Sulfur respiration in A. fulgidus ? Kinetics and Structure of an A. fulgidus NAD‐dependent Poly/Persulfide Reductase.

PNDORs are a large class of homodimeric NADH and FAD‐dependent enzymes that generally function to reduce small molecule substrates at a catalytic cysteine positioned at the si face of the isoalloxazine ring. The substrate preference and reactivity vary greatly depending on surrounding catalytic residues and additional structural elements. Two closely related PNDORs have been found to have in vitro S 0 reducing activities and have been correlated with in vivo S 0 reduction – CoADR and Npsr. Both enzymes catalyze the general reaction of S 8 0 , S n 2− or CoA‐S n H + NAD(P)H‐> S 2− + NAD(P) + + CoA‐SH. The most obvious structural difference between the two enzymes is the addition of a rhodanase‐like domain on the Npsr which contains a surface‐located cysteine that we have shown to be the initial sulfur‐accepting site. CoADR is widely distributed throughout the thermophilic and hyperthermophilic archaea, while the Npsr is not found in the archaea, with the exception of an Npsr homologue found in the sulfate‐reducing thermophile Archaeoglobus fulgidus . Unlike the other CoADR and Npsr‐containing organisms, A. fulgidus cannot respire S 0 , and is actually inhibited by S 0 . Sequence homology suggests that this A. fulgidus Npsr should be a per/polysulfide reductase, however, the lack of a sulfur‐reducing phenotype suggests 1) the catalytic function of this enzyme is changed, and it fulfills a new function, or 2) the polysulfide reductase activity of the enzyme in vivo is not sufficient to confer the ability to respire S 0 in the context of the A. fulgidus cell, and the enzyme's persulfide/polysulfide reductase activity fulfills a different role, perhaps acting in an antioxidant or detoxification reaction. To differentiate between these two possibilities and better understand archael sulfur respiration processes here we report the kinetics, substrate preferences and structure A. fulgidus Npsr. Support or Funding Information This material is based upon work supported by the National Science Foundation under Grant No. 1518306 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .