Phosphate binding by llama and camel hemoglobin

Abstract

1. We have estimated the binding constant of 2,3-diphosphoglycerate (DPG) to llama and camel hemoglobin at 37°C and pH 6.7, 7.2, and 7.7 from the DPG induced change in the half saturation pressure (P50). The values obtained were compared with the ones derived from similar measurements on human fetal and adult hemoglobin. At pH 7.2 the binding constant of DPG to llama deoxy-hemoglobin is smaller by a factor of 3 in comparison to camel hemoglobin whilst human fetal hemoglobin binds DPG six times less firmly than the human adult pigment. Camel and human adult hemoglobin have about the same affinity for DPG. 2. It is concluded that in llama hemoglobin an amino acid replacement at the DPG binding site explains the reduced phosphate affinity as compared to camel hemoglobin. In human fetal hemoglobin, in addition to an amino acid exchange, there are alterations of the anatomy of the DPG binding site which result in a poorer fit of the phosphate molecule. 3. Our data show that llama hemoglobin has a lower intrinsic oxygen affinity than camel hemoglobin so that despite the reduced DPG binding constant the oxygen affinity of llama and camel blood is very similar at equal phosphate concentration. Possibly, the amino acid exchange at the DPG binding site has compensated for the low oxygen affinity of llama hemoglobin in the course of phylogeny