Cuprous Oxidase Activity of CueO from Escherichia coli. Original paper

What was studied?

This study defined the cuprous oxidase activity of CueO and explained how this periplasmic multicopper oxidase protects Escherichia coli from copper stress. The authors showed that CueO directly oxidizes toxic Cu(I) to the less reactive Cu(II), working in the same pathway as the CopA P-type ATPase that pumps Cu(I) into the periplasm. They quantified kinetics across pH and compared performance with yeast Fet3 and human ceruloplasmin, finding higher catalytic rates for CueO that fit a primary role in copper detox in the aerobic gut niche where E. coli lives. By placing CueO within the CueR-activated cue operon and alongside the CusCFBA efflux system, the paper links enzyme activity to a complete copper homeostasis circuit with clinical microbiome relevance.

Who was studied?

Researchers purified recombinant wild-type CueO and a C500S mutant that lacks the type-1 copper needed for catalysis, then tested oxygen consumption as a readout of Cu(I) oxidation. They used a stabilized Cu(I) donor to avoid spontaneous air oxidation and measured kinetic constants at pH 5.0 and 7.0. They also assessed ferroxidase activity and the effect of added Cu(II), using the inactive mutant to set background rates. For context, the authors compared catalytic efficiency to Fet3 and ceruloplasmin and set CueO within the copper regulons that include CopA (cytosol-to-periplasm Cu(I) export), CueR (Cu(I)-sensing regulator), and CusCFBA (periplasm-to-outside efflux), all of which E. coli uses to endure copper pulses in the gut.

Most important findings

CueO showed robust cuprous oxidase catalysis that exceeded homologs, with higher turnover numbers and favorable efficiency at acidic pH that matches the upper gastrointestinal environment. Cu(I) served as the only substrate that did not require added Cu(II) for activity, supporting a direct detox role in which CopA feeds periplasmic Cu(I) to CueO for oxidation to Cu(II). The C500S mutant lacked activity and confirmed that the observed oxygen consumption reflected CueO catalysis. Kinetic analysis showed greater activity at pH 5.0 than at pH 7.0, which aligns with prior phenoloxidase behavior and suggests enhanced protection in acidic mucosa.

Although CueO can oxidize Fe(II) or catecholates, the authors clarified that some reported catecholate “oxidase” signals likely arise because catecholates reduce Cu(II) to Cu(I), which CueO then oxidizes; the central biology remains Cu(I) removal. Placing these data into the full copper circuit, the study confirmed that E. coli senses and responds to Cu(I) via CueR, moves Cu(I) to the periplasm via CopA, oxidizes it via CueO, and expels excess copper through CusCFBA. For a microbiome signatures database, the combined presence of cueO, copA, cueR, and cusCFBA marks a copper-tolerant Enterobacterales profile suited to metal-stressed niches such as inflamed gut, where low pH, reactive oxygen species, and dietary copper raise Cu(I) pressure.

Key implications

Clinicians can read the cuprous oxidase activity of CueO as a mechanistic anchor for E. coli survival under host and dietary copper stress. Genomic detection of cueO with copA and cusCFBA in gut isolates signals a strain more likely to manage copper spikes, persist during inflammation, and compete against copper-sensitive commensals. Reporting these loci in microbiome results can help explain Enterobacterales expansion when gastric acidity rises or when feeds or supplements raise copper exposure. Because CueO activity peaks at lower pH, acid suppression and diet could alter copper risk and reshape community structure. Therapeutic copper use on devices or in feeds may select for cueO-positive strains; balanced strategies that limit unnecessary copper while preserving nutritional immunity may curb expansion of these pathobionts without harming beneficial taxa.