Carbohydrate-binding modules (CBMs) are non-catalytic proteins found appended to carbohydrate-active enzymes. Soil and marine bacteria secrete such enzymes to scavenge nutrition, and they often use CBMs to improve reaction rates and retention of released sugars. Here we present a structural and functional analysis of the recently established CBM family 92. All proteins analysed bind preferentially to β−1,6-glucans. This contrasts with the diversity of predicted substrates among the enzymes attached to CBM92 domains. We present crystal structures for two proteins, and confirm by mutagenesis that tryptophan residues permit ligand binding at three distinct functional binding sites on each protein. Multivalent CBM families are uncommon, so the establishment and structural characterisation of CBM92 enriches the classification database and will facilitate functional prediction in future projects. We propose that CBM92 proteins may cross-link polysaccharides in nature, and might have use in novel strategies for enzyme immobilisation.

碳水化合物结合模块（CBM）是附加在碳水化合物活性酶上的非催化蛋白质。土壤和海洋细菌分泌此类酶来清除营养，它们经常使用煤层气来提高反​​应速率和释放糖的保留。在这里，我们对最近建立的 CBM 家族 92 进行了结构和功能分析。分析的所有蛋白质均优先与 β−1,6-葡聚糖结合。这与 CBM92 结构域连接的酶中预测底物的多样性形成对比。我们展示了两种蛋白质的晶体结构，并通过诱变证实色氨酸残基允许配体结合在每种蛋白质上的三个不同的功能结合位点。多价 CBM 家族并不常见，因此 CBM92 的建立和结构表征丰富了分类数据库，并将有助于未来项目的功能预测。我们认为 CBM92 蛋白可能在自然界中交联多糖，并且可能用于酶固定化的新策略。