Rice ( Oryza) hemoglobins

Peroxidase activity of rice (Oryza sativa) hemoglobin: distinct role of tyrosines 112 and 151

Five non-symbiotic hemoglobins (nsHb) have been identified in rice (Oryza sativa). Previous studies have shown that stress conditions can induce their overexpression, but the role of those globins is still unclear. To better understand the functions of nsHb, the reactivity of rice Hb1 toward hydrogen peroxide (H2O2) has been studied in vitro. Our results show that recombinant rice Hb1 dimerizes through dityrosine cross-links in the presence of H2O2. By site-directed mutagenesis, we suggest that tyrosine 112 located in the FG loop is involved in this dimerization. Interestingly, this residue is not conserved in the sequence of the five rice non-symbiotic hemoglobins. Stopped-flow spectrophotometric experiments have been performed to measure the catalytic constants of rice Hb and its variants using the oxidation of guaiacol. We have shown that Tyrosine112 is a residue that enhances the peroxidase activity of rice Hb1, since its replacement by an alananine leads to a decrease of guaiacol oxidation. In contrast, tyrosine 151, a conserved residue which is buried inside the heme pocket, reduces the protein reactivity. Indeed, the variant Tyr151Ala exhibits a higher peroxidase activity than the wild type. Interestingly, this residue affects the heme coordination and the replacement of the tyrosine by an alanine leads to the loss of the distal ligand. Therefore, even if the amino acid at position 151 does not participate to the formation of the dimer, this residue modulates the peroxidase activity and plays a role in the hexacoordinated state of the heme.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Protection of root apex meristem during stress responses

By regulating the levels of nitric oxide (NO) in a cell and tissue specific fashion, Phytoglobins (Pgbs), plant hemoglobin-like proteins, interfere with many NO-mediated pathways participating in developmental and stress-related responses. Recent evidence reveals that one of the functions of Pgbs is to protect the root apical meristem (RAM) from stress conditions by retaining the viability and function of the quiescent center (QC), required to maintain the stem cells in an undifferentiated state and ensure proper tissue patterning and root viability. Based on this and other evidence, it is suggested that Pgbs regulate cell fate by modulating NO homeostasis.

Effect of the synthesis of rice non-symbiotic hemoglobins 1 and 2 in the recombinant Escherichia coli TB1 growth

Non-symbiotic hemoglobins (nsHbs) are widely distributed in land plants, including rice. These proteins are classified into type 1 (nsHbs-1) and type 2. The O2-affinity of nsHbs-1 is very high mostly because of an extremely low O2-dissociation rate constant resulting in that nsHbs-1 apparently do not release O2after oxygenation. Thus, it is possible that theinvivofunction of nsHbs-1 is other than O2-transport. Based on the properties of multiple Hbs it was proposed that nsHbs-1 could play diverse roles in rice organs, however theinvivoactivity of rice nsHbs-1 has been poorly analyzed. Aninvivoanalysis for rice nsHbs-1 is essential to elucidate the biological function(s) of these proteins. Rice Hb1 and Hb2 are nsHbs-1 that have been generated in recombinantEscherichiacoliTB1. The rice Hb1 and Hb2 amino acid sequence, tertiary structure and rate and equilibrium constants for the reaction of O2are highly similar. Thus, it is possible that rice Hb1 and Hb2 function similarlyinvivo. As an initial approach to test this hypothesis we analyzed the effect of the synthesis of rice Hb1 and Hb2 in the recombinantE.coliTB1 growth. Effect of the synthesis of the O2-carrying soybean leghemoglobina, cowpea leghemoglobin II andVitreoscillaHb in the recombinantE.coliTB1 growth was also analyzed as an O2-carrier control. Our results showed that synthesis of rice Hb1, rice Hb2, soybean Lba, cowpea LbII andVitreoscillaHb inhibits the recombinantE.coliTB1 growth and that growth inhibition was stronger when recombinantE.coliTB1 synthesized rice Hb2 than when synthesized rice Hb1. These results suggested that rice Hb1 and Hb2 could function differentlyin vivo.