近日，国际著名杂志《生物化学杂志》（Journal of Biological Chemistry，JBC）在线刊登了中科院上海生科院营养所向嵩研究组的最新研究成果“Crystal structure of ureacarboxylase provides insights into the carboxyltransfer reaction，”。该研究解析了尿素羧化酶(urea carboxylase， UC）的三维结构，揭示了其CarboxylTransferase（CT）结构域催化反应的机制。
UC属于依赖生物素的羧化酶家族，该家族成员能羧化形式多样的底物分子，在多条代谢途径中发挥重要功能。UC分布于多种微生物中，它催化的由尿素生成脲(allophanate)的转化反应是这些微生物获得氮元素的必需环节，氮元素在生物圈的循环中发挥重要功能。向嵩研究员指导的博士研究生樊晨等利用结晶学的方法解析了UC的晶体结构，捕捉到其CT结构域催化反应的一个中间态，并结合结构信息指导的功能实验结果，揭示了CT结构域催化反应的机制。另外，分析表明UC的CT结构与其它依赖生物素的羧化酶有相似的地方，因此对UC结构和功能的了解也有助于理解依赖生物素羧化酶家族其它成员的催化机理。该课题获得国家科技部和中国科学院的经费资助。（生物谷Bioon.com）
doi:10.1074/jbc.M111.319475
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Crystal structure of urea carboxylase provides insights into the carboxyltransfer reaction
Chen Fan1, Chi-Yuan Chou2, Liang Tong3 and Song Xiang1,*
Urea carboxylase (UC) is conserved in many bacteria, algae and fungi, and catalyzes the conversion of urea to allophanate, an essential step in the utilization of urea as a nitrogen source in these organisms. UC belongs to the biotin-dependent carboxylase superfamily, and shares the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains with these other enzymes, but its carboxyltransferase (CT) domain is distinct. Currently there is no information on the molecular basis of catalysis by UC. We report here crystal structure of the Kluyveromyces lactis UC and biochemical studies to assess the structural information. Structural and sequence analyses indicate the CT domain of UC belongs to a large family of proteins with diverse functions, including the Bacillus subtilis KipA-KipI complex, which has important functions in sporulation regulation. A structure of the KipA-KipI complex is not currently available, and our structure provides a framework to understand the function of this complex. Most interestingly, in the structure the CT domain interacts with the BCCP domain, with biotin and a urea molecule bound at its active site. This structural information and our follow-up biochemical experiments provided molecular insights into the UC carboxyltransfer reaction. Several structural elements important for the UC carboxyltransfer reaction are found in other biotin-dependent carboxylases and might be conserved within this family, and our data could shed light on the mechanism of these enzymes' catalysis.
