Spectroscopic and Molecular Dynamics Simulation Study
of Lysozyme in the Aqueous Mixture of Ethanol: Insights into the Nonmonotonic
Change of the Structure of Lysozyme
Structural
change of lysozyme in the aqueous mixture of ethanol
(ETH) has been investigated using emission, circular dichroism spectroscopy,
free-energy molecular dynamics (MD) simulation, and contact map analysis
methods. The emission and circular dichroism data of protein depict
the nonmonotonic change suggesting that the structure as well as local
environment near the Trp of lysozyme modifies differently for different
compositions of the ETH–water mixture. The free-energy MD simulation
shows that the change in the average size of lysozyme in the aqueous
mixture of ETH also shows a nonmonotonic nature. The free-energy profile
of lysozyme in the low concentration of ETH suggests that the distance
between helices increases (χETH ≈ 0.07) first
and decreases again (χETH ≈ 0.2) becoming
almost similar to the native structure. Around χETH ≈ 0.5, the size of lysozyme increases significantly probably
leading to the unfolding of the protein. With further increase of
ETH (χETH ≈ 0.7), size of lysozyme decreases
suggesting the refolding of the protein. Contact map as well as solvent
organization analysis depicts that ETH gets preferentially solvated
by the hydrophobic core of lysozyme which weaken the hydrophobic interactions
of protein, resulting in the increase in its size. The aggregation
of ETH dominated at the higher concentration of ETH (χETH ≈ 0.7); hence, the, weakening of hydrophobic core of protein
by ETH is reduced, which probably lead to the refolding of lysozyme.