Haemoglobin (Hb) is the complex protein that fills our red blood cells. Inside the red blood cells, haemoglobin exists in a stable environment containing the enzymes it requires to carry out its routine functions - transporting oxygen around the body.

The Hb protein consists of four subunit chains, two a and two b chains. Each of these chains contains an iron atom, to which oxygen can bind reversibly. One a and one b chain (monomers) combine to form one of the two ab dimers that comprise the Hb tetramer.

It is the breakdown of the Hb tetramer to its constituent monomers and dimers that causes the kidney damage often seen after injection of cell free Hb into the bloodstream. Such smaller products, if present in high enough concentrations, can damage the kidneys as it attempts to clear them from the bloodstream.

Haemoglobin's oxygen delivery

Oxygen binds actively to the 4 iron containing haem groups of the Hb tetramer in a reversible nature. This process produces what is now established as the characteristic sigmoid binding curve of oxygen to Hb, shown below. The curve is produced as the affinity for a haem group on a Hb molecule to bind to and also release oxygen varies in accordance with the oxygen saturation of the other haem groups on the molecule.

The mechanism for Hb oxygen delivery to our tissues and organs depends on a key chemical present inside the microenvironment of the red blood cell, called 2,3-diphosphoglycerate (2,3-DPG). This enzyme helps bring about a conformational change in the Hb tetramer that allows for its characteristic binding to oxygen.

In the presence of increased 2,3-DPG, Hb gives up its oxygen more readily (see graph). It is for this reason that the effect of poor oxygen delivery to the tissues is seen when cell free Hb is injected into the bloodstream, since the 2,3-DPG enzyme is not present and oxygenated Hb will not readily give up its oxygen molecules.

Problems of vasoconstriction

Many Hb-based products have shown to cause increased mean arterial blood pressure (MAP) and constriction of blood vessels. This can reduce the ability of Hb-based substitutes to deliver oxygen to the tissues. The mechanism by which the Hb in these products causes this effect is still under debate but a lot of evidence points towards the scavenging of nitric oxide (NO) gas.

Some companies are currently developing products that have increased molecular size and oxygen affinity properties. These have so far shown promising results and do not produce the vasoconstriction effects seen in other Hb-based substitutes.