Interplay of Multiple Interaction Forces: Binding of Norfloxacin
to Human Serum Albumin
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Abstract
Herein, the binding interaction of
a potential chemotherapeutic antibacterial drug norfloxacin (NOF)
with a serum transport protein, human serum albumin (HSA), is investigated.
The prototropic transformation of the drug (NOF) is found to be remarkably
modified following interaction with the protein as manifested through
significant modulations of the photophysics of the drug. The predominant
zwitterionic form of NOF in aqueous buffer phase undergoes transformation
to the cationic form within the protein-encapsulated state. This implies
the possible role of electrostatic interaction force in NOF–HSA
binding. This postulate is further substantiated from the effect of
ionic strength on the interaction process. To this end, the detailed
study of the thermodynamics of the drug–protein interaction
process from isothermal titration calorimetric (ITC) experiments is
found to unfold the signature of electrostatic as well as hydrophobic
interaction forces underlying the binding process. Thus, interplay
of more than one interaction forces is argued to be responsible for
the overall drug–protein binding. The ITC results reveal an
important finding in terms of enthalpy–entropy compensation
(EEC) characterizing the NOF–HSA binding. The effect of drug-binding
on the native protein conformation has also been evaluated from circular
dichroism (CD) spectroscopy which unveils partial rupture of the protein
secondary structure. In conjunction to this, the functionality of
the native protein (in terms of esterase-like activity) is found to
be lowered as a result of binding with NOF. The AutoDock-based docking
simulation unravels the probable binding location of NOF within the
hydrophilic subdomain IA of HSA. The present program also focuses
on exploring the dynamical aspects of the drug–protein interaction
scenario. The rotational-relaxation dynamics of the protein-bound
drug reveals the not-so-common “dip-and-rise” pattern