Yamada Chihaya, Gotoh Aina, Sakanaka Mikiyasu, Hattie Mitchell, Stubbs Keith A, Katayama-Ikegami Ayako, Hirose Junko, Kurihara Shin, Arakawa Takatoshi, Kitaoka Motomitsu, Okuda Shujiro, Katayama Takane, Fushinobu Shinya
Department of Biotechnology, The University of Tokyo, Tokyo 113-8657, Japan; Graduate School of Biostudies, Kyoto University, Kyoto 606-8052, Japan.
Graduate School of Biostudies, Kyoto University, Kyoto 606-8052, Japan.
Cell Chem Biol. 2017 Apr 20;24(4):515-524.e5. doi: 10.1016/j.chembiol.2017.03.012. Epub 2017 Apr 6.
Breast-fed infants generally have a bifidobacteria-rich microbiota with recent studies indicating that human milk oligosaccharides (HMOs) selectively promote bifidobacterial growth. Bifidobacterium bifidum possesses a glycoside hydrolase family 20 lacto-N-biosidase for liberating lacto-N-biose I from lacto-N-tetraose, an abundant HMO unique to human milk, while Bifidobacterium longum subsp. longum has a non-classified enzyme (LnbX). Here, we determined the crystal structure of the catalytic domain of LnbX and provide evidence for creation of a novel glycoside hydrolase family, GH136. The structure, in combination with inhibition and mutation studies, provides insight into the molecular mechanism and broader substrate specificity of this enzyme. Moreover, through genetic studies, we show that lnbX is indispensable for B. longum growth on lacto-N-tetraose and is a key genetic factor for persistence in the gut of breast-fed infants. Overall, this study reveals possible evolutionary routes for the emergence of symbiosis between humans and bifidobacterial species in the infant gut.
母乳喂养的婴儿通常拥有富含双歧杆菌的微生物群，最近的研究表明，人乳寡糖（HMOs）能选择性地促进双歧杆菌生长。两歧双歧杆菌拥有一个糖苷水解酶家族20的乳糖-N-生物酶，可从乳糖-N-四糖中释放出乳糖-N-二糖I，乳糖-N-四糖是母乳中特有的一种丰富的HMO，而长双歧杆菌亚种则有一种未分类的酶（LnbX）。在此，我们确定了LnbX催化结构域的晶体结构，并为一个新的糖苷水解酶家族GH136的产生提供了证据。该结构结合抑制和突变研究，为该酶的分子机制和更广泛的底物特异性提供了深入了解。此外，通过遗传学研究，我们表明lnbX对于长双歧杆菌在乳糖-N-四糖上的生长不可或缺，并且是其在母乳喂养婴儿肠道中持续存在的关键遗传因素。总体而言，这项研究揭示了婴儿肠道中人类与双歧杆菌物种之间共生关系出现的可能进化途径。
