title: Facilitation of Hb S polymerization by the substitution of Glu for Gln at beta 121.
abstract: In an effort to clarify the role of Glu-beta 121 of Hb S molecules in polymerization, we studied the solubility and kinetics of polymerization of various mixtures of deoxyhemoglobins S (Glu-beta 6----Val) and D Los Angeles (Glu-beta 121----Gln). It is known that patients with Hb S-D Los Angeles have a relatively severe clinical course. Mixtures of Hb S and Hb D Los Angeles polymerized after a distinct delay time, the length of which depended on the initial hemoglobin concentration and the fraction of Hb S in the mixture. There was a linear relationship between the logarithmic plot of delay time and initial hemoglobin concentration. The line for a 1:1 mixture of Hb S and Hb D Los Angeles shifted to the right of that for deoxy-Hb S by 0.08. This shift is much smaller than the shift of 0.32 for 1:1 AS mixtures. From these data, the probability factor for nucleation of S-D Los Angeles hybrid hemoglobin was calculated to be 1.16, which is higher than that of Hb S (1.0) and AS hybrid hemoglobin (0.5). The degree of co-polymerization of Hb D Los Angeles in S-D Los Angeles mixtures was similar to that of Hb A in AS mixtures. The critical concentration for the polymerization of Hb D Los Angeles was between that of Hb A and Hb Machida, which has the same amino acid substitution (Glu----Gln) at the beta 6 position. These results suggest that the protein interaction of Hb S molecules during nucleation involves at least two steps. First, the Val-beta 6 of a Hb S molecule interacts hydrophobically with the Phe-beta 85 and the Leu-beta 88 of an adjacent Hb S molecule. In the second step, Glu-beta 121 weakens the interaction with His-beta 116 and Pro-alpha 114. The substitution of Glu-beta 121----Gln may strengthen this second reaction and facilitate nucleation as well as polymerization.
mesh_terms: D005971:Glutamates; D018698:Glutamic Acid; D005973:Glutamine; D006451:Hemoglobin, Sickle / Q000378:metabolism*; D006455:Hemoglobins, Abnormal / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D011108:Polymers; D011336:Probability; D055503:Protein Multimerization; D012995:Solubility; D013329:Structure-Activity Relationship
chemical_list: D005971:Glutamates; D006451:Hemoglobin, Sickle; D006455:Hemoglobins, Abnormal; D011108:Polymers; D005973:Glutamine; D018698:Glutamic Acid; C026237:hemoglobin D Punjab