The Thiol Groups of Human Hemoglobin Carrying Heme only on the α‐Chains

Hemoglobin ICII is a derivative of human HbA, carrying the heme prosthetic group only on the α‐chains. Reaction of ICII in the cyanmet form with p ‐mercuribenzoic acid or iodoacetamide yields one titratable thiol group per αβ dimer as in HbA. The kinetics of the thiol modification of HbA and ICII with iodoacetamide indicate that ICII reacts more than 10‐times faster than HbA under pseudo‐first‐order conditions. The site of modification was determined from the tryptic peptides following chain separation. The modified thiol group in both HbA and ICII was identified as cysteine β‐93, whereas cysteine α‐104 and β‐112 remained unmodified. These results indicate a different conformation of the heme‐free and heme‐bound β‐chains, which seems to be independent of the nature of the ligands of the α‐chains.