McAdam M E, Fox R A, Lavelle F, Fielden E M
Biochem J. 1977 Jul 1;165(1):71-9. doi: 10.1042/bj1650071.
The enzymic reaction mechanism of a manganese-containing superoxide dismutase from Bacillus stearothermophilus was studied by using pulse radiolysis. During catalysis (pH 8.9; 25 degrees C), changes occurring in the kinetics of substrate disappearance and in the visible absorption of the enzyme at 480 nm established that the simple two-step mechanism found for copper- and iron-containing superoxide dismutases is not involved. At a low ratio (less than 15) of substrate concentration to enzyme concentration the decay of O2--is close to exponetial, whereas at much higher ratios (greater than 100) the observed decay is predominantly zero-order. The simplest interpretation of the results invokes a rapid one-electron oxidation-reduction cycle ('the fast cycle') and, concurrently, a slower reaction giving a form of the enzyme that is essentially unreactive towards O2-- but which undergoes a first-order decay to yield fully active native enzyme ('the slow cycle'). The fast cycle involves the native enzyme EA and a form of the enzyme EB which can be obtained also by treating the form EA with H2O2. Computer calculations made with such a simple model predict behaviour in excellent agreement with the observed results.
利用脉冲辐解研究了嗜热脂肪芽孢杆菌含锰超氧化物歧化酶的酶促反应机制。在催化过程中（pH 8.9；25℃），底物消失动力学和酶在480nm处的可见吸收变化表明，含铜和含铁超氧化物歧化酶的简单两步机制并不适用。在底物浓度与酶浓度的低比例（小于15）下，O₂⁻的衰减接近指数形式，而在高得多的比例（大于100）下，观察到的衰减主要是零级的。对结果的最简单解释是涉及一个快速的单电子氧化还原循环（“快速循环”），同时，一个较慢的反应产生一种对O₂⁻基本无反应但经历一级衰减以产生完全活性天然酶的酶形式（“慢速循环”）。快速循环涉及天然酶EA和一种酶形式EB，也可以通过用H₂O₂处理EA形式获得。用这样一个简单模型进行的计算机计算预测的行为与观察结果非常一致。
