Redox properties of the sulfhydrogenase from Pyrococcus furiosus

The sulfhydrogenase from the extreme thermophile Pyrococcus furiosus has been re-investigated. The alpha beta gamma delta 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 = 1/2 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 disappeared again at Em7.5 = -328 mV. Also, two other signals appear upon reduction: a near-axial (g = 2.02, 1.95, 1.92) S = 1/2 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.