Sulfoquinovose (SQ) is a key component of plant sulfolipids (sulfoquinovosyl diacylglycerols) and a major environmental reservoir of biological sulfur. Breakdown of SQ is achieved by bacteria through the pathways of sulfoglycolysis. The sulfoglycolytic sulfofructose transaldolase (sulfo-SFT) pathway is used by gut-resident firmicutes and soil saprophytes. After isomerization of SQ to sulfofructose (SF), the namesake enzyme catalyzes the transaldol reaction of SF transferring dihydroxyacetone to 3C/4C acceptors to give sulfolactaldehyde and fructose-6-phosphate or sedoheptulose-7-phosphate. We report the 3D cryo-EM structure of SF transaldolase from Bacillus megaterium in apo and ligand bound forms, revealing a decameric structure formed from two pentameric rings of the protomer. We demonstrate a covalent “Schiff base” intermediate formed by reaction of SF with Lys89 within a conserved Asp-Lys-Glu catalytic triad and defined by an Arg-Trp-Arg sulfonate recognition triad. The structural characterization of the signature enzyme of the sulfo-SFT pathway provides key insights into molecular recognition of the sulfonate group of sulfosugars.

磺基奎诺糖 (SQ) 是植物硫脂（磺基奎诺糖二酰基甘油）的关键成分，也是生物硫的主要环境储存库。SQ 的分解是由细菌通过磺糖酵解途径实现的。磺基糖酵解磺基果糖转醛酶 (sulfo-SFT) 途径被肠道驻留的厚壁菌门和土壤腐生菌使用。SQ 异构化为磺果糖 (SF) 后，同名酶催化 SF 的转醛醇反应，将二羟基丙酮转移至 3C/4C 受体，得到磺基乳醛和果糖 6-磷酸或景庚酮糖-7-磷酸。我们报告了来自巨大芽孢杆菌的 SF 转醛酶的 3D 冷冻电镜结构在 apo 和配体结合形式中，揭示了由原聚体的两个五聚环形成的十聚体结构。我们证明了在保守的 Asp-Lys-Glu 催化三联体中由 SF 与 Lys89 反应形成的共价“席夫碱”中间体，并由 Arg-Trp-Arg 磺酸盐识别三联体定义。sulfo-SFT 途径的特征酶的结构表征为分子识别磺基糖的磺酸盐基团提供了关键见解。