Marsh E N
Cambridge Centre for Molecular Recognition, UK.
Bioessays. 1995 May;17(5):431-41. doi: 10.1002/bies.950170511.
Free radicals are generally perceived as highly reactive species which are harmful to biological systems. There are, however, a number of enzymes that use carbon-based radicals to catalyse a variety of important and unusual reactions. The most prominent example is ribonucleotide reductase, an enzyme which is crucial for the synthesis of DNA. In general, radicals are used to remove hydrogen from unreactive positions in the substrate, and in this way the substrate is activated to undergo chemical transformations that would otherwise be difficult to achieve. Several different mechanisms have evolved which allow enzymes to generate and maintain radicals in increasingly aerobic environments. An unexpected finding is the existence of stable protein-based radicals, residing on a variety of amino-acid side chains, which serve to link the radical-generating and catalytic sites and to store the radical between turnovers.
自由基通常被认为是对生物系统有害的高反应性物种。然而，有许多酶利用基于碳的自由基来催化各种重要且不寻常的反应。最突出的例子是核糖核苷酸还原酶，这是一种对DNA合成至关重要的酶。一般来说，自由基用于从底物中无反应性的位置去除氢，通过这种方式，底物被激活以进行否则难以实现的化学转化。已经进化出几种不同的机制，使酶能够在越来越有氧的环境中产生和维持自由基。一个意想不到的发现是存在稳定的基于蛋白质的自由基，它们存在于各种氨基酸侧链上，用于连接自由基产生位点和催化位点，并在周转之间储存自由基。
