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Proton nmr investigation of cross linked asymmetrically modified hemo globins influence of the salt bridges on tertiary and quaternary structures of hemo globin



Proton nmr investigation of cross linked asymmetrically modified hemo globins influence of the salt bridges on tertiary and quaternary structures of hemo globin



Biochemistry 23(11): 2492-2499



Asymmetrically modified Hb [.alpha.(des-Arg) .beta.]A[.alpha.beta.]CXL, [.alpha.(des-Arg-Tyr).beta.]A[.alpha.beta.]CXL, [.alpha.(des-Arg).beta.(NES)]A[.alpha.beta.]CXL, and [.alpha.(des-Arg).beta.]A[.alpha.beta.(NES)]CXL, were prepared from chemically modified human normal adult Hb A and mutant Hb C (.beta.6Glu .fwdarw. Lys), where the subscript A or C denotes that the .alpha.beta. dimer is from either Hb A or Hb C, respectively, and XL symbolizes a cross-linked Hb prepared by reaction with a bifunctional cross-linking reagent, bis(3,5-dibromosalicyl) fumarate. It was shown by X-ray crystallography that this bifunctional reagent cross-links the .epsilon.-amino group of the lysyl residue at position 82 of the 2 .beta. chains. Proton NMR spectra of these asymmetrically modified Hb together with their parent Hb, des-Arg(.alpha.141) Hb A, des-Arg(.alpha.141)-Tyr(.alpha.140) Hb A, NES-Hb A and NES-des-Arg(.alpha.141) Hb A, were obtained over the spectral region 5-10 ppm downfield from H2O for the exchangeable proton resonances and 50-80 ppm downfield from H2O for the hyperfine-shifted proximal histidyl N.delta.H exchangeable proton resonances. The effects on the hyperfine-shifted proximal histidyl N.delta.H exchangeable proton resonances at pH 6.0 of removing Arg(.alpha.141) or Arg(.alpha.141)-Tyr(.alpha.140) from 1 of the 2 .alpha. subunits are limited to within the .alpha. subunit from which the carboxyl-terminal amino acids are specifically removed. These 2 asymmetrically modified Hb have the exchangeable proton resonance at 9.3 ppm form H2O, which was assigned to the hydrogen bond between .alpha.42 tyrosine and .beta.99 aspartic acid located at the .alpha.1.beta.2 subunit interface. These asymmetrically modified Hb preserve the deoxy-like quaternary structure in the .alpha.1.beta.2 subunit interface as manifested by the presence of this intersubunit hydrogen bond. The proton NMR spectra of [.alpha.(des-Arg).beta.-(NES)]A[.alpha.beta.]CXL and [.alpha.(des-Arg).beta.]A[.alpha.beta.(NES)]CXL at low pH cannot be explained simply as a sum of the spectral features specific for the deoxy-like and the oxy-like quaternary structures. There apparently exist intermediate structures in which the tertiary and the quaternary structural transitions occur asymmetrically about the diad axis of the Hb molecule during the course of the successive removal of the salt bridges.

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