Nitrogenases catalyze the reduction of dinitrogen (N₂) gas to ammonium at a complex heterometallic cofactor. This most commonly occurs at the FeMo cofactor (FeMoco), a [Mo–7Fe–9S–C] cluster whose exact reactivity and substrate-binding mode remain unknown. Alternative nitrogenases replace molybdenum with either vanadium or iron and differ in reactivity, most prominently in the ability of vanadium nitrogenase to reduce CO to hydrocarbons. Here we report the 1.35-Å structure of vanadium nitrogenase from Azotobacter vinelandii. The 240-kDa protein contains an additional α-helical subunit that is not present in molybdenum nitrogenase. The FeV cofactor (FeVco) is a [V–7Fe–8S–C] cluster with a homocitrate ligand to vanadium. Unexpectedly, it lacks one sulfide ion compared to FeMoco, which is replaced by a bridging ligand, likely a μ-1,3-carbonate. The anion fits into a pocket within the protein that is obstructed in molybdenum nitrogenase, and its different chemical character helps to rationalize the altered chemical properties of this unique N₂- and CO-fixing enzyme.

中文翻译：

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钒氮酶的结构揭示了一个不寻常的桥联配体

氮酶在复杂的杂金属辅因子上催化将二氧化氮（N ₂）还原为铵。这最常见于FeMo辅助因子（FeMoco），[Mo-7Fe-9S-C]团簇，其确切的反应性和底物结合模式仍然未知。替代的固氮酶用钒或铁替代钼，并且反应性不同，最主要的是钒固氮酶将CO还原为碳氢化合物的能力。在这里，我们报道来自葡萄固氮菌的钒氮酶的1.35-Å结构。240 kDa蛋白质还包含一个额外的α-螺旋亚基，不存在于钼固氮酶中。FeV辅因子（FeVco）是[V-7Fe-8S-C]簇，具有钒的均柠檬酸盐配体。出乎意料的是，与FeMoco相比，它缺少一个硫化物离子，而FeMoco被桥连配体（可能是μ-1,3-碳酸盐）取代。阴离子适合被钼固氮酶阻塞的蛋白质的口袋，其不同的化学特性有助于合理化这种独特的N _2-和CO固定酶的化学性质。