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Three-dimensional structure of (1,4)--D-mannan mannanohydrolase from tomato fruit

¹ Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1, Canada² School of Biomedical and Chemical Sciences/School of Medicine and Pharmacology University of Western Australia, Crawly WA6009, Australia

(RECEIVED December 17, 2004; FINAL REVISION January 16, 2005; ACCEPTED January 19, 2005)

The three-dimensional crystal structure of tomato (Lycopersicon esculentum) -mannanase 4a (LeMAN4a) has been determined to 1.5 Å resolution. The enzyme adopts the (/)₈ fold common to the members of glycohydrolase family GH5. The structure is comparable with those of the homologous Trichoderma reesei and Thermomonospora fusca -mannanases: There is a conserved three-stranded -sheet located near the N terminus that stacks against the central -barrel at the end opposite the active site. Three noncanonical -helices surround the active site. Similar helices are found in T. reesei but not T. fusca -mannanase. By analogy with other -mannanases, the catalytic acid/base residue is E204 and the nucleophile residue is E318. The active site cleft of L. esculentum -mannanase most closely resembles that of the T. reesei isozyme. A model of substrate binding in LeMAN4a is proposed in which the mannosyl residue occupying the –1 subsite of the enzyme adopts the ¹S₅ skew-boat conformation.

Keywords: 1,4--D-mannan mannanohydrolase; -mannanase; mannan; ripening; Lycopersicon esculentum; tomato; glycoside hydrolase; glycohydrolase family GH5; (/)₈ fold; TIM barrel; crystal structure; molecular dynamics

Abbreviations: LeMAN4a, Lycopersicon esculentum -mannanase 4a • TrMAN, Trichoderma reesei -mannanase • TfMAN, Thermomonospora fusca -mannanase • PcMAN, Pseudomonas cellulosa -mannanase • DFDM, 2-deoxy-2-fluoro-dinitrophenyl-mannotrioside • MD, molecular dynamics • RMSD, root mean square deviation • MWCO, molecular-weight cutoff

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.041260905.

Reprint requests to: Aaron J. Oakley, Research School of Chemistry, Australian National University, Acton ACT0200, Australia; e-mail: oakley{at}rsc.anu.edu.au; fax: +61-2-6125-0750.

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