Trong ba trung gian trong quá trình này hai gốc tự do (các gốc hydroxyl và superoxide) và thứ ba (peroxide) có xu hướng tạo ra các gốc tự do trong các phản ứng như được thảo luận sau trong bài viết này. Giảm bốn điện tử oxy xảy ra trong hệ thống vận chuyển điện tử ty lạp thể của tất cả các tế bào hô hấp hiếu k | RADICAL REACTIONS OF HAEM PROTEINS Of the three intermediates in this process two are free radicals superoxide and hydroxyl radicals and the third peroxide has a tendency to generate free radicals in reactions as discussed later in this article. The four-electron reduction of oxygen occurs in the mitochondrial electron transport system of all aerobically respiring cells. The enzyme which catalyses this reaction cytochrome c oxidase contains the transition metals iron and copper in its active site. These ions can be paramagnetic and contain stable unpaired electrons in their d-orbitals. By using the unpaired electrons in these transition metals to control the oxygen reactions mitochondria prevent the unwanted release of oxygen-derived free Reactions of free radicals Although free radical reactions are generally considered detrimental it has long been known that enzymes use the reactivity of free radicals to catalyse biological chemistry for example respiration thyroid hormone synthesis prostaglandin metabolism and DNA synthesis to name but a few. More recently signalling roles have been discovered for free radicals. Therefore the perception that formation of free radicals in vivo necessarily represents a pathological event is changing to encompass the idea that these reactive species can in fact regulate numerous physiological processes. The classic example is the free radical nitric oxide which has diverse physiological roles in the vasculature in host immune responses and in the nervous Nitric oxide stimulation of soluble guanylate cyclase in the vascular smooth muscle activates a signalling cascade that eventually leads to relaxation of the vessel or in platelets to an inhibition of aggregation. These properties of nitric oxide have defined key roles for this free radical in the mechanisms that maintain vascular homeostasis. However one should not neglect the dark side of free radical reactivity. A number of biological processes have the .
