Crystal structure of a novel putative sugar isomerase from the psychrophilic bacterium Paenibacillus sp. R4.

Sugar isomerases (SIs) catalyze the reversible conversion of aldoses to ketoses. A novel putative SI gene has been identified from the genome sequence information on the psychrophilic bacterium Paenibacillus sp. R4. Here, we report the crystal structure of the putative SI from Paenibacillus sp. R4 (Pb SI) at 2.98 Å resolution. It was found that the overall structure of Pb SI adopts the triose-phosphate isomerase (TIM) barrel fold. Pb SI was also identified to have two heterogeneous metal ions as its cofactors at the active site in the TIM barrel, one of which was confirmed as a Zn ion through X-ray anomalous scattering and inductively coupled plasma mass spectrometry analysis. Structural comparison with homologous SI proteins from mesophiles, hyperthermophiles, and a psychrophile revealed that key residues in the active site are well conserved and that dimeric Pb SI is devoid of the extended C-terminal region, which tetrameric SIs commonly have. Our results provide novel structural information on the cold-adaptable SI, including information on the metal composition in the active site. • The structure of Pb SI adopts the triose-phosphate isomerase barrel fold. • Pb SI has two heterogeneous metal ions as its cofactor at the active site. • Residues in the active site are well conserved. • Dimeric Pb SI is devoid of the extended C-terminal region. [ABSTRACT FROM AUTHOR]