Dynamics of NO interacting with soluble guanylate cyclase from 1 ps to 0.1 s and induced structural transitions

Abstract

Results We investigated the interaction between purified soluble guanylate cyclase (sGC) from beef lung and NO by timeresolved spectroscopy in a time-range which encompasses eleven orders of magnitude, from 1 ps [1] to 0.1 s [2]. After its dissociation from the heme, NO either recombines geminately to th e4 -coordinate heme within τG1 = 7 ps [1] (96 ± 1 % of the population) or exits the heme pocket (4 ± 1 %), allowing the proximal histidine to rebind within 62 ± 10 ps. Then, NO is distributed in two approximately equal populations (~2 %). One geminately rebinds to the 5-coordinate heme (τG2 = 6.5 ns) while the other migrates into the solution. NO can rebind from the solution (bimolecular rebinding, τB =0 .25 ms with [NO] = 20 µM), forming a 6-coordinate heme with a rate constant of 2 x 10 8 M -1 s -1 (in vitro purified protein) very close to that measured in platelets (3 × 10 8 M -1 s -1 ) [3]. The cleavage of Fe-His bond and subsequent formation of 5-coordinate NO-heme occurs with different time constants for NO which geminately rebinds (τ5C1 = 0.66 µs) and for NO which binds from the solution (τ5C2 = 43 ms). Thus, because the same structural event occurs with rates separated by more than 4 orders of magnitude, we must infer that sGC is not in the same structural state in both cases, with a different strain exerted on the Fe-His bond. This allosteric transition between both states occurs in the time range 0.66 µs < τR < 250 µs in sGC after His rebinding and NO release. Conclusion