Haemoglobin oxygen binding

In the concerted model, the first binding of substrate increases the affinity of the other active sites, but further bindings have no effect. This doesn t contradict the haemoglobin/oxygen binding constants presented earlier, as the increased affinities at ever increasing oxygen concentrations could be due to the increased proportion of relaxed form, rather than to individual increases in each protein molecule (Fig. 6.16). 

Fig. 6.2. Haemoglobin-oxygen binding curve. Note the increase in sigmoidicity as the Hill coeflicient (h) increases from I to 5.

Figure 13.5 (a) The structure of sperm whale myoglobin. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.) (b) The oxygen-binding curves of myoglobin and haemoglobin. (Reprinted with permission from Collman et al., 2004. Copyright (2004) American Chemical Society.)... 

In addition to altered oxygen-binding characteristics, free haemoglobin in plasma disassociates rapidly into afi dimers, which are in turn rapidly oxidized and cleared by the kidneys. Indeed, high plasma concentrations can result in kidney toxicity. Development of a... 

There are two types of control shown by allosteric enzymes. They can be modulated by a molecule other than a substrate of the enzyme (termed heterotrophic enzymes, e.g. threonine dehydratase), or by the substrate itself (termed homotrophic enzymes, e.g. oxygen binding to haemoglobin). They contain two or more binding sites for the substrate, and activity is modulated by the number of binding sites which are filled. 

The oxygen-binding curves for myoglobin(Mb) and haemoglobin(Hb), showing also the pH-dependence(Bohr effect). 

Thus, for haemoglobin (Hb) the oxygen binding curves are sigmoidal as is shown in the above figure. The fact that n exceeds the unit can be ascribed physically to the fact that attachment of 02 to one haem group increases the binding constant for the next 02, which in turn increases the constant for the next one and so... 

Alternatively, if this agent is not available, the cyanide can be made to bind to the haemoglobin in the patient s blood, which will bind cyanide if it is in a particular (known as oxidised) form. Some of the patient s haemoglobin can be converted to the oxidised form by giving him/her a substance called nitrite. The cyanide in the blood then binds to the oxidised haemoglobin. Oxygen can also be given with these treatments and seems to help. 

Homotropic this is where the sites are identical, and each sites is allosteric to the others. This is like the cooperative interactions seen in oxygen binding by haemoglobin - four (essentially) identical oxygen binding sites interacting with each other allosterically. 

Hemerythrin is an oxygen carrier iron protein found in certain species of marine invertebrates within the Annelida, Brachiopoda, Priapulida and Sipunculoidea. This respiratory pigment can be considered a rather remote non-haem evolutionary relative of haemoglobin. It binds one... 

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