Contribution of Time-Resolved Absorption Spectroscopy to Study Biological Questions

In this report, we illustrate through the study of two allosteric heme proteins the contribution of time-resolved absorption spectroscopy to the understanding of fundamental biological mechanisms. The first studied protein is the endogenous nitric oxide receptor (guanylate cyclase, sGC) whose activation and deactivation mechanisms are not yet fully resolved. We show that the rebinding of the proximal histidine occurs in ~100 picoseconds in sGC, which is the very first step of its deactivation following NO release. We also show that synergistic action of CO together with an allosteric activator induces the cleavage of the bond between heme iron and proximal histidine. The second one is the prototype of allosteric protein, the dioxygen transporter hemoglobin (Hb). In Hb, we show that the motion of the iron atom, central to the heme, moves in ~18 picoseconds after NO binding; this motion represents the very first step of the allosteric T → R transition.