Mechanistic Insight into the Nitrosylation of the [4Fe-4S] Cluster of WhiB-like Proteins

摘要

The reactivity of protein bound iron-sulfuer clusters with nitric oxide (NO) is well documented, but little is known about the actual mechanism of cluster nitrosylation. Here, we report studies of members of the Wbl family of [4Fe-4S] containing proteins, which play key roles in regulating developmental processes in actinomycetes, including Streptomyces and Mycobacteria, and have been shown to be NO responsive. Streptomyces coelicolor WhiD and Mycobacterium tuberculosis WhiBl react extremely rapidly with NO in a multiphasic reaction involving, remarkably, 8 NO molecules per [4Fe-4S] cluster. The reaction is 10~4-fold faster than that observed with O_2 and is by far the most rapid iron-sulfuer cluster nitrosylation reaction reported to date. An overall stoichiometry of [Fe_4S_4(Cys)_4]~(2-) + 8NO → 2[Fe~I_2(NO)_4(Cys)_2] + S + 3S has been established by determination of the sulfuer products and their oxidation states. Kinetic analysis leads to a four-step mechanism that accounts for the observed NO dependence. DFT calculations suggest the possibility that the nitrosylation product is a novel cluster [Fe~1_4(NO)_8(Cys)_4]~0 derived by dimerization of a pair of Roussin's red ester (RRE) complexes.