Department of Biology, Memorial University of Newfoundland, St. John's, NL, Canada.
IUBMB Life. 2011 Mar;63(3):146-52. doi: 10.1002/iub.439.
Nonsymbiotic class 1 plant hemoglobins are induced under hypoxia. Structurally they are protein dimers consisting of two identical subunits, each containing heme iron in a weak hexacoordinate state. The weak hexacoordination of heme-iron binding to the distal histidine results in an extremely high avidity to oxygen, with a dissociation constant in the nanomolar range. This low dissociation constant is due to rapid oxygen binding resulting in protein conformational changes that slow dissociation from the heme site. Class 1 hemoglobins are characterized by an increased rate of Fe³(+) reduction which is likely mediated by cysteine residue. This cysteine can form a reversible covalent bond between two monomers as shown by mass spectrometry analysis and, in addition to its structural role, prevents the molecule from autoxidation. The structural properties of class 1 hemoglobins allow them to serve as soluble electron transport proteins in the enzymatic system scavenging nitric oxide produced in low oxygen via reduction of nitrite. During oxygenation of nitric oxide to nitrate, oxidized ferric hemoglobin is formed (methemoglobin), which can be reduced by an associated reductase. The identified candidate for this reduction is monodehydroascorbate reductase. It is suggested that hemoglobin functions as a terminal electron acceptor during the hypoxic turnover of nitrogen, the process aided by its extremely high affinity for oxygen.
非共生类 1 植物血红蛋白在缺氧条件下诱导产生。从结构上看，它们是由两个相同亚基组成的二聚体蛋白质，每个亚基都含有处于弱六配位状态的血红素铁。血红素铁与远端组氨酸的弱六配位导致其对氧气具有极高的亲和力，解离常数在纳摩尔范围内。这种低解离常数是由于快速的氧结合导致蛋白质构象发生变化，从而减缓了从血红素部位的解离。类 1 血红蛋白的特征是 Fe³⁺还原速率增加，这可能是由半胱氨酸残基介导的。该半胱氨酸可以通过质谱分析显示在两个单体之间形成可逆的共价键，除了其结构作用外，还可以防止分子自氧化。类 1 血红蛋白的结构特性使其能够作为可溶性电子传递蛋白，在低氧条件下通过还原亚硝酸盐来清除产生的一氧化氮的酶系统中发挥作用。在一氧化氮氧化为硝酸盐的过程中，形成氧化的高铁血红蛋白（高铁血红蛋白），可以被相关的还原酶还原。该还原酶的候选物是单脱氢抗坏血酸还原酶。有研究表明，血红蛋白在缺氧条件下的氮循环中作为末端电子受体发挥作用，这一过程得益于其对氧气的极高亲和力。
