Alberty Robert A
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, 02139, USA.
Biophys Chem. 2005 Apr 22;114(2-3):115-20. doi: 10.1016/j.bpc.2004.11.009. Epub 2004 Dec 10.
The fixation of molecular nitrogen by nitrogenase requires a lot of energy because 16 mol of ATP are hydrolyzed per mole of nitrogen converted to ammonia. Kim and Dees determined the crystallograpic structure of nitrogenase and this has led to a three-step mechanism that involves Feprotein and MoFeprotein in addition to ferredoxin. Each of these steps can be interpreted in terms of two half reactions that are connected through their transfer of electrons. Estimates can be made of the standard apparent reduction potentials of these three steps and their dependencies on pH and ionic strength. This mechanism is compared with the same type of analysis of an alternative three-step mechanism in which the hydrolysis of ATP is coupled with the reduction of molecular nitrogen, rather than the reduction of Feprotein. The problem with the first mechanism is that the second step produces 12 mol of hydrogen ions per mole of nitrogen fixed and the third step consumes 10 mol of hydrogen ions per mole of nitrogen fixed. The alternative mechanism does not have this problem.
固氮酶固定分子氮需要大量能量，因为每将一摩尔氮转化为氨，就有16摩尔ATP被水解。金和迪斯确定了固氮酶的晶体结构，这导致了一种三步机制，除了铁氧化还原蛋白外，还涉及铁蛋白和钼铁蛋白。这些步骤中的每一步都可以用通过电子转移连接的两个半反应来解释。可以对这三个步骤的标准表观还原电位及其对pH值和离子强度的依赖性进行估计。将该机制与另一种三步机制的相同类型分析进行比较，在另一种机制中，ATP的水解与分子氮的还原相偶联，而不是与铁蛋白的还原相偶联。第一种机制的问题是，第二步每固定一摩尔氮会产生12摩尔氢离子，第三步每固定一摩尔氮会消耗10摩尔氢离子。另一种机制没有这个问题。
