Monitoring the kinetics of ion-dependent protein folding by time-resolved NMR spectroscopy at atomic resolution

The kinetics of protein refolding have been monitored by time-resolved NMR spectroscopy. It is shown that refolding of metal binding proteins can be induced by photolysis of photo labile ion chelators, the subsequent release of Ca2+ ions can induce protein folding, and the changes in resonance positions can be monitored by time-resolved NMR spectroscopy. The feasibility of the approach is demonstrated by characterizing the refolding of alpha-lactalbumin, or protein containing a Ca2+ binding site, unfolded in 4 M urea at pH 7 in the absence of Ca2+. The refolding kinetics of the methyl groups of residues Leu15 and Val21 in the core of the protein have been determined to be mono-exponential with rates of 0.28 s(-1) and 0.23 s(-1), respectively at 300 K.