13.html Bucci , E., Razynska , A., Kwansa , H., Gryczynski , Z., Collins, 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.