Article

Structural and functional properties of class 1 plant hemoglobins.

Department of Biology, Memorial University of Newfoundland, St. John's, NL, Canada.
International Union of Biochemistry and Molecular Biology Life (Impact Factor: 2.79). 03/2011; 63(3):146-52. DOI: 10.1002/iub.439
Source: PubMed

ABSTRACT 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.

0 Bookmarks
 · 
119 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Lingonberry (Vaccinium vitis-idaea L. ssp. vitis-idaea Britton) cultivars Regal, Splendor, and Erntedank were obtained by conventional softwood cuttings (taken as a control), by in vitro shoot proliferation of node explants, and by adventitious shoot regeneration from excised leaves of micropropagated shoots. In the plants propagated in vitro, the total ascorbate content increased and its pool was more oxidized, the total glutathione content also increased but its pool became more reduced. The leaves of plants obtained from the in vitro culture showed significantly higher antioxidant enzyme activities except for dehydroascorbate reductase which was at a similar level in all plants. Total soluble phenolics, tannins, and flavonoids were enhanced in fruits of in vitro-propagated plants whereas in leaves, the levels of these metabolites (except flavonoids) were higher in ex vitro derived plants. The total radical scavenging capacity was enhanced in berries of the in vitro propagated plants. It is suggested that the active morphogenetic process, characterized by intensive formation and scavenging reactive oxygen species is reflected in the activities of antioxidant enzymes and metabolites. The reduction potential of glutathione is the most important parameter which determines patterns of growth and differentiation in the investigated plants.
    Biologia Plantarum 12/2013; · 1.69 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nitric oxide (NO) is emerging as an important regulatory player in the Rhizobium-legume symbiosis. The occurrence of NO during several steps of the symbiotic interaction suggests an important, but yet unknown, signaling role of this molecule for root nodule formation and functioning. The identification of the molecular targets of NO is key for the assembly of the signal transduction cascade that will ultimately help to unravel NO function. We have recently shown that the key nitrogen assimilatory enzyme glutamine synthetase (GS) is a molecular target of NO in root nodules of Medicago truncatula, being post-translationally regulated by tyrosine nitration in relation to nitrogen fixation. In functional nodules of M. truncatula NO formation has been located in the bacteroid containing cells of the fixation zone, where the ammonium generated by bacterial nitrogenase is released to the plant cytosol and assimilated into the organic pools by plant GS. We propose that the NO-mediated GS post-translational inactivation is connected to nitrogenase inhibition induced by NO and is related to metabolite channeling to boost the nodule antioxidant defenses. Glutamate, a substrate for GS activity is also the precursor for the synthesis of glutathione (GSH), which is highly abundant in root nodules of several plant species and known to play a major role in the antioxidant defense participating in the ascorbate/GSH cycle. Existing evidence suggests that upon NO-mediated GS inhibition, glutamate could be channeled for the synthesis of GSH. According to this hypothesis, GS would be involved in the NO-signaling responses in root nodules and the NO-signaling events would meet the nodule metabolic pathways to provide an adaptive response to the inhibition of symbiotic nitrogen fixation by reactive nitrogen species.
    Frontiers in Plant Science 01/2013; 4:372. · 3.60 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Significance: During the Legume - Rhizobium symbiosis, hydrogen peroxide (H2O2) and nitric oxide (NO) appear to play an important signalling role in the establishment and the functioning of this interaction. Modifications of the levels of these reactive species in both partners impair either the development of the nodules (new root organs formed upon the interaction), or their N2-fixing activity. Recent Advances: NADPH oxidases have been recently described as major sources of H2O2 production, via superoxide dismutation, during symbiosis. Nitrate reductases and electron transfer chains from both partners were found to significantly contribute to NO production in N2-fixing nodules. Both S-sulfenylated and S-nitrosylated proteins have been detected during early interaction and in functioning nodules, linking ROS/NO production to redox-based protein regulation. NO was also found to play a metabolic role in nodule energy metabolism. Critical Issues: H2O2 may control the infection process and the subsequent bacterial differentiation into the symbiotic form. NO is required for an optimal establishment of symbiosis and appears to be a key player in nodule senescence. Future Directions: A challenging question is to define more precisely when and where reactive species are generated and to develop adapted tools to detect their production in vivo. To investigate the role of NADPH oxidases and nitrate reductases in the production of H2O2 and NO, respectively, the use of mutants under the control of organ-specific promoters will be of crucial interest. The balance between ROS and NO production appears to be a key point to understand the redox regulation of symbiosis.
    Antioxidants & Redox Signaling 12/2012; · 8.20 Impact Factor

Full-text

View
82 Downloads
Available from
May 16, 2014