Identification and functional characterization of sorbitol-6-phosphate dehydrogenase protein from rice and structural elucidation by in silico approach

The sorbitol-6-phosphate dehydrogenase (S6PDH) is a key enzyme for sorbitol synthesis and plays an important role in the alleviation of salinity stress in plants. Despite the huge significance, the structure and the mode of action of this enzyme are still not known. In the present study, sequence analysis, cloning, expression, activity assays and enzyme kinetics using various substrates (glucose-6-phosphate, sorbitol-6-phosphate and mannose-6-phosphate) were performed to establish the functional role of S6PDH protein from rice (Oryza sativa). For the structural analysis of the protein, a comparative homology model was prepared on the basis of percentage sequence identity and substrate similarity using the crystal structure of human aldose reductase in complex with glucose-6-phosphate and NADP⁺ (PDB ID: 2ACQ) as a template. Molecular docking was performed for studying the structural details of substrate binding and possible enzyme mechanism. The cloned sequence resulted into an active recombinant protein when expressed into a bacterial expression system. The purified recombinant protein was found to be active with glucose-6-phosphate and sorbitol-6-phosphate; however, activity against mannose-6-phosphate was not found. The K _m values for glucose-6-phosphate and sorbitol-6-phosphate were found to be 15.9 ± 0.2 and 7.21 ± 0.5 mM, respectively. A molecular-level analysis of the active site of OsS6PDH provides valuable information about the enzyme mechanism and requisite enantioselectivity for its physiological substrates. Thus, the fundamental studies of structure and function of OsS6PDH could serve as the basis for the future studies of bio-catalytic applications of this enzyme.