Redox chemistry of biological tungsten: An EPR study of the aldehyde oxidoreductase from Pyrococcus furiosus

Aldehyde:ferredoxin oxidoreductase (AOR) from the hyperthermophilic archaeon Pyrococcus furiosus is a homodimeric protein. Each subunit carries one [4Fe-4S] cubane and a novel tungsten cofactor containing two pterins. A single iron atom bridges between the subunits. AOR has previously been studied with EPR spectroscopy in an inactive form known as the red tungsten protein (RTP): reduced RTP exhibits complex EPR interaction signals. We have now investigated the active enzyme AOR with EPR, and we have found an S = 1/2 plus S = 3/2 spin mixture from a non-interacting [4Fe-4S](1+) cluster in the reduced enzyme. Oxidized AOR affords EPR signals typical for W(V) with g-values of 1.982, 1.953, and 1.885. The W(V) signals disappear at a reduction potential E(m,7.5) of + 180 mV. This unexpectedly high value indicates that the active-site redox chemistry is based on the pterin part of the cofactor.