Alginate, a major component of the cell wall matrix in brown seaweeds, is degraded by alginate lyases through a -elimination reaction. Almost all alginate lyases act endolytically on substrate, thereby yielding unsaturated oligouronic acids having 4-deoxy-L-erythro-hex-4-enepyranosyluronic acid at the nonreducing end. In contrast, Agrobacterium tumefaciens alginate lyase Atu3025, a member of polysaccharide lyase family 15, acts on alginate polysaccharides and oligosaccharides exolytically and releases unsaturated monosaccharides from the substrate terminal. The crystal structures of Atu3025 and its inactive mutant in complex with alginate trisaccharide (H531A/⌬GGG) were determined at 2.10-and 2.99-Å resolutions with final R-factors of 18.3 and 19.9%, respectively, by x-ray crystallography. The enzyme is comprised of an ␣/␣-barrel ؉ anti-parallel -sheet as a basic scaffold, and its structural fold has not been seen in alginate lyases analyzed thus far. The structural analysis of H531A/⌬GGG and subsequent site-directed mutagenesis studies proposed the enzyme reaction mechanism, with His 311 and Tyr 365 as the catalytic base and acid, respectively. Two structural determinants, i.e. a short ␣-helix in the central ␣/␣-barrel domain and a conformational change at the interface between the central and C-terminal domains, are essential for the exolytic mode of action. This is, to our knowledge, the first report on the structure of the family 15 enzyme.Carbohydrate-active enzymes such as glycoside hydrolases, polysaccharide lyases, glycosyl transferases, and carbohydrate esterases are categorized into more than 200 families based on amino acid sequences in the Carbohydrate-Active enZYmes (CAZy) data base (1). Polysaccharide lyases (PLs) 2 are classified into 21 PL families and they play an important role in degrading acidic polysaccharides such as polygalacturonan, rhamnogalacturonan, alginate, chondroitin, hyaluronan, heparin, heparan, and xanthan. These lyases commonly recognize uronic acid residues in polysaccharides, catalyze -elimination, and produce unsaturated saccharides with CϭC double bonds at nonreducing terminal uronate residues.Alginate, produced by brown seaweeds as a major component of the cell wall matrix, is a linear polysaccharide composed of ␣-L-guluronate (G) and its C5 epimer -D-mannuronate (M). Three blocks, such as poly-␣-L-guluronate (poly(G)), poly--D-mannuronate (poly(M)), and heteropolymeric random sequences (poly(MG)), constitute the alginate polymer (2). The alginate degradation pathway has been characterized in some bacteria, virus, and abalone. In the bacterium Sphingomonas sp. strain A1, alginate is directly incorporated into the cytoplasm by the cell surface pit, periplasmic binding proteins, and ATPbinding cassette transporter. The incorporated alginate is subsequently depolymerized into unsaturated disaccharides, trisaccharides, and tetrasaccharides through the action of three cytoplasmic endotype alginate lyases, A1-I, A1-II, and A1-III (Fig. 1A) (3). The unsaturated oligosaccha...