Non-symbiotic plant hemoglobins (nsHbs) have been found in various plant tissues and plant species, especially in crop plants. Different classes of nsHbs have been identified and divided into class-1 (Hb1) and class-2 (Hb2) based on phylogenetic characteristics, gene expression pattern and oxygen binding properties. The extremely high affinity of Hb1 for oxygen makes it unlikely that this protein acts as an oxygen sensor, oxygen carrier, oxygen storage or electron transport molecule. Expression of Hb1 is very low under normal conditions, but is strongly induced by hypoxic stress. Studies using Arabidopsis, maize and alfalfa lines over-expressing Hb1 indicate a role of class-1 Hbs in scavenging nitric oxide (NO) that is produced under severe hypoxia. Relatively little is known about the function of Hb2 in plants. In Arabidopsis, AHb2 expression is resilient to hypoxic stress, but shows induction in response to low temperatures indicating a possible yet undefined role under cold-stress. AHb2 is also induced in response to the plant hormone cytokinin. The underlying mechanisms of AHb2-induced responses are still unresolved, and may differ from AHb1. Since the oxygen binding characteristics of AHb2 are comparable to leghemoglobin, a specific function of AHb2 in facilitating oxygen diffusion cannot be excluded. It has recently been reported that deoxy AHb1 and AHb2 reduce nitrite to form NO via a mechanism analogous to that observed for hemoglobin, myoglobin and neuroglobin. To test the hypothesis that a change in the equilibrium between the six- and five-coordinate heme mediates the control of the nitrite reduction rate, we have generated distal pocket mutants of the two AHbs and the resulting proteins have been examined for nitrite reductase activity, nitrite affinity and spectroscopic features. Our data indicate that nitrite reductase activity is not entirely determined by heme coordination, but also by a unique distal heme pocket in each AHb.
Residues in the distal heme pocket of Arabidopsis non-symbiotic hemoglobins:Implication for nitrite reductase activity
KUMAR, Nitin;ASTEGNO, Alessandra;Paola Dominici
2012-01-01
Abstract
Non-symbiotic plant hemoglobins (nsHbs) have been found in various plant tissues and plant species, especially in crop plants. Different classes of nsHbs have been identified and divided into class-1 (Hb1) and class-2 (Hb2) based on phylogenetic characteristics, gene expression pattern and oxygen binding properties. The extremely high affinity of Hb1 for oxygen makes it unlikely that this protein acts as an oxygen sensor, oxygen carrier, oxygen storage or electron transport molecule. Expression of Hb1 is very low under normal conditions, but is strongly induced by hypoxic stress. Studies using Arabidopsis, maize and alfalfa lines over-expressing Hb1 indicate a role of class-1 Hbs in scavenging nitric oxide (NO) that is produced under severe hypoxia. Relatively little is known about the function of Hb2 in plants. In Arabidopsis, AHb2 expression is resilient to hypoxic stress, but shows induction in response to low temperatures indicating a possible yet undefined role under cold-stress. AHb2 is also induced in response to the plant hormone cytokinin. The underlying mechanisms of AHb2-induced responses are still unresolved, and may differ from AHb1. Since the oxygen binding characteristics of AHb2 are comparable to leghemoglobin, a specific function of AHb2 in facilitating oxygen diffusion cannot be excluded. It has recently been reported that deoxy AHb1 and AHb2 reduce nitrite to form NO via a mechanism analogous to that observed for hemoglobin, myoglobin and neuroglobin. To test the hypothesis that a change in the equilibrium between the six- and five-coordinate heme mediates the control of the nitrite reduction rate, we have generated distal pocket mutants of the two AHbs and the resulting proteins have been examined for nitrite reductase activity, nitrite affinity and spectroscopic features. Our data indicate that nitrite reductase activity is not entirely determined by heme coordination, but also by a unique distal heme pocket in each AHb.
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