Stereochemistry of carbon monoxide binding to normal human adult and Cowtown haemoglobins.

The structures of carbonmonoxyhaemoglobins A and Cowtown (His146 beta----Leu) have been refined at 2.2 A (1 A = 0.1 nm) and 2.3 A resolution, respectively. The least squares fit to the Fe-C-O line makes an angle to the haem normal of about 6 degrees. The Fe-C-O group is bent from linearity by about 7 degrees. The porphyrins in the CO liganded haemoglobins are ruffled. This deformation of the haem and the distortion of the Fe-C-O group may explain the low CO affinity of haemoglobin. The electron density for the C-terminal residues is low but sufficient to distinguish the histidyl and leucyl residues clearly. The similarity between these two structures, apart from 146 beta, means that the reduced alkaline Bohr effect is due solely to the replacement of histidine by a leucine.