Laminarin is an abundant algal polysaccharide that serves as carbon storage and fuel to meet the nutrition demands of heterotrophic microbes. Laminarin depolymerization catalyzed by microbial extracellular enzymes initiates remineralization, a key process in ocean biogeochemical cycles. Here, we described a glycoside hydrolase 16 (GH16) family laminarinase from a marine alga-associated Flavobacterium at the biochemical and structural levels. We found that the endolytic enzyme cleaved laminarin with a preference for β-1,3-glycoside linkages and showed transglycosylation activity across a broad range of acceptors. We also solved and compared high-resolution crystal structures of laminarinase in the apo form and in complex with β-1,3-tetrasaccharides, revealing an expanded catalytic cleft formed following substrate binding. Moreover, structure and mutagenesis studies identified multiple specific contacts between the enzyme and glucosyl residues essential for the substrate specificity for β-1,3-glucan. These results provide novel insights into the structural requirements for substrate binding and catalysis of GH16 family laminarinase, enriching our understanding of bacterial utilization of algal laminarin.

中文翻译：

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海洋细菌laminarinase识别和催化底物的分子基础。

层流素是一种丰富的藻类多糖，可作为碳储存和燃料，以满足异养微生物的营养需求。微生物细胞外酶催化的层流素解聚反应引发了矿化作用，这是海洋生物地球化学循环的关键过程。在这里，我们描述了一种来自海藻相关黄杆菌的糖苷水解酶16（GH16）家族laminarinase。在生化和结构水平上。我们发现，内切酶以优先选择β-1,3-糖苷键的方式切割了层板蛋白，并在广泛的受体范围内显示了转糖基化活性。我们还解决并比较了apo形式和与β-1,3-四糖复合的层板层裂酶的高分辨率晶体结构，揭示了底物结合后形成的扩展催化裂隙。此外，结构和诱变研究确定了酶和葡糖基残基之间的多个特定接触，这对于β-1,3-葡聚糖的底物特异性至关重要。这些结果提供了对底物结合和GH16家族laminarinase催化的结构要求的新见解，丰富了我们对藻类laminarin细菌利用的理解。