Was the loss of the D helix in α globin a functionally neutral mutation?

Abstract

PROTEINS in the globin family are found in a variety of species from bacteria to man^{1, 3}. From the many globin sequences known, evolutionary trees have been constructed showing that α and β globins diverged from a common ancestor between 425 and 500 million years ago, after vertebrate species had appeared and roughly when sharks and bony vertebrates diverged^4–6. The αand β globins assemble to form tetrameric haemoglobin,α₂/ β₂, which can switch between quaternary states having high and low oxygen affinity⁷. This allows the protein to bind oxygen cooperatively and therefore efficiently transport oxygen from the lungs to respiring tissues. The αand β globins have closely related tertiary structures, being α-helical proteins with similar haem-binding sites. Most globins consist of eight helices, designated A to H from the N terminus, connected by short nonhelical segments, but all known vertebrate α globins lack a D helix. Because the loss of this helix by α globin occurred shortly before tetrameric haemoglobin appeared, it might be a functionally important mutation required for a tetramer assembly or allostery. We have now tested this idea by engineering human haemoglobins containing β subunits without a D helix and α subunits with a D helix. Both of these mutations have little effect on the oxygen-binding properties of the molecule. Thus it is possible that deletion of the D helix in the α subunit was caused by a neutral mutation⁸.