Haemoglobin is about the most thoroughly studied molecule larger than water, but it still turns up surprises. The job of delivering oxygen around the body is a tricky balancing act, and the balance depends on whose body, and where it is. Recently, Roy Weber and co-workers have shown that haemoglobin from the frog species Telmatobius peruvianus, which lives in snow melt at altitudes above 3500 metres in the Andes, has exceptionally high oxygen affinity.
In itself this is not unexpected – some bird species can fly at three times that altitude thanks to high oxygen affinity haemoglobin variants– but the frog has evolved rather differently. Since the classic studies of Perutz on human haemoglobin, it is well understood that the oxygen affinity of the protein can be altered radically by gaining or losing bonds that stabilise the preferred conformations of the deoxy and liganded states. Even very minor changes can affect the equilibrium between these T and R states significantly. For many animal haemoglobins, the low affinity T conformation is bound more strongly by heterotropic (non-oxygen) ligands, including organic phosphates such as diphosphoglycerate (DPG) or ATP, chloride and hydrogen ions, and Nature has found that DPG provides opportunities for fine-tuning the properties of the protein. Mammals living at high altitude tend to have haemoglobins that bind DPG more weakly than those close to sea-level, and the amino acid sequences of the proteins show alterations around the DPG binding site, between the two beta chains and in the central cavity of the tetramer. Weaker heterotropic ligand binding means the T state is less stabilised, and oxygen affinity is higher.
The first surprise with the new analysis of the frog haemoglobin is that its oxygen binding is almost completely independent of chloride concentration,the first time this means of altitude adaptation has been found. The second surprise is the protein sequence; although about 56% of the residues are identical to those of human haemoglobin, none of the usual suspects, the positively charged beta chain residues in the central cavity, are altered. This result appears to conflict with Perutz's view that chloride functions through general electrostatic effects within the protein, rather than binding at discrete sites. Instead, alpha chain residues Val 1 and Ser 131, implicated by other studies in chloride binding, are found to have mutated in the Telmatobius protein. Reanalyzing the sequences of other high-altitude mammalian haemoglobins (and human fetal haemoglobins) in this light suggests that they too may have weakened chloride effects. Whether or not this particular twist in the tale of haemoglobin belongs solely to a little frog living in mountain streams, or whether we used it before we were born, will be interesting to find out.