Spontaneous deamidation of amino acids is a physiologically
important
process, particularly for protein aging and diseases. Despite its
widespread occurrence, the mechanism of glutamine deamidation particularly
within proteins remains poorly understood. We have used a multiscale
computational approach to investigate glutamine deamidation in the
tripeptide Glycine–Glutamine–Glycine (Gly–Gln–Gly)
and γS-Crystallin protein. Specifically, both the 5- and 6-membered
water-assisted deamidation pathways in the tripeptide have been elucidated
and compared. Both are found to occur in three stages: iminol formation,
cyclization, and deamination. The rate-limiting step in each mechanism
is nucleophilic attack of the backbone iminol nitrogen, formed in
the first stage, at the glutamine’s side-chain carbonyl carbon.
For the 6- and 5-membered mechanisms, this occurs with a free energy
cost of 136.4 and 179.5 kJ mol–1, respectively.
Thus, overall, in the Gly–Gln–Gly tripeptide, the 6-membered
pathway is preferred. Furthermore, the free energies for forming cyclic
intermediates and products at selected Gln residues (based on experimentally
reported % deamidation) in γS-Crystallin have been obtained.
It is found that the 5-membered product complex is exergonic at −25.3
kJ mol–1, while the 6-membered product complex is
calculated to be endergonic at 90.7 kJ mol–1. Thus,
the deamidation pathway in folded and constrained proteins may not
exclusively follow the 6-membered route. Molecular dynamics (MD) simulations
of γS-Crystallin indicate that deamidation is more likely to
occur when two or more water molecules are in the proximity of the
glutamine residue. Consequently, significant conformational changes
are found to accompany Gln120 deamidation in γS-Crystallin.
This in turn can influence water availability at the other Gln residues
considered and hence potentially their deamidation. Collectively,
these results provide comprehensive insights into spontaneous water-assisted
deamidation of glutamine residues in peptides and into the role and
impact of Gln deamidation in proteins.