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l-Rhamnose isomerase (l-RhI) catalyzes the reversible isomerization of l-rhamnose to l-rhamnulose. Pseudomonas stutzeril-RhI, with a broad substrate specificity, can catalyze not only the isomerization ofl-rhamnose, but also that betweend-allose andd-psicose. For the aldose–ketose isomer-ization by l-RhI, a metal-mediated hydride-shift mechanism has been proposed, but the catalytic mechanism is still not entirely understood. | ỊFEBS Journal Catalytic reaction mechanism of Pseudomonas stutzeri L-rhamnose isomerase deduced from X-ray structures Hiromi Yoshida1 Masatsugu Yamaji1 2 Tomohiko Ishii2 Ken Izumori3 and Shigehiro Kamitori1 1 Life Science Research Center and Faculty of Medicine Kagawa University Japan 2 Faculty of Technology Kagawa University Japan 3 Rare Sugar Research Center and Faculty of Agriculture Kagawa University Japan Keywords catalytic mechanism hydride-shift L-rhamnose isomerase rare sugar X-ray structure Correspondence S. Kamitori Life Science Research Center and Faculty of Medicine Kagawa University 1750-1 Ikenobe Miki-cho Kita-gun Kagawa 761-0793 Japan Fax 81 87 891 2421 Tel 81 87 891 2421 E-mail kamitori@med.kagawa-u.ac.jp Received 27 October 2009 revised 7 December 2009 accepted 15 December 2009 doi 10.1111 j.1742-4658.2009.07548.x L-Rhamnose isomerase L-RhI catalyzes the reversible isomerization of L-rhamnose to L-rhamnulose. Pseudomonas stutzeri L-RhI with a broad substrate specificity can catalyze not only the isomerization of L-rhamnose but also that between D-allose and D-psicose. For the aldose-ketose isomerization by L-RhI a metal-mediated hydride-shift mechanism has been proposed but the catalytic mechanism is still not entirely understood. To elucidate the entire reaction mechanism the X-ray structures of P. stutzeri L-RhI in an Mn2 -bound form and of two inactive mutant forms of P. stutzeri L-RhI S329K and D327N in a complex with substrate product were determined. The structure of the Mn2 -bound enzyme indicated that the catalytic site interconverts between two forms with the displacement of the metal ion to recognize both pyranose and furanose ring substrates. Solving the structures of S329K-substrates allowed us to examine the metal-mediated hydride-shift mechanism of L-RhI in detail. The structural analysis of D327N-substrates and additional modeling revealed Asp327 to be responsible for the ring opening of furanose and a water molecule coordinating .
