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Interaction between poly( L ‐lysine 48 , L ‐histidine 52 ) and DNA
Author(s) -
Santella Regina M.,
Li Hsueh Jei
Publication year - 1977
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1977.360160905
Subject(s) - histidine , chemistry , protonation , imidazole , deprotonation , dna , circular dichroism , side chain , crystallography , lysine , stereochemistry , polylysine , absorbance , base pair , biochemistry , organic chemistry , amino acid , chromatography , polymer , ion
Poly(Lys 48 , His 52 ), a random copolypeptide of L ‐lysine (48%) and L ‐histidine (52%), was used as a model protein for investigating the effects of protonation on the imidazole group of histidines on protein binding to DNA. The complexes formed between poly(Lys 48 , His 52 ) and DNA were examined using absorbance, circular dichroism (CD), and thermal denaturation. Although increasing pH reduces the charges on histidine side chains in the model protein, the protein still binds the DNA with approximately one positive charge per negative charge in protein‐bound regions. Nevertheless, CD and melting properties of poly(Lys 48 , His 52 )‐DNA complexes still depend upon the solution pH which determines the protonation state of imidazole group of histidine side chains. At pH 7.0, the complexes show two characteristic melting bands with a t m (46–51°C) for free base pairs and a t ′ m (94°C) for protein‐bound base pairs. The t ′ m of the complexes is reduced to 90°C at pH 9.2, although at this pH there is still one lysine per phosphate in protein‐bound regions. Presumably, the presence of deprotonated histidine residues destabilizes the native structure of protein‐bound DNA. The binding of this model protein to DNA causes a red shift of the crossover point and both a red shift and a reduction of the positive CD band of DNA near 275 nm. This phenomenon is similar to that caused by polylysine binding. These effects, however, are greatly diminished when histidine side chains in the model protein are deprotonated. The structure of already formed poly(Lys 48 , His 52 )·DNA complexes can be perturbed by changing the solution pH. However, the results suggest a readjustment of the complex to accommodate charge interactions rather than a full dissociation of the complex followed by reassociation between the model protein and DNA.