9ia1
Crystal Structure of AgcA bound to 4-ethylguaiacol
Structural highlights
FunctionPublication Abstract from PubMedLignin is a sustainable alternative to petroleum as a feedstock for the chemical industry. Emergent strategies for lignin valorization involve tandem processes in which biomass is chemo-catalytically fractionated, followed by bioconversion of the depolymerized lignin by microbial cell factories. A rate-limiting step in this bioconversion is O-demethylation of the lignin-derived monomers. The reductive catalytic fractionation of hardwood biomass generates high yields of two classes of monomers: 4-alkylguaiacols and 4-alkylsyringols. The former are O-demethylated by AgcA, a cytochrome P450, and AgcB, the cognate reductase, but there are no known enzymes that convert the latter. To develop a biocatalyst that can efficiently transform these monomers, we studied and rationally engineered AgcAB. A 1.82 A resolution crystal structure of AgcA(EP4) from Rhodococcus rhodochrous EP4 in complex with 4-ethylguaiacol identified residues Leu78, Ala293, and Phe166 as potential specificity determinants. Substitution of Ala293 and Leu78 decreased the specificity of AgcA(EP4) for alkylguaiacols. Substitution of Phe166 yielded a variant that bound 4-propylsyringol but did not transform it. In contrast, the corresponding variant in the Rhodococcus aromaticivorans RHA1 homologue, AgcA(RHA1) Y166A, catalyzed the O-demethylation of both methoxy groups of 4-propylsyringol with a k(cat)/K(m) of 8500 M(-1) s(-1) for the first O-demethylation, nearly 7-fold higher than WT AgcA(RHA1). Engineering RHA1 to express the variant yielded a strain that transformed 4-propylsyringol and 4-propylguaiacol simultaneously. Moreover, the engineered strain converted some of the 4-propylsyringol to pentanoyl-CoA, consistent with catabolism via the meta-cleavage pathway that catabolizes 4-alkylguaiacols. Exometabolomics validated the conversion of 4-propylsyringol via this pathway and identified O-demethylation and extradiol ring cleavage as bottlenecks for its transformation. These studies improve our understanding of a critical lignin-degrading enzyme system and significantly advance the development of a biocatalyst to convert these monomers. Engineering a Cytochrome P450 O-Demethylase for the Bioconversion of Hardwood Lignin.,Wolf ME, Hinchen DJ, Zahn M, McGeehan JE, Eltis LD ACS Synth Biol. 2026 Jul 6. doi: 10.1021/acssynbio.6c00337. PMID:42406683[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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