Bound ligand motion in crystalline carboxypeptidase A

Area of Science:

• Biochemistry
• Structural Biology
• Spectroscopy

Background:

• Understanding protein-ligand interactions is crucial in enzymology.
• Phenyl ring dynamics play a role in enzyme function and substrate binding.
• Carboxypeptidase A is a well-studied metalloprotease involved in peptide hydrolysis.

Purpose of the Study:

• To investigate the dynamics of phenyl ring deuterons in various ligands bound to carboxypeptidase A.
• To analyze the influence of water content on ligand mobility within the enzyme's active site.
• To elucidate the mechanisms of phenyl ring motion, including pi-flips and ring jumps.

Main Methods:

• Deuterium Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
• Spectra were recorded for D-phenylalanine, L-phenylalanine, phenylacetic acid, and phenyl propionic acid in carboxypeptidase A crystals.
• Data analysis utilized a two-site jump model and considered ligand exchange with unbound environments.

Main Results:

• Complete phenyl ring pi-flip motion is consistent with spectra for all ligands across varying water contents.
• Pi-flip rates at 298 K range from 7.5 x 10^5 to 4.0 x 10^6 S^-1 and increase with water activity.
• Phenylacetic acid crystals exhibited complex spectra consistent with a superposition of ring jump motions, not pi-flips.

Conclusions:

• The binding pocket of carboxypeptidase A allows for complete pi-flip motion of bound phenyl-containing ligands.
• Ligand pi-flip rates are modulated by water content, suggesting a role for hydration in enzyme dynamics.
• Constrained environments, like in phenylacetic acid crystals, lead to different motional patterns, highlighting the influence of packing on ligand dynamics.