Abstract
β-mannans and xylans are important components of the plant cell wall and they are acetylated to be protected from degradation by glycoside hydrolases. β-mannans are widely present in human and animal diets as fiber from leguminous plants and as thickeners and stabilizers in processed foods. There are many fully characterized acetylxylan esterases (AcXEs); however, the enzymes deacetylating mannans are less understood. Here we present two carbohydrate esterases, RiCE2 and RiCE17, from the Firmicute Roseburia intestinalis, which together deacetylate complex galactoglucomannan (GGM). The three-dimensional (3D) structure of RiCE17 with a mannopentaose in the active site shows that the CBM35 domain of RiCE17 forms a confined complex, where the axially oriented C2-hydroxyl of a mannose residue points toward the Ser41 of the catalytic triad. Cavities on the RiCE17 surface may accept galactosylations at the C6 positions of mannose adjacent to the mannose residue being deacetylated (subsite -1 and +1). In-depth characterization of the two enzymes using time-resolved NMR, high-performance liquid chromatography (HPLC), and mass spectrometry demonstrates that they work in a complementary manner. RiCE17 exclusively removes the axially oriented 2-O-acetylations on any mannose residue in an oligosaccharide, including double acetylated mannoses, while the RiCE2 is active on 3-O-, 4-O-, and 6-O-acetylations. Activity of RiCE2 is dependent on RiCE17 removing 2-O-acetylations from double acetylated mannose. Furthermore, transacetylation of oligosaccharides with the 2-O-specific RiCE17 provided insight into how temperature and pH affects acetyl migration on manno-oligosaccharides.
Highlights
Β-mannans and xylans are important components of the plant cell wall and they are acetylated to be protected from degradation by glycoside hydrolases. β-mannans are widely present in human and animal diets as fiber from leguminous plants and as thickeners and stabilizers in processed foods
N-acetylation of lysine in histones is crucial for the control of gene transcription [1]; acetylation or deacetylation of N-acetylglucosamine and N-acetylmuramic acid in microbial peptidoglycan affects its degradation by lysozyme [2]
The efficacy of penicillins is linked to acetylations, e.g., penicillin resistance caused by bacteria expressing cephalosporin esterases [3]
Summary
Β-mannans and xylans are important components of the plant cell wall and they are acetylated to be protected from degradation by glycoside hydrolases. β-mannans are widely present in human and animal diets as fiber from leguminous plants and as thickeners and stabilizers in processed foods. Acetylations are important constituents in some of the most abundant polymers on Earth, like chitin, pectin, xylan, and β-mannan [4, 5]. Enzymes that process these polymers have adapted to a wide array of structurally diverse targets. Studying the structure-function relationships of acetyl esterases and transferases involved in these processes is needed to shed light on this important and very common covalent modification. A unique characteristic of the 2-O-acetyl groups of mannans is their relative orientation: 2-O-acetylations are axial, as opposed to other common structural polysaccharides, like xylan, pectin, and chitin, that all have acetylations in the equatorial plane of the sugar ring. Acetylations affect the solubility of the polysaccharides by restricting the formation of hydrogen
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