S‐adenosyl‐L‐methionine (SAM)‐dependent methyltransferases (MTases) are widely distributed among almost all organisms and often characterized with conserved Rossmann fold, TIM barrel, and D×G×G×G motif. However, some MTases show no methyltransferase activity. In the present study, the crystal structure of LepI, one MTase‐like enzyme isolated from A. flavus that catalyzes pericyclic reactions, was investigated to determine its structure‐function relationship. The overall structure of LepI in complex with the SAM mimic S‐adenosyl‐L‐homocysteine (SAH) (PDB ID: 6IV7) indicated that LepI is a tetramer in solution. The residues His133, Arg197, Arg295, and Asp296 located near the active site can form hydrogen bonds with the substrate, thus participating in catalytic reactions. The binding of SAH in LepI is almost identical to that in other resolved MTases; however, the location of catalytic residues differs significantly. Phylogenetic trials suggest that LepI proteins share a common ancestor in plants and algae, which may explain the conserved SAM‐binding site. However, the accelerated evolution of A. flavus has introduced both functional and structural changes in LepI. More importantly, the residue Arg295, which is unique to LepI, might be a key determinant for the altered enzymatic behavior. Collectively, the differences in the composition of catalytic residues, as well as the unique tetrameric form of LepI, define its unique enzymatic behavior. The present work provides an additional understanding of the structure‐function relationship of MTases and MTase‐like enzymes.

中文翻译：

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黄曲霉的S-腺苷-L-蛋氨酸依赖性O-甲基转移酶样酶的晶体结构。

S-腺苷-L-蛋氨酸（SAM）依赖性甲基转移酶（MTases）广泛分布于几乎所有生物中，通常具有保守的Rossmann折叠，TIM桶和D×G×G×G图案。然而，一些MTase显示无甲基转移酶活性。在本研究中，LepI的晶体结构是一种从黄曲霉分离的MTase样酶研究了催化周环反应的方法，以确定其结构与功能的关系。LepI的整体结构与SAM模拟S-腺苷L-同型半胱氨酸（SAH）（PDB ID：6IV7）复杂，表明LepI是溶液中的四聚体。位于活性位点附近的残基His133，Arg197，Arg295和Asp296可与底物形成氢键，从而参与催化反应。LepI中SAH的结合与其他已解析的MTase中的结合几乎相同。但是，催化残基的位置明显不同。系统发育研究表明，LepI蛋白在植物和藻类中具有相同的祖先，这可能解释了保守的SAM结合位点。然而，黄曲霉的加速进化已经介绍了LepI的功能和结构变化。更重要的是，LepI特有的残基Arg295可能是改变酶促行为的关键决定因素。总的来说，催化残基组成的差异以及LepI的独特四聚体形式定义了其独特的酶促行为。本工作提供了对MTase和类MTase的结构-功能关系的进一步理解。