Division of Chemical Sciences, Lawrence Berkeley National Laboratory, and Department of Chemistry , University of California , Berkeley , California 94720 , United States.
Acc Chem Res. 2019 Feb 19;52(2):326-335. doi: 10.1021/acs.accounts.8b00586. Epub 2019 Jan 29.
Enzymes have evolved to catalyze a range of biochemical transformations with high efficiencies and unparalleled selectivities, including stereoselectivities, regioselectivities, chemoselectivities, and substrate selectivities, while typically operating under mild aqueous conditions. These properties have motivated extensive research to identify or create enzymes with reactivity that complements or even surpasses the reactivity of small-molecule catalysts for chemical reactions. One of the limitations preventing the wider use of enzymes in chemical synthesis, however, is the narrow range of bond constructions catalyzed by native enzymes. One strategy to overcome this limitation is to create artificial metalloenzymes (ArMs) that combine the molecular recognition of nature with the reactivity discovered by chemists. This Account describes a new approach for generating ArMs by the formal replacement of the natural iron found in the porphyrin IX (PIX) of hemoproteins with noble metals. Analytical techniques coupled with studies of chemical reactivity have demonstrated that expression of apomyoglobins and apocytochrome P450s (for which "apo-" denotes the cofactor-free protein) followed by reconstitution with metal-PIX cofactors in vitro creates proteins with little perturbation of the native structure, suggesting that the cofactors likely reside within the native active site. By means of this metal substitution strategy, a large number of ArMs have been constructed that contain varying metalloporphyrins and mutations of the protein. The studies discussed in this Account encompass the use of ArMs containing noble metals to catalyze a range of abiological transformations with high chemoselectivity, enantioselectivity, diastereoselectivity, and regioselectivity. These transformations include intramolecular and intermolecular insertion of carbenes into C-H, N-H, and S-H bonds, cyclopropanation of vinylarenes and of internal and nonconjugated alkenes, and intramolecular insertions of nitrenes into C-H bonds. The rates of intramolecular insertions into C-H bonds catalyzed by thermophilic P450 enzymes reconstituted with an Ir(Me)-PIX cofactor are now comparable to the rates of reactions catalyzed by native enzymes and, to date, 1000 times greater than those of any previously reported ArM. This reactivity also encompasses the selective intermolecular insertion of the carbene from ethyl diazoacetate into C-H bonds over dimerization of the carbene to form alkenes, a class of carbene insertion or selectivity not reported to occur with small-molecule catalysts. These combined results highlight the potential of well-designed ArMs to catalyze abiological transformations that have been challenging to achieve with any type of catalyst. The metal substitution strategy described herein should complement the reactivity of native enzymes and expand the scope of enzyme-catalyzed reactions.
酶已经进化到能够高效且无与伦比地催化各种生化转化，包括立体选择性、区域选择性、化学选择性和底物选择性，同时通常在温和的水相条件下进行。这些特性促使人们广泛研究识别或创造具有反应性的酶，这些反应性可以补充甚至超过小分子催化剂在化学反应中的反应性。然而，限制酶在化学合成中更广泛应用的一个因素是天然酶催化的键结构范围狭窄。克服这一限制的一种策略是创建人工金属酶 (ArM)，将自然的分子识别与化学家发现的反应性结合起来。本账户描述了一种通过正式取代血红素蛋白中卟啉 IX (PIX) 中天然铁来生成 ArM 的新方法。分析技术与化学反应性研究相结合，证明表达去辅基肌红蛋白和细胞色素 P450（其中“apo-”表示无辅因子的蛋白质），然后在体外用金属-PIX 辅因子重建，可创建对天然结构几乎没有扰动的蛋白质，这表明辅因子可能位于天然活性部位内。通过这种金属取代策略，已经构建了大量包含不同金属卟啉和蛋白质突变的 ArM。本账户中讨论的研究包括使用包含贵金属的 ArM 催化一系列具有高化学选择性、对映选择性、非对映选择性和区域选择性的非生物转化。这些转化包括卡宾插入 C-H、N-H 和 S-H 键的分子内和分子间插入、乙烯基芳烃和内部及非共轭烯烃的环丙烷化、以及氮宾插入 C-H 键的分子内插入。用 Ir(Me)-PIX 辅因子重建的嗜热 P450 酶催化的分子内 C-H 键插入反应的速率现在与天然酶催化的反应速率相当，迄今为止，比任何以前报道的 ArM 都快 1000 倍。这种反应性还包括乙基重氮乙酸酯的卡宾选择性地插入 C-H 键，而不是卡宾二聚化形成烯烃，这是一类以前报道的小分子催化剂未发生的卡宾插入或选择性。这些综合结果突出了精心设计的 ArM 催化具有挑战性的非生物转化的潜力，这些转化用任何类型的催化剂都难以实现。本文所述的金属取代策略应补充天然酶的反应性，并扩展酶催化反应的范围。
