二甲基巯基丙酸内盐（DMSP）年产量高达10亿吨，是海洋中重要的有机硫。海洋细菌对DMSP的代谢是全球碳、硫循环的重要过程，主要包括去甲基化代谢和裂解代谢，其中去甲基化代谢约占DMSP总代谢通量的90%，是DMSP代谢的主要途径。DMSP去甲基化代谢包括四个关键酶：DmdA、DmdB、DmdC和DmdD，其中DmdB、DmdC的结构及催化机制至今未被揭示。本项目以代谢DMSP主要细菌类群——玫瑰杆菌类群细菌为材料，开展如下研究：（1）解析DmdB及其与底物、产物复合物的结构，结合生化研究，揭示其催化MMPA与CoA反应生成MMPA-CoA的分子机制。（2）解析DmdC及其与底物、产物复合物的结构，结合生化研究，揭示其催化MMPA-CoA脱氢生成MTA-CoA的分子机制。上述研究结果将完善海洋细菌参与的DMSP去甲基化代谢的分子机制，有助于更好的认识地球碳、硫循过程，具有重要的理论意义。

Dimethylsulfoniopropionate (DMSP), produced on a scale of 1000 Tg a year, is an important organic sulfur in the ocean. Microbial cleavage of DMSP is an important step in the global sulfur and carbon cycles. Most DMSP is degraded by marine bacteria via either a demethylation pathway or a cleavage pathway. Between them, demethylation pathway which accounts for about 90% of total metabolism of DMSP, is the major metabolism pathway. There are four key enzymes in the demethylation pathway, DmdA, DmdB, DmdC, and DmdD. However, till now, the structures and catalytic mechanisms of DmdB and DmdC have not been revealed. Roseobacter is one of the major bacterial groups that metabolize DMSP. This program taking Roseobacter as the material will carry out the following studies: (1) Solve the structures of DmdB, DmdB in complex with substrate/product. By combining structural analysis with biochemical experiments, reveal the molecular mechanism that DmdB catalyzes the reaction of MMPA and CoA to produce MMPA-CoA. (2) Solve the structures of DmdC, DmdC in complex with substrate/product.By combining structural analysis with biochemical experiments, reveal the molecular mechanism that DmdC catalyzes the dehydrogenation of MMPA-CoA to MTA-CoA. The research results will consummate the molecular mechanism of DMSP demethylation metabolism and help us to better understand the process of carbon and sulfur cyclings on the earth.