Structural basis for the Root effect in haemoglobin

The remarkable ability of Root effect haemoglobins to pump oxygen against high O 2 gradients results from extreme, acid-induced reductions in O 2 affinity and cooperativity. The long-sought mechanism for the Root effect, revealed by the 2 Å crystal structure of the ligand-bound haemoglobin from Leiostomus xanthurus at pH 7.5, unexpectedly involves modulation of the R-state. Key residues strategically assemble positive-charge clusters across the allosteric β1β2-interface in the R-state. At low βH, protonation of the βN terminus and His 147(HC3)β within these clusters is postulated to destabilize the R-state and promote the acid-triggered, allosteric R→T switch with concomitant O 2 release. Surprisingly, a set of residues specific to Root effect haemoglobins recruit additional residues, conserved among most haemoglobins, to produce the Root effect.