Squaring a Cube

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Science  16 Feb 2007:
Vol. 315, Issue 5814, pp. 914
DOI: 10.1126/science.315.5814.914a

The detection of environmental changes is of paramount importance for microbes, and a host of mechanisms have evolved in which signal detection is coupled to an amplification step in order to increase sensitivity and to reduce response time. The bacterial regulator of fumarate and nitrate reduction (FNR) regulates the transcription of more than 100 genes in response to the transition from anaerobic to aerobic growth; molecular oxygen triggers a conversion of its [4Fe-4S]2+ cluster into a [2Fe-2S]2+ cluster, leading to conformational changes in FNR and dissociation from DNA.

Crack et al. report that this reaction not only detects dioxygen, but actually uses it to amplify the signal, consuming some in doing so. The first step in signal detection is a one-electron oxidation of the [4Fe-4S]2+ cluster that transforms it into a [3Fe-4S]1+ cluster, and a kinetic analysis confirmed the oxygen-dependent coincident loss of the [4Fe-4S] cluster and the appearance of the ejected Fe2+ and the superoxide anion (O2⚫). The second step is about 10 times slower, and a second Fe departs (as Fe3+) in the conversion of the [3Fe-4S]1+ cluster into a [2Fe-2S]2+ cluster. Superoxide is known to undergo dismutation into oxygen and hydrogen peroxide, which can itself dismutate into oxygen and water. In sum, one O2 molecule can trigger the disassembly of four [4Fe-4S] clusters. — GJC

Proc. Natl. Acad. Sci. U.S.A. 104, 2092 (2007).

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