Bacterial Glutathione S‐Transferases (GSTs) Involved in Breaking the β‐Aryl Ether Bond of Lignin Reveal Novel Catalytic Abilities and Reaction Mechanisms within the GST Superfamily

Abstract

The glutathione S‐transferase (GST) protein superfamily contains thousands of known or predicted members, all of which contain an N‐terminal thioredoxin domain that binds a glutathione (GSH) molecule and activates it for catalysis. GSTs are important in many organisms for conjugating GSH to toxic molecules, making them more soluble and easier to excrete from cells. However, GSTs are also capable of catalyzing a wide and continually expanding range of other reactions using GSH as cofactor.Sphingomonad bacteria are capable of metabolizing low molecular weight oligomers and monomers derived from lignin, a heterogeneous polymer of aromatic subunits that is a major component of plant biomass. These bacteria and their enzymes have potential applications in converting naturally abundant lignin into valuable commodities. Sphingomonads use GSTs (called β‐etherases and GSH lyases) to catalyze two consecutive steps in their pathway for breaking the β‐aryl ether bond commonly found between aromatic subunits in the lignin polymer: β‐etherases stereospecifically break the β‐aryl ether bond by replacing it with a thioether bond involving GSH, and GSH lyases remove the GSH moiety from the resulting β‐thioether GSH conjugate.We have identified a heterodimeric β‐etherase (called BaeAB) that provides insight into the diversity of GSTs capable of acting as β‐etherases and into the evolution of the GST ability to break the β‐aryl ether bond. In addition, BaeAB has properties that are unique amongst characterized GSTs in general, including the majority of its catalytic activity apparently residing in only one of its subunits, and using an active site asparagine for catalysis (whereas most characterized GSTs use an amino acid residue with a side chain hydroxyl for catalysis).We have also identified a GSH lyase (called NaGSTNu), which has close homologues in many different organisms, though the physiological roles of these homologues are largely unknown in other organisms besides sphingomonads. We found that the two homologues from Escherichia coli were able to catalyze the GSH lyase reaction in the pathway for breaking the β‐aryl ether bond, despite the fact that E. coli is not known to be able to metabolize lignin‐derived oligomers. This suggests that GSH lyase activity may be a common feature of these homologues (and that they likely act on GSH‐conjugated compounds naturally found within their respective organisms). In addition, the reaction mechanism of NaGSTNu involves binding two GSH molecules (whereas most characterized GSTs can only bind one GSH), as well as amino acid residues located in a channel leading from the active site to the protein exterior.Altogether, besides helping to identify novel GSTs that could potentially be used to generate value from lignin, our work is helping to uncover unique features of the large and widely studied, but relatively uncharacterized, GST superfamily.Support or Funding InformationThis work was supported by U.S. Department of Energy (DOE) Great Lakes Bioenergy Research Center grants (DOE Office of Science BER DE‐FC02‐07ER64494 and DE‐SC0018409).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.