Estación Experimental de Aula Dei, Apdo. 202, 50080 Zaragoza, Spain.
Plant Physiol. 1992 Apr;98(4):1217-21. doi: 10.1104/pp.98.4.1217.
Reactions involving changes that affect the function of leghemoglobin (Lb) are reviewed. The chemical nature of Lb and conditions inside nodules, such as slightly acid pH and the presence of metal ions, chelators, and toxic metabolites (nitrite, superoxide radical, peroxides), are conducive for oxidation of ferrous Lb (Lb(2+)) or its oxygenated form (LbO(2)) to nonfunctional ferric Lb (Lb(3+)) and ferryl Lb. Because Lb(3+) is nearly nonexistent in nodules and undergoes observable reduction in vivo, mechanisms must operate in nodules to maintain Lb in the Lb(2+) state. Redox reactions of Lb are mediated, for the most part, by activated oxygen species: (a) oxidation of LbO(2) to Lb(3+) involves superoxide; (b) excess peroxide oxidizes LbO(2) and Lb(3+) to ferryl Lb and may cause breakdown of heme, release of iron, and generation of hydroxyl radicals (protein radicals may be formed in this process); (c) enzymatic reduction of Lb(3+) requires active flavin and thiol groups and involves formation of peroxide; and (d) direct reduction of Lb(3+) by NADH is mediated by superoxide and peroxide. Transition metal ions and certain small molecules of nodules such as flavins may act as intermediate electron carriers between NADH and Lb(3+), increasing the rate of reaction, which then proceeds via superoxide or flavin radicals, respectively.
本文综述了影响豆血红蛋白（Lb）功能的反应。Lb 的化学性质以及根瘤内的条件，如略酸性 pH 值和金属离子、螯合剂和有毒代谢物（亚硝酸盐、超氧自由基、过氧化物）的存在，有利于亚铁 Lb（Lb(2+)）或其氧合形式（LbO(2)）氧化为非功能的铁 Lb（Lb(3+)）和高铁 Lb。由于 Lb(3+)在根瘤中几乎不存在，并在体内发生可观察到的还原，因此根瘤中必须存在机制来维持 Lb 处于 Lb(2+)状态。Lb 的氧化还原反应主要由活性氧物质介导：（a）LbO(2)氧化为 Lb(3+)涉及超氧自由基；（b）过氧化物过量氧化 LbO(2)和 Lb(3+)为高铁 Lb，并可能导致血红素分解、铁释放和羟基自由基生成（在此过程中可能形成蛋白质自由基）；（c）Lb(3+)的酶促还原需要活性黄素和巯基基团，并涉及过氧化物的形成；（d）NADH 对 Lb(3+)的直接还原由超氧自由基和过氧化物介导。过渡金属离子和根瘤中的某些小分子，如黄素，可能在 NADH 和 Lb(3+)之间充当中间电子载体，增加反应速率，然后分别通过超氧自由基或黄素自由基进行。
