期刊
JOURNAL OF BIOLOGICAL CHEMISTRY
卷 293, 期 35, 页码 13636-13649出版社
ELSEVIER
DOI: 10.1074/jbc.RA118.002374
关键词
glycoside hydrolase; enzyme; enzyme catalysis; protein structure; protein conformation; site-directed mutagenesis; exo--mannanase; GH2 family; molecular mechanism; Xanthomonas axonopodis pv; citri
资金
- Sao Paulo Research Foundation (FAPESP) [2015/26982-0]
- FAPESP [2013/24622-0, 2016/06509-0, 2016/19995-0]
The classical microbial strategy for depolymerization of -mannan polysaccharides involves the synergistic action of at least two enzymes, endo-1,4--mannanases and -mannosidases. In this work, we describe the first exo--mannanase from the GH2 family, isolated from Xanthomonas axonopodis pv. citri (XacMan2A), which can efficiently hydrolyze both manno-oligosaccharides and -mannan into mannose. It represents a valuable process simplification in the microbial carbon uptake that could be of potential industrial interest. Biochemical assays revealed a progressive increase in the hydrolysis rates from mannobiose to mannohexaose, which distinguishes XacMan2A from the known GH2 -mannosidases. Crystallographic analysis indicates that the active-site topology of XacMan2A underwent profound structural changes at the positive-subsite region, by the removal of the physical barrier canonically observed in GH2 -mannosidases, generating a more open and accessible active site with additional productive positive subsites. Besides that, XacMan2A contains two residue substitutions in relation to typical GH2 -mannosidases, Gly(439) and Gly(556), which alter the active site volume and are essential to its mode of action. Interestingly, the only other mechanistically characterized mannose-releasing exo--mannanase so far is from the GH5 family, and its mode of action was attributed to the emergence of a blocking loop at the negative-subsite region of a cleft-like active site, whereas in XacMan2A, the same activity can be explained by the removal of steric barriers at the positive-subsite region in an originally pocket-like active site. Therefore, the GH2 exo--mannanase represents a distinct molecular route to this rare activity, expanding our knowledge about functional convergence mechanisms in carbohydrate-active enzymes.
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