Effect of free L-cysteine on the structure and function of α-chymotrypsin

L-cysteine (L-Cys) is one of 20 alpha-amino acids, which is connected by peptide and disulfide bonds in proteins and polypeptides. The L-Cys plays a very considerable role in stabilization of protein structure at a higher level due to disulfide bridges. The interaction of small chemical molecules to proteins has been much studied in recent years. Distinguishing the thermodynamic and kinetic properties of a-chymotrypsin (alpha-Chy) helps us in understanding this protein. Spectroscopic and computational approaches were applied to investigate the effect of L-Cys on the structure and the activity of the alpha-Chy. The UV-Vis results are revealed that the most absorption peaks were found at 260-300 nm due to Trp residues. Hyperchromism shift was seen in the presence of this ligand. This was because of the forming of the ground state complex between alpha-Chy and L-Cys. Static quenching was seen by emission intensity changes. The more polar environment for Trp residue was recommended by the fluorescence quenching. The secondary structure alterations were slight. A reduction in the content of beta-sheet structure and an increase in the a-helix were shown. Kinetic parameters display that L-Cys inhibited the activity of the enzyme by the mixed mode. Molecular docking results show a negative value for the Gibbs free energy of the binding of L-Cys to alpha-Chy with hydrophobic interactions. The molecular dynamic simulation revealed alpha-Chy becomes more stable in the presence of L-Cys. (C) 2019 Published by Elsevier B.V.