Alexander F. Arendsen

Redox properties of the sulfhydrogenase from Pyrococcus furiosus

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 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 E m,7.5 = −90 mV. Moreover, the signal dissappeared again at E m,7.5 = −328 mV. Also, two other signals appear upon reduction: a near‐axial (g = 2.02, 1.95, 1.92) spectrum (E m,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 (E m,7.5 = −310 mV). We hypothesize that the latter signal is caused by magnetic interaction of the rhombic signal and a third cluster.

Discovery of novel iron clusters in proteins by Mossbauer spectroscopy.

Iron clusters are the catalytically active groups of many proteins. The basis building blocks of these clusters are few; they are all variants of three archetypal patterns: [Fe–O–Fe], [2Fe–2S], and [4Fe–4S]. Mössbauer spectroscopy, in conjunction with EPR spectroscopy, susceptometry and EXAFS, plays an important role in characterizing type, number, oxidation states and magnetic properties of such clusters in newly discovered proteins, and in detecting how valency and magnetism vary by passing through the catalytical cycle of these proteins. In this contribution we present the characterization of a novel iron cluster with unusual bridging and terminal ligands and metal coordinations.