Inhibition of α-chymotrypsin by pristine single-wall carbon nanotubes: Clogging up the active site.

The preferred spatial orientation of single-wall carbon nanotubes (SWCNTs) in their interaction with enzymes determines their behavior either as nano-supports or as inhibitors. α -chymotrypsin (α-CT) is considered a serine protease model for studying nanomaterial/proteases interactions. The interaction of α-CT with pristine single-wall carbon nanotubes is still unknown. Here α-CT/SWCNT hybrids are synthesized and characterized. Spectroscopic, microscopic and kinetic measurements, coupled to molecular dynamics simulations, provide a detailed description of the interaction between α-CT and SWCNTs. The SWCNT binding pocket was unambiguously identified. A perfect match is observed with the crevice structure of the α-CT substrate binding pocket. The activity of α-CT, upon SWCNT binding, is dramatically reduced, as expected by the interaction of the SWCNT in the active site of the protein. π-π stacking between aromatic residues and the conjugated surface of SWCNT governs α-CT/SWCNT interactions. An important role in the bonding appears also for purely hydrophobic residues and with residues able to establish surfactant-like interactions. The secondary structure of α-CT and the catalytic triad structure are not perturbed by the complex formation, on the contrary the volume of the substrate binding pocket is strongly reduced by SWCNT binding because SWCNT occupies the α-CT substrate binding site, clogging the active site.