Structure and function of Hb Saint-Jacques (alpha 2 beta 2 140 (H18) Ala----Thr): a new high-oxygen-affinity variant with altered bisphosphoglycerate binding.
A low P50 value in a fresh red blood cell suspension was discovered in a polycythemic patient (Hb 19 g X dl-1). Routine acid and alkaline electrophoreses of the hemolysate were identical to normal hemolysate. Isoelectrofocusing (pH gradient 6-8) did not reveal any abnormal band whether performed with the fully liganded or deoxygenated samples. Precise analyses of the oxygen dissociation curves of the propositus' red cells demonstrated a biphasic Hill plot, a normal Bohr effect and low interaction with 2,3-bisphosphoglycerate (2,3-DPG). Studies on the unfractionated hemolysate confirmed these observations and the inhibition of the effect of organic phosphates. Structural studies were carried out on the mixture of beta A + beta X chains and revealed the presence of two beta Tp14 peptides. Sequencing the abnormal beta Tp14 peptide showed the substitution Ala----Thr of the beta 140 (H18) residue. This new variant was named Hb Saint-Jacques. Examination of the three dimensional model of HbAo indicates that the substitution beta 140 (H18) Ala----Thr induces van der Waals interactions with the nearby lysine-82 (EF6) and leucine-81 (EF5) and a displacement of the EF corner of the beta chains. This is likely to change the normal position of the lysine-82 (EF6), a major anionic binding site in the central cavity between the two beta chains. Functional studies confirm the interpretation of a steric hindrance inhibiting the binding of large organic phosphates to Hb Saint-Jacques.
D019794:2,3-Diphosphoglycerate; D000328:Adult; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D002712:Chlorides / Q000494:pharmacology; D004163:Diphosphoglyceric Acids / Q000378:metabolism*; D006455:Hemoglobins, Abnormal / Q000032:analysis*; D006801:Humans; D008297:Male; D010100:Oxygen / Q000378:metabolism; D011086:Polycythemia / Q000097:blood; D014357:Trypsin / Q000378:metabolism
D000596:Amino Acids; D002712:Chlorides; D004163:Diphosphoglyceric Acids; D006455:Hemoglobins, Abnormal; D019794:2,3-Diphosphoglycerate; C040760:hemoglobin Saint Jacques; D014357:Trypsin; D010100:Oxygen