Vinha a caminhar para casa a falar com a Gemma e acabámos por discutir "qual o nosso enzima preferido". O meu é Superóxido Dismutase, especialmente pela sua acção face às ROSs. O da Gemma é Nitrogenase (enzima que catalisa a reacção N2 + 6H + energia → 2NH3). Eu não sou grande fã de moléculas ou seja o que for que tenha a ver com Azoto (aka Nitrogénio), não me perguntem porquê e não é nada pessoal (ou molecular?), não gosto e prontos!
The enzyme superoxide dismutase (SOD, EC 22.214.171.124), catalyzes the dismutation of superoxide into oxygen and hydrogen peroxide. As such, it is an important antioxidant defense in nearly all cells exposed to oxygen.
Cell damage is induced by reactive oxygen species (ROS). ROS are either free radicals, reactive anions containing oxygen atoms, or molecules containing oxygen atoms that can either produce free radicals or are chemically activated by them. Under normal conditions, ROS are cleared from the cell by the action of superoxide dismutase (SOD), catalase, or glutathione (GSH) peroxidase. The main damage to cells results from the ROS-induced alteration of macromolecules such as polyunsaturated fatty acids in membrane lipids, essential proteins, and DNA. Additionally, oxidative stress and ROS have been implicated in disease states, such as Alzheimer's disease, Parkinson’s disease, cancer, and aging.
Simply-stated, SOD outcompetes damaging reactions of superoxide, thus protecting the cell from superoxide toxicity. The reaction of superoxide with non-radicals is spin forbidden. In biological systems, this means its main reactions are with itself (dismutation) or with another biological radical such as nitric oxide (NO). The superoxide anion radical (O2-) spontaneously dismutes to O2 and hydrogen peroxide (H2O2) quite rapidly (~105 M-1 s-1 at pH 7). SOD is biologically necessary because superoxide reacts even faster with certain targets such as NO radical, which makes peroxynitrite. Similarly, the dismutation rate is second order with respect to initial superoxide concentration. Thus, the half-life of superoxide, although very short at high concentrations (e.g. 0.05 seconds at 0.1mM) is actually quite long at low concentrations (e.g. 14 hours at 0.1 nM). In contrast, the reaction of superoxide with SOD is first order with respect to superoxide concentration. Moreover, superoxide dismutase has the fastest turnover number (reaction rate with its substrate) of any known enzyme (~109 M-1 s-1), this reaction being only limited by the frequency of collision between itself and superoxide. That is, the reaction rate is "diffusion limited".