Rational Design for Broadened Substrate Specificity and Enhanced Activity of a Novel Acetyl Xylan Esterase from Bacteroides thetaiotaomicron

Enhancing Catalytic Activity of a Baeyer-Villiger monooxygenase from Oceanicola granulosus: Simultaneous Engineering of the Distal Site and Active Site

Enzymatic synthesis of δ-lactones using Baeyer-Villiger monooxygenases (BVMOs) has potential in the fragrance and flavor industries but is constrained by poor activity toward ortho-alkyl-substituted cyclopentanones, key δ-lactone precursors. We determined the crystal structure of a BVMO derived from Oceanicola granulosus (OgBVMO), uncovering a unique loop adjacent to key catalytic residue R335. Site-saturation mutagenesis of loop residues A338 and A339 identified the A339E variant, obtaining a 2.4- to 3-fold increase in catalytic activity toward ortho-alkyl-substituted cyclopentanones (2-methyl-, 2-ethyl-, 2-hexyl-, and 2-heptylcyclopentanone). Further engineering the substrate-binding pocket yielded the Q442N variant, improving activity by 2.7-3.8-fold. Remarkably, the combinatorial mutant A339E/Q442N achieved 3.3-5.2-fold higher activity than the wild-type. Molecular dynamics simulations indicated that these improvements were driven by more favorable nucleophilic attack distances, underscoring the synergistic effects of distal and active-site mutations. These findings offer valuable insights into enhancing the catalytic activity of BVMOs, supporting the green manufacturing of high-value flavor compounds.

Characterisation of a Novel Acetyl Xylan Esterase (BaAXE) Screened from the Gut Microbiota of the Common Black Slug (Arion ater)

Acetyl xylan esterases (AXEs) are enzymes capable of hydrolysing the acetyl bonds in acetylated xylan, allowing for enhanced activity of backbone-depolymerizing enzymes. Bioprospecting novel AXE is essential in designing enzyme cocktails with desired characteristics targeting the complete breakdown of lignocellulose. In this article, we report the characterisation of a novel AXE identified as Gene_id_40363 in the metagenomic library analysed from the gut microbiota of the common black slug. The conserved domain description was identified with an NCBI BLASTp search using the translated nucleotide sequence as a query. The activity of the recombinant enzyme was tested on various synthetic substrates and acetylated substrates. The protein sequence matched the conserved domain described as putative hydrolase and aligned closely to an uncharacterized esterase from Buttiauxella agrestis, hence the designation as BaAXE. BaAXE showed low sequence similarity among characterized CE family proteins with an available 3D structure. BaAXE was active on 4-nitrophenyl acetate, reporting a specific activity of 78.12 U/mg and a Km value of 0.43 mM. The enzyme showed optimal activity at 40 °C and pH 8 and showed high thermal stability, retaining over 40% activity after 2 h of incubation from 40 °C to 100 °C. BaAXE hydrolysed acetyl bonds, releasing acetic acid from acetylated xylan and β-D-glucose pentaacetate. BaAXE has great potential for biotechnological applications harnessing its unique characteristics. In addition, this proves the possibility of bioprospecting novel enzymes from understudied environments.