Evolutionary analysis of a novel zinc ribbon in the N-terminal region of threonine synthase

Threonine synthase (TS) catalyzes the terminal reaction in the biosynthetic pathway of threonine and requires pyridoxal phosphate as a cofactor. TSs share a common catalytic domain with other fold type II PALP dependent enzymes. TSs are broadly grouped into two classes based on their sequence, quaternary structure, and enzyme regulation. We report the presence of a novel zinc ribbon domain in the N-terminal region preceding the catalytic core in TS. The zinc ribbon domain is present in TSs belonging to both classes. Our sequence analysis reveals that archaeal TSs possess all zinc chelating residues to bind a metal ion that are lacking in the structurally characterized homologs. Phylogenetic analysis suggests that TSs with an N-terminal zinc ribbon likely represents the ancestral state of the enzyme while TSs without a zinc ribbon must have diverged later in specific lineages. The zinc ribbon and its N- and C-terminal extensions are important for enzyme stability, activity and regulation. It is likely that the zinc ribbon domain is involved in higher order oligomerization or mediating interactions with other biomolecules leading to formation of larger metabolic complexes.

苏氨酸合酶（Threonine synthase, TS）催化苏氨酸生物合成途径的末端反应，且需以磷酸吡哆醛（pyridoxal phosphate）作为辅因子。TS与其他II型PALP依赖型酶共享保守的催化结构域。依据序列特征、四级结构及酶调控机制，可将TS划分为两大类。本研究首次报道，在TS的催化核心区域上游的N端区域中，存在一个全新的锌指带状结构域（zinc ribbon domain），该结构域在两类TS中均有分布。序列分析结果显示，古菌来源的TS含有所有用于结合金属离子的锌螯合残基，而这类残基在已解析结构的同源TS中并未发现。系统发育分析表明，带有N端锌指带状结构域的TS大概率代表该酶的祖先演化状态，而不含锌带状结构域的TS则是在特定演化支中后期分化产生的。锌指带状结构域及其N端、C端延伸区域对酶的稳定性、催化活性及调控功能均具有关键作用。推测该锌指带状结构域可能参与高阶寡聚化过程，或介导与其他生物分子的相互作用，进而促成更大尺寸的代谢复合物的形成。