Respiration

Respiratory pigments:
Coloured proteins that contain a metabolic element in their constitution and have the property of forming loose combination with O2 and CO2.
They increase the oxygen carrying capacity of the blood.
Haemoglobin increases the oxygen carrying capacity of the blood between 65 and 70 times.
Each red blood cell has about 250 million haemoglobin molecules, and each milliliter of blood contains 1.25 x 1015 haemoglobin molecules.
Oxygen concentration in cells is low (when leaving the lungs blood is 97% saturated with oxygen), so oxygen diffuses from the blood to cells when it reaches the capillaries.

Functions of Respiratory Pigments
Gas transport (O2 and CO2)
Gas (O2) storage
pH buffers
Enhancing gradients for gas diffusion
Non-respiratory transport (e.g. NO)
Possible enzymatic function (e.g., NO)

Structure of haemoglobin
Haemoglobin consists of 4 globular polypeptide subunits 2ά and 2ß chains.
Each subunit is composed of a polypeptide chain tightly associated with a non-protein heme group.
Each heme has a central iron (Fe++) atom held in a heterocyclic ring, known as porphyrin. The iron atom which binds a single O2 molecule, binds with the 4 nitrogens in the center of the ring and also is bound strongly to the globular protein.
Mwt. Of Hb is 68,000 daltons, lifespan is 120 days.

The iron atom may either be in the Fe2+ or Fe3+ state, but ferrihemoglobin (methemoglobin Fe3+) can not bind oxygen.
In binding O2 temporarily oxidizes Fe2+ to Fe3+ so iron must exist in the +2 oxidation state in order to bind oxygen.
Methemoglobin (Fe3+), is reduced back to Fe2+ by methemoglobin reductase in RBC.
Oxygen combine loosely with heme of Hb forming ox-hemoglobin.
Hemoglobin reversibly binds O2

Hb (deoxyhemoglobin) + O2  HbO2 (oxyhemoglobin)

loading vs. unloading determined by:
O2 tension in the plasma
affinity of Hb for O2

Oxygen binding to hemoglobin
The binding affinity of Hb to O2 is increased by the O2 concentration of the molecule, the first O2 bound influences the shape of the binding sites for the next O2 ‘s in a way favorable for binding.
As a consequence, the oxygen binding curve of Hb is sigmoidal, or S-shaped, vs normal hyperbolic curve associated with non cooperative binding.
The sigmoid shape of the O2 dissociation curve is a result of the cooperative binding of O2 to the four polypeptide chains.
Cooperative binding is characteristic of a Hb to have a greater ability to bind O2 after a subunit has bound O2. Thus, Hb is most attracted to O2 when 3 of the 4 chains are bound to O2.
Hb- O2 binding is affected by molecules such as carbon monoxide –CO (from tobacco smoking, cars and furnaces).

CO competes with O2 at the same heme binding site –carboxyhemoglobin. Hb binding affinity for CO is 200 times greater than its affinity for O2 meaning that small amounts of CO reduces Hb ability to transport O2.
Inspired air containing CO levels as low as 0.02%, cause headache and nausea; if the CO concentration is increased to 0.1% unconsciousness follows.
In heavy smokers up to 20% of the oxygen-active sites can be blocked by CO.
Carboxyhemoglobin is very toxic at high concentrations.
Hb also has competitive binding affinity for cyanide (CN), SO, NO2 and S2- including H2S. All of these bind to iron in heme inhibit oxygen-binding, causing grave toxicity.

Oxygen dissociation curve
Oxygen dissociation curve is a graph that shows the percent saturation of Hb at various partial pressure of oxygen.
The purpose of an O2 dissociation curve is to show the equilibrium of oxyhemoglobin and non-bonded hemoglobin at various partial pressures.
At high partial pressures of O2 usually in the lungs, Hb load O2 at respiratory surface to 95%.
When the blood is fully saturated the RBC are in the form of oxyhemoglobin.
P50
PO2 at which 50% of the Hb is saturated with O2
index of Hb affinity:  P50,  affinity


the curve relating percentage saturation of the O2-carry power of hemoglobin to the PO2.
As RBC travels to tissues deprived of O2 the partial pressure of O2 decrease to 40%.
Consequently, the oxyhemoglobin unload O2 in the tissue to form Hb.
Hb with high affinity facilitate movement of O2 into blood from environment because it binds at low PO2.Hb with high affinity does not release O2 to tissues until PO2 of tissue is very low.
Functionally:
Hb should have high affinity to O2 at respiratory surface and low affinity to O2 in the tissue.
Hb - O2 affinity is affected by changes in chemical and physical factors in the blood that favour O2 binding at respiratory surfaces and O2 release in the tissues.
Changes in CO2, pH, temperature and organic phosphates like 2,3-Diphosphoglycerate (DPG) directly affects the dissociation of oxygen.
Carbondioxide:
Presence of CO2 reduces O2 affinity of pigment with more readiness to release O2 .
At comparable pressures, the O2 dissociation curve is shifted to the right in presence of CO2 –Bohrs’s shift/effects
Factors that influence O2 binding –pH
pH:
A decrease in pH (increase in acidity) by addition of CO2 or other acids causes a right shift of the curve characterized by causing more O2 to be given up as O2 pressure increases.CO2
More pronounced in animals of smaller size due to the increase in sensitivity to acids.





Factors Affecting Affinity: Organic Phosphates
Erythrocytes (RBCs)
carry hemoglobin
contain phosphates (ATP, DPG, IP5, GTP, etc.)
DPG - decreases O2 affinity of Hb
induces unloading of O2

Factors that influence oxygen binding -2,3-DPG
Organic phosphates -2,3-Diphosphoglycerate (DPG) is the primary organic phosphate in mammals.
DPG binds to Hb which rearranges the Hb into the T-state, thus decreasing the affinity of oxygen for Hb. The curve shifts to the right.
Normal RBC always contain DPG. The higher the concentration of DPG the greater the release O2 by Hb molecule.






Thyroxine, growth hormone, epinephrine, androgens and high blood ph increases concentration of DPG.
Production of DPG decrease with RBC’s age. Stored blood has decreased 2,3-DPG.
When DPG is too low Hb bind to the O2 firmly and blood become useless for transfusion, because the RBC will not release O2 to periphery tissue.




REFERENCES

• Gorge B. Johnson 2000. The living world. Second edition