Structure of hemoglobins Zürich [His E7(63)beta replaced by Arg] and Sydney [Val E11(67)beta replaced by Ala] and role of the distal residues in ligand binding.

Abstract

In [human] Hb Zurich the side chain of the distal arginine attaches itself to the propionate of the heme, leaving the heme pocket wide open, allowing sulfanilamides easy access to the iron, and doubling the partition coefficient between CO and O2. The replacement of the distal valine by alanine in Hb Sydney leaves a large gap inside the heme pocket, which is partly filled by a water molecule bonded to the distal histidine. Hb Sydney has the same partition coefficient between CO and O2 as Hb A. Replacement of the distal histidine increases the stretching frequency of CO linked to the .beta. heme by 6 cm-1, but replacement of the distal valine increases it by only 3 cm-1. Replacement of the distal histidine leaves the O.sbd.O stretching frequency unchanged.