School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, Essex, CO4 3SQ (U, K.
Department of Biotechnology, Chemistry and Pharmaceutical Sciences, University of Siena, Via A. Moro, 2, 53100, Siena, Italy.
Angew Chem Int Ed Engl. 2017 Jun 1;56(23):6502-6506. doi: 10.1002/anie.201701270. Epub 2017 May 2.
The concerted redox action of a metal ion and an organic cofactor is a unique way to maximize the catalytic power of an enzyme. An example of such synergy is the fungal galactose 6-oxidase, which has inspired the creation of biomimetic copper oxidation catalysts. Galactose 6-oxidase and its bacterial homologue, GlxA, possess a metalloradical catalytic site that contains a free radical on a covalently linked Cys-Tyr and a copper atom. Such a catalytic site enables for the two-electron oxidation of alcohols to aldehydes. When the ability to form the Cys-Tyr in GlxA is disrupted, a radical can still be formed. Surprisingly, the radical species is not the Tyr residue but rather a copper second-coordination sphere Trp residue. This is demonstrated through the introduction of a new algorithm for Trp-radical EPR spectra simulation. Our findings suggest a new mechanism of free-radical transfer between aromatic residues and that the Cys-Tyr cross-link prevents radical migration away from the catalytic site.
金属离子和有机辅因子的协同氧化还原作用是最大限度地提高酶催化能力的独特方法。真菌半乳糖 6-氧化酶就是这种协同作用的一个例子，它启发了仿生铜氧化催化剂的创造。半乳糖 6-氧化酶及其细菌同源物 GlxA 具有一个金属自由基催化位点，其中包含一个通过共价连接的 Cys-Tyr 和一个铜原子连接的自由基。这样的催化位点能够实现醇的两电子氧化为醛。当 GlxA 中形成 Cys-Tyr 的能力被破坏时，仍然可以形成自由基。令人惊讶的是，自由基物种不是 Tyr 残基，而是铜的第二配位层 Trp 残基。这通过引入一种新的 Trp 自由基 EPR 光谱模拟算法得到了证明。我们的发现表明了芳香族残基之间自由基转移的新机制，并且 Cys-Tyr 交联阻止了自由基从催化位点的迁移。
