Department of Chemistry, University of California, Davis, Davis, CA 95616, USA.
Science. 2013 Oct 25;342(6157):472-5. doi: 10.1126/science.1241859.
The radical S-adenosylmethionine (SAM) enzyme HydG lyses free l-tyrosine to produce CO and CN(-) for the assembly of the catalytic H cluster of FeFe hydrogenase. We used electron paramagnetic resonance spectroscopy to detect and characterize HydG reaction intermediates generated with a set of (2)H, (13)C, and (15)N nuclear spin-labeled tyrosine substrates. We propose a detailed reaction mechanism in which the radical SAM reaction, initiated at an N-terminal 4Fe-4S cluster, generates a tyrosine radical bound to a C-terminal 4Fe-4S cluster. Heterolytic cleavage of this tyrosine radical at the Cα-Cβ bond forms a transient 4-oxidobenzyl (4OB(•)) radical and a dehydroglycine bound to the C-terminal 4Fe-4S cluster. Electron and proton transfer to this 4OB(•) radical forms p-cresol, with the conversion of this dehydroglycine ligand to Fe-bound CO and CN(-), a key intermediate in the assembly of the 2Fe subunit of the H cluster.
激进的 S-腺苷甲硫氨酸（SAM）酶 HydG 将游离 l-酪氨酸裂解为 CO 和 CN（-），用于 FeFe 氢化酶催化 H 簇的组装。我们使用电子顺磁共振波谱法检测和表征了一组（2）H、（13）C 和（15）N 核自旋标记的酪氨酸底物生成的 HydG 反应中间体。我们提出了一个详细的反应机制，其中自由基 SAM 反应在 N 端 4Fe-4S 簇上引发，生成与 C 端 4Fe-4S 簇结合的酪氨酸自由基。这种酪氨酸自由基在 Cα-Cβ 键上的异裂形成瞬态 4-氧化苄基（4OB（•））自由基和与 C 端 4Fe-4S 簇结合的脱氢甘氨酸。向该 4OB（•）自由基的电子和质子转移形成对甲酚，同时将该脱氢甘氨酸配体转化为 Fe 结合的 CO 和 CN（-），这是 H 簇 2Fe 亚基组装的关键中间体。
