Layer Gunhild, Moser Jürgen, Heinz Dirk W, Jahn Dieter, Schubert Wolf-Dieter
Institute of Microbiology, Technical University Braunschweig, Spielmannstrasse 7, D-38106 Braunschweig, Germany.
EMBO J. 2003 Dec 1;22(23):6214-24. doi: 10.1093/emboj/cdg598.
'Radical SAM' enzymes generate catalytic radicals by combining a 4Fe-4S cluster and S-adenosylmethionine (SAM) in close proximity. We present the first crystal structure of a Radical SAM enzyme, that of HemN, the Escherichia coli oxygen-independent coproporphyrinogen III oxidase, at 2.07 A resolution. HemN catalyzes the essential conversion of coproporphyrinogen III to protoporphyrinogen IX during heme biosynthesis. HemN binds a 4Fe-4S cluster through three cysteine residues conserved in all Radical SAM enzymes. A juxtaposed SAM coordinates the fourth Fe ion through its amide nitrogen and carboxylate oxygen. The SAM sulfonium sulfur is near both the Fe (3.5 A) and a neighboring sulfur of the cluster (3.6 A), allowing single electron transfer from the 4Fe-4S cluster to the SAM sulfonium. SAM is cleaved yielding a highly oxidizing 5'-deoxyadenosyl radical. HemN, strikingly, binds a second SAM immediately adjacent to the first. It may thus successively catalyze two propionate decarboxylations. The structure of HemN reveals the cofactor geometry required for Radical SAM catalysis and sets the stage for the development of inhibitors with antibacterial function due to the uniquely bacterial occurrence of the enzyme.
“自由基S-腺苷甲硫氨酸”（Radical SAM）酶通过使一个4Fe-4S簇和S-腺苷甲硫氨酸（SAM）紧密靠近来产生催化性自由基。我们报道了一种Radical SAM酶的首个晶体结构，即大肠杆菌中不依赖氧气的粪卟啉原III氧化酶HemN的晶体结构，分辨率为2.07埃。HemN在血红素生物合成过程中催化粪卟啉原III向原卟啉原IX的关键转化。HemN通过所有Radical SAM酶中保守的三个半胱氨酸残基结合一个4Fe-4S簇。一个并列的SAM通过其酰胺氮和羧酸盐氧来配位第四个铁离子。SAM的锍硫既靠近铁（3.5埃）又靠近簇的相邻硫（3.6埃），使得单电子从4Fe-4S簇转移至SAM的锍。SAM被裂解产生一个高氧化性的5'-脱氧腺苷自由基。引人注目的是，HemN紧邻第一个SAM结合第二个SAM。因此它可能相继催化两次丙酸脱羧反应。HemN的结构揭示了Radical SAM催化所需的辅因子几何结构，并为开发具有抗菌功能的抑制剂奠定了基础，因为该酶独特地存在于细菌中。
