Amino acids located in the outer-sphere of the trinuclear copper center in a multicopper oxidase, CueO as the putative electron donor in the four-electron reduction of dioxygen

Molecular cloning, expression, and bioinformatics analysis of the CueO laccase gene from Escherichia coli SDB2

BACKGROUND

Laccase CueO, a multicopper oxidase, possesses the capability to degrade phenolic compounds. In prior research, a strain of Escherichia coli named SDB2, isolated from chicken cecum, was found to degrade sinapine (a phenolic constituent of rapeseed meal) through the secretion of laccase CueO. Herein, the cloning, expression, and bioinformatics analysis of the CueO gene derived from E. coli SDB2 are reported.

METHODS AND RESULTS

Sequence analysis indicated that SDB2 CueO comprised 1551 bp, 516 amino acids, a putative molecular weight of 56.65 kDa, and an isoelectric point (pI) of 6.21. BLAST comparisons showed that the CueO protein sequence from E. coli SDB2 exhibited 65-90% identity with CueO from other bacteria. Multiple alignment analysis further confirmed the similarity and identity of SDB2 CueO with CueO from other species, and the amino acids surrounding the Cu-binding sites were highly conserved. A phylogenetic tree demonstrated a close evolutionary relationship between CueO from E. coli and CueO from Citrobacter amalonaticus. The three-dimensional (3D) structural model revealed four copper (Cu)-binding regions. Recombinant CueO was successfully obtained by expressing the CueO gene in E. coli BL21 after isopropyl β-D-1-thiogalactopyranoside (IPTG) induction. Bioinformatics analysis confirmed the similarity of recombinant CueO with native CueO.

CONCLUSIONS

These findings established a basis for understanding the characteristics and functions of laccase CueO from E. coli SDB2, paving the way for future research to explore the properties of recombinant CueO and its potential practical applications in optimizing feed resources, such as rapeseed meal, in the feed production industry.

Methionine-rich domains emerge as facilitators of copper recruitment in detoxification systems

Copper homeostasis mechanisms are critical for bacterial resistance to copper-induced stress. The Escherichia coli multicopper oxidase copper efflux oxidase (CueO) is part of the copper detoxification system in aerobic conditions. CueO contains a methionine-rich (Met-rich) domain believed to interact with copper, but its exact function and the importance of related copper-binding sites remain unclear. This study investigates these open questions by employing a multimodal and multiscale approach. Through the design of various E. coli CueO (EcCueO) variants with altered copper-coordinating residues and domain deletions, we employ biological, biochemical, and physico-chemical approaches to unravel in vitro CueO catalytic properties and in vivo copper resistance. Strong correlation between the different methods enables evaluation of EcCueO variants' activity as a function of Cu+ availability. Our findings demonstrate the Met-rich domain is not essential for cuprous oxidation, but it facilitates Cu+ recruitment from strongly chelated forms, acting as transient copper binding domain thanks to multiple methionines. They also indicate that the Cu6/7 copper-binding sites previously observed within the Met-rich domain play a negligible role. Meanwhile, Cu5, located at the interface with the Met-rich domain, emerges as the primary and sole substrate-binding active site for cuprous oxidation. The Cu5 coordination sphere strongly affects the enzyme activity and the in vivo copper resistance. This study provides insights into the nuanced role of CueO Met-rich domain, enabling the functions of copper-binding sites and the entire domain itself to be decoupled. This paves the way for a deeper understanding of Met-rich domains in the context of bacterial copper homeostasis.

Engineering of Laccase CueO for Improved Electron Transfer in Bioelectrocatalysis by Semi-Rational Design

Copper efflux oxidase (CueO) from Escherichia coli is a special bacterial laccase due to its fifth copper binding site. Herein, it is discovered that the fifth Cu occupancy plays a crucial and favorable role of electron relay in bioelectrocatalytic oxygen reduction. By substituting the residues at the four coordinated positions of the fifth Cu, 11 beneficial variants are identified with ≥2.5-fold increased currents at -250 mV (up to 6.13 mA cm-2 ). Detailed electrocatalytic characterization suggests the microenvironment of the fifth Cu binding site governs the electrocatalytic current of CueO. Additionally, further electron transfer analysis assisted by molecular dynamics (MD) simulation demonstrates that an increase in localized structural stability and a decrease of distance between the fifth Cu and the T1 Cu are two main factors contributing to the improved kinetics of CueO variants. It may guide a novel way to tailor laccases and perhaps other oxidoreductases for bioelectrocatalytic applications.