Oxygen (O₂) Carrying Functions of Hemoglobin!

When Fe²⁺ binds O₂, the confirmation of heme is altered such that Fe²⁺ is forced out of the plane of the Hb molecule sufficiently such that other compounds will not bind to it.  This makes it harder for most other substances to displace O₂ from Hb.  Because the Fe²⁺ is bound to the globin portions of the Hb molecule, O₂ binding to the Fe²⁺ causes the Fe²⁺ to move the globin chains like levers such that the structure of the Hb molecule changes and protects Fe²⁺ from binding to just about anything other than O₂.  Hb molecules in RBC's rotate rapidly to allow O₂ to be picked up near the cell membrane and transferred from Hb to Hb towards the center of the RBC. Thus, O₂ is distributed to all of the Hb molecules within the RBC.

Each hemoglobin molecule is made up of a "tetramer" of globin subunits with a heme ring at the center of each globin subunit.  That means that each Hb can carry a maximum of 4 O₂'s!  Notice that each hemogobin (Hb) has 2 alpha subunits and 2 beta subunits!  The alpha subunits have a higher affinity for O₂ than do the beta subunits.  This means that the alpha subunits are the first subunits to load O₂ in the lungs and the last subunits to release O₂ to the tissues!  Recall that O₂ unloading from the alpha subunits normally requires the assistance of 2,3-DPG.  This difference in affinity for O₂ between subunit types results in a sigmoid O₂ saturation curve.  The curve is sometimes referred to as the "cooperative" curve as O₂ loads onto the alphas before the betas and the betas must unload O₂ at the tissues before DPG can assist in O₂ unloading from the alpha subunits of Hb.

O₂ binds to alpha subunits with a higher affinity than to beta subunits of Hb. Thus, O₂ binding occurs in a stepwise fashion, first to alpha subunits and then to beta subunits. Therefore, we see a "sigmoid-shaped cooperative curve" for O₂ loading onto Hb. The sigmoid curve is a cooperative curve because when alpha globin binds O₂, allosteric changes to Hb increase the affinity of the beta subunits for O₂.  If Hb were a monomer, rather than a tetramer, we would see a saturation curve similar to that of myoglobin (ie. without the effects of allosteric interaction).  Remember, myoglobin is the O₂ carrying molecule in muscle and each myoglobin binds a single O₂.

The sigmoid "cooperative curve" for Hb gives Hb an advantage during O₂ unloading at the tissues.  Let's look at the curve at the 50% saturation point.  Note that Hb can be 20-90% saturated with O₂ within a narrow range of partial pressures of O₂ in the blood (the steepest part of the curve).  Thus, it is possible to readily unload O₂ at the tissues with only a small decrease in the partial pressure of O₂ (PO₂).  The steep central portion of the curve also means that, in the lungs, at high PO₂, you can hold your breath or hyperventilate and change the partial pressure of O₂ a lot.  Hb is loaded 100% with O₂ in the lungs (top of saturation curve).

How much O₂ can Hb bind? - 1 g of Hb binds 1.34 ml of O₂ at 37C

It is not uncommon to have 15 g of Hb in 100 ml of blood, thus; 20 ml of O₂/100 ml blood or : 150 g Hb/l with 200 ml O₂.

Under normal atmospheric conditions, only about 3 ml of O₂ can effectively dissolve in 100 mL of blood (exclusive of that bound to Hb) such that Hb is of great importance to the O₂ carrying capacity of blood (ie. Hb transports over 98% of the O₂ in the blood).  The remaining 2% is delivered to the tissues in a diffused form carried in the plasma!

Mechanism one for CO₂ transport in the blood:

The above reaction proceeds to the right (as depicted) in the tissues in order to disperse O₂ to the tissues and to remove CO₂ from the tissues!  The above reaction proceeds to the left (as depicted) in the lungs in order to eliminate CO₂ and pick up O₂.

CO₂ is a byproduct of metabolism and must be transported to the lung where it can be "blown off" in the expiratory gas.  Hb binds CO₂ mainly on the alpha globin (carbaminoHb); 20% of CO₂ in blood is transported to the lungs bound to Hb in the form of carbaminoHb.  CO₂ binds more readily to deoxygenated Hb (the Haldane effect).  That is, the affinity of Hb for O₂ is greater than for CO₂.  Therefore, the binding of O₂ in the lungs facilitates the release of CO₂ from Hb; at the tissues, the release of O₂ facilitates the binding of CO₂ (once again, this is known as the Haldane Effect).

Mechanism two for CO₂ transport in the blood:

As CO₂ passes through RBC membranes, the "carbonic anhydrase enzyme" promotes conversion of CO₂ to carbonic acid (H₂CO₃) via the following reaction:

Approximately 60% of the CO₂ picked up from the tissues undergoes the carbonic anhydrase reaction (above) within the RBC membranes.  That means that 60% of the CO₂ picked up in the tissues and delivered to the lungs is transported in the form of carbonic acid!  Looking at the above equation you can see the danger of hypercarbia is the potential for acidosis with an increase in H⁺.  This is not normally a problem.  Even though the carbonic anhydrase reaction produces lots of H⁺ ion in solution, a minimum of pH change occurs in the blood as H⁺ is buffered as a result of being picked up by Hb.  We will talk about the reasons that Hb is such a good buffer when we get to the section on acid-base balance!  However, any protein can act as a buffer; Hb is present in large quantities in blood; the ability of Hb to transport H⁺ with almost no change in blood pH is a phenomenon referred to as "isohydric transport".

If you are a real chemistry afficianado, you may be interested in the following effects of O2 binding on the pk_a of Hb.  Recall that pk_a is the pH at which a substance is 50% ionized and is, thus, the pH at which a substance works best as a buffer!  When Hb releases O₂ at the tissues, the pK_a of Hb goes from 6.68 to 7.93; in other words, as Hb releases O₂, Hb becomes a weak base (H⁺ acceptor) at the tissue such that Hb will bind H⁺ with greater affinity at the tissues.  Let's add H⁺ to the oxyhemoglobin <-> carbaminohemoglobin equation:

In the tissues, this reaction proceeds to the right, as depicted.  In the lungs, the reaction proceeds to the left.  This means that...

In the tissues: where Hb releases the O₂ it is carrying, becoming deoxyhemoglobin, the pka of Hb jumps up to 7.92, making it a weak base which can pick up H⁺ and remove it from the tissue!

In the lungs: where Hb picks up O₂ to become oxyhemoglobin, the pK_a of Hb falls to 6.68, making it a weak acid which releases H⁺ as it picks up O₂!

Mechanism three for CO₂ transport in the blood:

Approximately 20% of the CO₂ carried in the blood is simply dissolved in plasma; CO₂ is highly diffusible and readily dissolves in aqueous solutions (for example, in carbonated beverages like soda pop).  Recall that as little as 2% of O₂ is transported in the dissolved form in plasma!