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J Biol Chem. 2018 Dec 12. pii: jbc.RA118.006548. doi: 10.1074/jbc.RA118.006548. [Epub ahead of print]

A heterodimeric glutathione S-transferase that stereospecifically breaks lignin's β(R)-aryl ether bond reveals the diversity of bacterial β-etherases.

Author information

1
Wisconsin Energy Institute, University of Wisconsin-Madison, United States.
2
University of Wisconsin-Madison, United States.
3
GLBRC, Wisconsin Energy Institute, University of Wisconsin-Madison, United States.
4
Wisconsin Energy Institute, University of Wisconsin-Madison.
5
Bacteriology, University of Wisconsin-Madison, United States.

Abstract

Lignin is a heterogeneous polymer of aromatic subunits that is a major component of lignocellulosic plant biomass. Understanding how microorganisms deconstruct lignin is important for understanding the global carbon cycle, and could aid in developing systems for processing plant biomass into valuable commodities. Sphingomonad bacteria use stereospecific glutathione S-transferases (GSTs) called β-etherases to cleave the β-aryl ether (β-O-4) bond, the most common bond between aromatic subunits in lignin. Previously characterized bacterial β-etherases are homodimers that fall into two distinct GST subclasses: LigE homologues, which cleave the β(R) stereoisomer of the bond, and LigF homologues, which cleave the β(S) stereoisomer. Here, we report on a heterodimeric β-etherase (BaeAB) from the sphingomonad Novosphingobium aromaticivorans that stereospecifically cleaves the β(R)-aryl ether bond of the di-aromatic compound β-(2-methoxyphenoxy)-γ-hydroxypropiovanillone (MPHPV). BaeAB's subunits are phylogenetically distinct from each other and from other β-etherases, though they are evolutionarily related to LigF, despite the fact that BaeAB and LigF cleave different β-aryl ether bond stereoisomers. We identify amino acid residues in BaeAB's BaeA subunit important for substrate binding and catalysis, including an asparagine that is proposed to activate the glutathione cofactor. We also show that BaeAB homologues from other sphingomonads can cleave β(R)-MPHPV, and that they may be as common in bacteria as LigE homologues. Our results suggest that the ability to cleave the β-aryl ether bond arose independently at least twice in GSTs, and that BaeAB homologues may be important for cleaving the β(R)-aryl ether bonds of lignin-derived oligomers in nature.

KEYWORDS:

Novosphingobium aromaticivorans; bacterial metabolism; enzyme mutation; glutathione S-transferase; lignin degradation; protein evolution; sphingomonads; stereoselectivity; β-aryl ether bond; β-etherase

PMID:
30541921
DOI:
10.1074/jbc.RA118.006548
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