Cosper Michele M, Cosper Nathaniel J, Hong Wei, Shokes Jacob E, Broderick William E, Broderick Joan B, Johnson Michael K, Scott Robert A
Department of Chemistry, University of Georgia, Athens, GA 30602-2556, USA.
Protein Sci. 2003 Jul;12(7):1573-7. doi: 10.1110/ps.0302203.
The diverse reactions catalyzed by the radical-SAM superfamily of enzymes are thought to proceed via a set of common mechanistic steps, key among which is the reductive cleavage of S-adenosyl-L-methionine (SAM) by a reduced [4Fe-4S] cluster to generate an intermediate deoxyadenosyl radical. A number of spectroscopic studies have provided evidence that SAM interacts directly with the [4Fe-4S] clusters in several of the radical-SAM enzymes; however, the molecular mechanism for the reductive cleavage has yet to be elucidated. Selenium X-ray absorption spectroscopy (Se-XAS) was used previously to provide evidence for a close interaction between the Se atom of selenomethionine (a cleavage product of Se-SAM) and an Fe atom of the [4Fe-4S] cluster of lysine-2,3-aminomutase (KAM). Here, we utilize the same approach to investigate the possibility of a similar interaction in pyruvate formate-lyase activating enzyme (PFL-AE) and biotin synthase (BioB), two additional members of the radical-SAM superfamily. The results show that the latter two enzymes do not exhibit the same Fe-Se interaction as was observed in KAM, indicating that the methionine product of reductive cleavage of SAM does not occupy a well-defined site close to the cluster in PFL-AE and BioB. These results are interpreted in terms of the differences among these enzymes in their use of SAM as either a cofactor or a substrate.
自由基-SAM超家族酶催化的各种反应被认为是通过一系列共同的机制步骤进行的,其中关键步骤是由还原型[4Fe-4S]簇对S-腺苷-L-甲硫氨酸(SAM)进行还原裂解,生成中间体脱氧腺苷自由基。多项光谱研究已提供证据表明,SAM在几种自由基-SAM酶中直接与[4Fe-4S]簇相互作用;然而,还原裂解的分子机制尚未阐明。先前已使用硒X射线吸收光谱法(Se-XAS)来证明硒代甲硫氨酸(Se-SAM的裂解产物)的硒原子与赖氨酸-2,3-氨基变位酶(KAM)的[4Fe-4S]簇的铁原子之间存在紧密相互作用。在此,我们采用相同的方法来研究丙酮酸甲酸裂解酶激活酶(PFL-AE)和生物素合酶(BioB)(自由基-SAM超家族的另外两个成员)中类似相互作用的可能性。结果表明,后两种酶未表现出与KAM中观察到的相同的铁-硒相互作用,这表明SAM还原裂解的甲硫氨酸产物在PFL-AE和BioB中并未占据靠近簇的明确位点。这些结果根据这些酶在将SAM用作辅因子或底物方面的差异进行了解释。