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Differential pathways in oxy and deoxy HbC aggregation/crystallization
Author(s) -
Hirsch Rhoda Elison,
Samuel Raymond E.,
Fataliev Nazim A.,
Pollack Michael J.,
Galkin Oleg,
Vekilov Peter G.,
Nagel Ronald L.
Publication year - 2000
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/1097-0134(20010101)42:1<99::aid-prot100>3.0.co;2-r
Subject(s) - crystallization , differential (mechanical device) , chemistry , physics , organic chemistry , thermodynamics
CC individuals, homozygous for the expression of β C ‐globin, and SC individuals expressing both β S and β C ‐globins, are known to form intraerythrocytic oxy hemoglobin tetragonal crystals with pathophysiologies specific to the phenotype. To date, the question remains as to why HbC forms in vivo crystals in the oxy state and not in the deoxy state. Our first approach is to study HbC crystallization in vitro, under non‐physiological conditions. We present here a comparison of deoxy and oxy HbC crystal formation induced under conditions of concentrated phosphate buffer (2g% Hb, 1.8M potassium phosphate buffer) and viewed by differential interference contrast microscopy. Oxy HbC formed isotropic amorphous aggregates with subsequent tetragonal crystal formation. Also observed, but less numerous, were twisted, macro‐ribbons that appeared to evolve into crystals. Deoxy HbC also formed aggregates and twisted macro‐ribbon forms similar to those seen in the oxy liganded state. However, in contrast to oxy HbC, deoxy HbC favored the formation of a greater morphologic variety of aggregates including polymeric unbranched fibers in radial arrays with dense centers, with infrequent crystal formation in close spatial relation to both the radial arrays and macroribbons. Unlike the oxy (R‐state) tetragonal crystal, deoxy HbC formed flat, hexagonal crystals. These results suggest: (1) the Lys substitution at β6 evokes a crystallization process dependent upon ligand state conformation [i.e., the R (oxy) or T (deoxy) allosteric conformation]; and (2) the oxy ligand state is thermodynamically driven to a limited number of aggregation pathways with a high propensity to form the tetragonal crystal structure. This is in contrast to the deoxy form of HbC that energetically equally favors multiple pathways of aggregation, not all of which might culminate in crystal formation. Proteins 2001;42:99–107. © 2000 Wiley‐Liss, Inc.