Methionine-Aromatic Interactions In Calmodulin: A Replica Exchange Md And Epr Spectroscopy Study

Methionine oxidation introduces methionine-aromatic interactions that play a significant role in secondary structure conformation and stabilization. Calmodulin (CaM) contains nine methionine residues that function as targets of reversible oxidation, serving as a mechanism through which the cell senses and responds to oxidative stress. Replica exchange molecular dynamics (REMD) simulations illustrated that methionine oxidatin of the N-terminal helix of CaM introduces two configurations that involve oxidized methionines at positions 144 and 145 interacting with tyrosine 138. As these configurations do not occur in the unoxidized helix, we propose the conformational change is induced by the stabilizing methionine-aromatic interaction. To verify the effect, electron paramagnetic resonance (EPR) spectroscopy performed with CaM at submicromolar [Ca2+] with probes near the residues of interest revealed two populations in the oxidized sample and only one in the unoxidized sample, agreeing well with the simulation and indicating that oxidation is responsible for a stabile conformational shift. This conformational change induced by the noncovalent interaction between oxidized methionine and tyrosine's aromatic ring could be the mechanism by which CaM responds to changes in the redox environment in a reversible manner.