Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Am Chem Soc. 2011 Apr 27;133(16):6158-61. doi: 10.1021/ja2013517. Epub 2011 Apr 4.
Cyanobacterial aldehyde decarbonylase (AD) catalyzes conversion of fatty aldehydes (R-CHO) to alka(e)nes (R-H) and formate. Curiously, although this reaction appears to be redox-neutral and formally hydrolytic, AD has a ferritin-like protein architecture and a carboxylate-bridged dimetal cofactor that are both structurally similar to those found in di-iron oxidases and oxygenases. In addition, the in vitro activity of the AD from Nostoc punctiforme (Np) was shown to require a reducing system similar to the systems employed by these O(2)-utilizing di-iron enzymes. Here, we resolve this conundrum by showing that aldehyde cleavage by the Np AD also requires dioxygen and results in incorporation of (18)O from (18)O(2) into the formate product. AD thus oxygenates, without oxidizing, its substrate. We posit that (i) O(2) adds to the reduced cofactor to generate a metal-bound peroxide nucleophile that attacks the substrate carbonyl and initiates a radical scission of the C1-C2 bond, and (ii) the reducing system delivers two electrons during aldehyde cleavage, ensuring a redox-neutral outcome, and two additional electrons to return an oxidized form of the cofactor back to the reduced, O(2)-reactive form.
蓝藻醛脱羧酶 (AD) 催化脂肪酸醛 (R-CHO) 转化为链烷烃 (R-H) 和甲酸盐。奇怪的是，尽管该反应似乎是氧化还原中性的，并且形式上是水解的，但 AD 具有类似于二铁氧化酶和加氧酶中发现的铁蛋白样蛋白结构和羧酸桥接的双核金属辅因子。此外，来自念珠藻 (Np) 的 AD 的体外活性表明需要类似于这些利用 O2 的二铁酶所使用的还原系统。在这里，我们通过表明 Np AD 的醛裂解也需要氧气并导致 (18)O 从 (18)O2 掺入甲酸盐产物来解决这个难题。因此，AD 对其底物进行加氧而不氧化。我们假设 (i) O2 加到还原的辅因子上以生成金属结合的过氧亲核试剂，该亲核试剂攻击底物羰基并引发 C1-C2 键的自由基断裂，和 (ii) 还原系统在醛裂解过程中提供两个电子，确保氧化还原中性结果，并提供另外两个电子将辅因子的氧化形式返回还原的、对 O2 反应的形式。
