Redox properties of the sulfhydrogenase fromPyrococcus furiosus

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Abstract
The sulfhydrogenase from the extreme thermophile Pyrococcus furiosus has been re-investigated. The αβγδ heterotetrameric enzyme of 153.3 kDa was found to contain 17 Fe, 17 S2−, and 0.74 Ni. The specific activity of the purified protein was 80 U/mg. Three EPR signals were found. A rhombic S = 12 signal (g = 2.07, 1.93, 1.89) was observed reminiscent in its shape and temperature dependence of spectra from [4Fe-4S](2+;1+) clusters. However, in reductive titrations the spectrum appeared at the unusually high potential Em,7.5 = −90 mV. Moreover, the signal dissappeared again at Em,7.5 = −328 mV. Also, two other signals appear upon reduction: a near-axial (g = 2.02, 1.95, 1.92) S = 12 spectrum (Em,7.5 = −303 mV) indicative for the presence of a [2Fe-2S](2+;1+) cluster, and a broad spectrum of unknown origin with effective g-values 2.25, 1.89 (Em,7.5 = −310 mV). We hypothesize that the latter signal is caused by magnetic interaction of the rhombic signal and a third cluster