Comparative kinetic analysis of anaerobic nitrite metabolism in phytoglobins
During the course of a plant’s life cycle, there are times when oxygen is a finite resource such as in rapidly growing and metabolizing tissue, in flooding, or in waterlogged root systems. When this occurs the plant develops alternative means of respiration for survival in order to cope with this hypoxic stress. The hypoxic plant cell will use nitrate and nitrite as alternative terminal electron acceptors. Increasing levels of nitrite during hypoxia are connected to higher NO levels within plants. However, in plants overexpressing Hbs there is decreased NO emission. Previous studies have confirmed that the Class 1 phytoglobin, Rice nonsymbiotic hemoglobin (nsHb) 1, could convert nitrite to NO (1). Earlier experiments have also shown the correlation between oxygen affinity and phytoglobin class (2), as well as the ability of hemoglobins to perform the NO dioxygenase reaction in hypoxic environments (3, 4). In fact, plants over-expressing class 1 phytoglobins during hypoxia released less NO (3), and had higher metabolic activity (5) as compared to WT plants. Taken all together, the nitrite reduction reaction presented a promising connection between NO detoxification and maintaining redox balance within the plant cell. This thesis project set out to investigate the relationship, if any, between phytoglobin class and nitrite reduction to NO. To further understand and delineate the functions of the distinct classes of phytoglobins, a comparative kinetic analysis of nitrite reduction across classes was performed. Overall, the capacity of phytoglobins to reduce nitrite to NO appears to cluster according to phytoglobin class, with class 1 being consistently high performers as compared to animal hemoglobins, and the recently evolved symbiotic and leghemoglobin classes being the least efficient at nitrite reduction.