Beta-propeller phytases in the aquatic environment

Phytate, which is one of the dominant organic phosphorus compounds in nature, is very stable in soils. Although a substantial amount of phytate is carried from terrestrial to aquatic systems, it is a minor component of organic phosphorus in coastal sediments. The ephemeral nature of phytate implies the rapid hydrolysis of phytate under aquatic conditions. Among the four classes of known phytases that have been identified in terrestrial organisms, only β-propeller phytase-like sequences have been identified in the aquatic environment. A novel β-propeller phytase gene (phyS), cloned from Shewanella oneidensis MR-1, was found to encode a protein with two beta-propeller phytase domains. The characterization of recombinant full-length PhyS and its domains demonstrated that Domain II was the catalytic domain responsible for phytate hydrolysis. The full-length PhyS displayed a K_m of 83 μM with a kcat of 175.9 min⁻¹ and the Domain II displayed a K_m of 474 μM with a kcat of 10.6 min⁻¹. These results confirm that the phyS gene encodes a functional β-propeller phytase, which is expressed in S. oneidensis under phosphorus deficienct condition. The presence of multiple sequences with a high similarity to phyS in aquatic environmental samples and the widespread occurrence of the Shewanella species in nature suggest that the β-propeller phytase family is the major class of phytases in the aquatic environment, and that it may play an important role in the recycling of phosphorus.