Functional Characterization of Multidomain LPMOs from Marine <i>Vibrio</i> Species Reveals Modulation of Enzyme Activity by Domain–Domain Interactions

Several bacterial pathogens secrete multidomain enzymes known as lytic polysaccharide monooxygenases (LPMOs) that are important for virulence. One example is the <i>Vibrio cholerae</i> virulence factor GbpA (<i>Vc</i>GbpA), in which an N-terminal LPMO domain is followed by two domains of unknown function called GbpA2 and GbpA3, and a C-terminal chitin-binding domain called CBM73. In-depth functional characterization of full-length and truncated variants of <i>Vc</i>GbpA and a homologue from <i>V. campbellii</i> (previously <i>V. harveyi</i>, <i>Vh</i>GbpA) showed that the catalytic LPMO domains of these proteins exhibit properties similar to natural single-domain LPMOs with established roles in chitin degradation. Interestingly, binding to chitin and efficient degradation of this substrate were affected by the presence of the GbpA2 and GbpA3 domains. Combined with structural predictions and analyses of sequence conservation, our data show that GbpA3 has evolved to interact with the reduced catalytic copper site in the LPMO domain to prevent off-pathway reactions in the absence of substrate. Substrate binding by CBM73 weakens this interaction, enabling the activation of the LPMO only when substrate is present. These observations shed new light into the functionality of these multidomain LPMOs and uncover a novel mechanism for regulating LPMO activity.