Kinetics of the Carbamylation of the Amino Groups of Sickle Cell Hemoglobin by Cyanate
Choong K. Lee 1 and James M. Manning 1
From the
1 From The Rockefeller University, New York, New York 10021
The NH2-terminal valine residues of both chains of oxyhemoglobin S are carbamylated 50 to 100 times faster at pH 7.4 than the 
-NH2 groups of the lysine residues of the protein up to a level of 1 carbamyl group per hemoglobin molecule. The pseudo-first order rate constants for the carbamylation of the NH2-terminal residues of carbonmonoxyhemoglobins A and S are identical for the proteins either in the tetrameric state or as the p-hydroxymercuribenzoate derivatives of the individual chains. The rate constant for the carbamylation of deoxyhemoglobin A is the same as that for deoxyhemoglobin S, and the deoxyhemoglobins are carbamylated about twice as rapidly as the liganded proteins. These results indicate that the pKa values of the NH2-terminal valine residues of hemoglobins A and S are probably identical. Carbon dioxide competes for the site of the carbamylation with deoxyhemoglobin more effectively than it does with carbonmonoxyhemoglobin. Since OCO is known to bind preferentially to deoxyhemoglobin, these data provide experimental support for the suggestion that HNCO, the reactive tautomer of cyanate, is a structural analog of OCO.
Submitted on November 17, 1972
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