Multimodular flavobacterial enzymes specialised in coordinated decomposition of cellulose and alginate in brown algal cell walls

Brown algal cell walls are complex matrices composed primarily of alginate, cellulose, and fucoidan. Their depolymerization is important in marine carbon cycling. Although numerous algal polysaccharide-degrading enzymes have been characterized, most studies focus on breaking down single, purified polysaccharides, leaving the degradation mechanisms of native cell walls containing mixed polysaccharides poorly understood. Here we report the integrated modular enzymes involved in brown algal cell wall polysaccharides (BACWPs) degradation. Using the marine flavobacterium Aquimarina sp. 2-A2 as a model, we isolated a bifunctional enzyme, CelAly, which integrates a glycoside hydrolase family 5 cellulase domain and a polysaccharide lyase family 31 alginate lyase domain within a single polypeptide, enabling the degradation of cellulose and alginate in brown algal cell walls. In vivo relevance of CelAly was confirmed by upregulation of its gene during growth on algal biomass. CelAly also contains three distinctive substrate-binding modules (B1, B2, UKD) that support its multimodular functionality; among these, UKD is notable for its dual substrate-binding capability. CelAly's modular architecture and interdomain flexibility may facilitate coordinated degradation of BACWPs. Bioinformatic analyses and biochemical validation revealed three additional types of such modular enzymes from marine microbes. CelAly and related modular enzymes are strongly associated with marine environments and exhibit conserved modular strategy for substrate recognition and catabolism. Thus, these enzyme architectures represent a previously unrecognized strategy specialised for BACWPs decomposition. This study elucidates the unique structural and functional adaptations of the integrated multimodular enzymes and highlights their ecological prevalence among marine bacteria, providing insights into natural biomass decomposition.