ABSTRACT
Argifin, a novel pentapeptide chitinase inhibitor isolated from Gliocladium fungal culture, is a promising candidate for the development of new fungicides, insecticides, and anti-asthma medications. In this study, we undertook rational molecular design of argifin-derivatives and tested them against chitinase B from Serratia marcescens (SmChiB). The work involved molecular dynamics simulation with explicit water molecules, the molecular docking calculation, and free-energy analysis using the molecular mechanics Poisson-Boltzmann surface area method. The custom-designed derivatives were synthesized via effective solid phase synthesis, developed recently in our laboratory, and their inhibitory activities were measured against SmChiB. Finally, we identified and obtained a derivative which exhibited 28-fold more inhibition than argifin itself, a compound in which the d-Ala(5) of argifin was replaced with d-Leu and the 4-benzylpiperdine was attached to l-Asp(4).
Subject(s)
Chitinases/antagonists & inhibitors , Enzyme Inhibitors/chemistry , Peptides, Cyclic/chemistry , Pesticides/chemistry , Serratia marcescens/enzymology , Amino Acid Sequence , Chitinases/metabolism , Computer-Aided Design , Crystallography, X-Ray , Drug Design , Enzyme Inhibitors/chemical synthesis , Enzyme Inhibitors/pharmacology , Gliocladium/chemistry , Kinetics , Peptides, Cyclic/isolation & purification , Peptides, Cyclic/pharmacology , Pesticides/chemical synthesis , Pesticides/pharmacology , Structure-Activity Relationship , ThermodynamicsABSTRACT
Molecular dynamics (MD) simulations and the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) method were applied to study the interaction of the natural-product cyclopentapeptide chitinase inhibitors argifin and argadin with chitinase B (ChiB) from Serratia marcescens. Argadin inhibited ChiB with an inhibition constant (K(i)) value of 20 nM, which was three orders of magnitude greater than that of argifin (K(i)=33,000 nM). The MM-PBSA free-energy analysis provided absolute binding free energies of -6.98 and -11.16 kcal/mol for the argifin and argadin complexes, respectively. These estimates were in good agreement with the free energies derived from the experimental K(i) values (-6.36 and -10.92 kcal/mol for the argifin and argadin complexes, respectively). The energetic analysis revealed that the van der Waals and nonpolar solvation energies drove the binding of both argifin and argadin. We found that the binding of argadin gained approximately 12 kcal/mol more van der Waals energy than that of argifin, which was mainly responsible for the difference in binding free energy between argifin and argadin. In particular, W220 and W403 of ChiB were found to contribute to the more favorable van der Waals interaction with argadin. We also designed argifin derivatives with better binding affinity, in which a constituent amino-acid residue of argifin was mutated to one with a bulky side chain. The derivative in which D-Ala of argifin was replaced with D-Trp appeared to possess a binding affinity that was equally potent to that of argadin.
