Interaction of multitryptophan protein with drug: an insight into the binding mechanism and the binding domain by time resolved emission, anisotropy, phosphorescence and docking.

The interaction of antibiotic Tetracycline hydrochloride (TC) with Alkaline Phosphatase (AP) from Escherichia coli, an important target enzyme in medicinal chemistry, having tryptophan (Trp) residues at 109, 220 and 268 has been studied using the steady state and time resolved emission of the protein and the enhanced emission of the bound drug. The association constant at 298 K (≈10(6) M) and the number of binding site (=1) were estimated using the quenched Trp emission of AP, the enhanced emission and the anisotropy of the bound drug. The values of ΔH(0) and ΔS(0) are indicative of electrostatic and H-bonding interaction. The low temperature phosphorescence of free AP and the protein- drug complex and molecular docking comprehensively prove the specific involvement of partially exposed Trp 220 in the binding process without affecting Trp 109 and Trp 268. The Förster energy transfer (ET) efficiency and the rate constant from the Trp residue to TC=0.51 and ≈10(8) s(-1) respectively. Arg 199, Glu 219, Trp 220, Lys 223, Ala 231, Arg 232 and Tyr 234 residues are involved in the binding process. The motional restriction of TC imposed by nearby residues is reflected in the observed life time and the rotational correlation time of bound TC.