Kinetics and mechanistic studies of the reactions of metleghemoglobin, ferrylleghemoglobin, and nitrosylleghemoglobin with reactive nitrogen species
Herold, Susanna ; Puppo, Alain
In: JBIC Journal of Biological Inorganic Chemistry, 2005, vol. 10, no. 8, p. 946-957
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- It is now established that nitrogen monoxide is produced not only in animals but also in plants. However, much less is known about the pathways of generation and the functions of $${\text{NO}}^ \bullet $$ in planta. One of the possible targets of $${\text{NO}}^ \bullet $$ is leghemoglobin (Lb), the hemoprotein found in high concentrations in the root nodules of legumes that establish a symbiosis with nitrogen-fixing bacteria. In analogy to hemoglobin and myoglobin, we have shown that different forms of Lb react not only with $${\text{NO}}^ \bullet $$ , but also with so-called reactive nitrogen species derived from it, among others peroxynitrite and nitrite. Because of the wider active-site pocket, the rate constants measured in this work for $${\text{NO}}^ \bullet $$ and for nitrite binding to metLb are 1order of magnitude larger than the corresponding values for binding of these species to metmyoglobin and methemoglobin. Moreover, we showed that reactive nitrogen species are able to react with two forms of Lb that are produced in vivo but that cannot bind oxygen: ferrylLb is reduced by $${\text{NO}}^ \bullet $$ and nitrite, and nitrosylLb is oxidized by peroxynitrite. The second-order rate constants of these reactions are on the order of 102, 106, and 105M−1s−1, respectively. In all cases, the final reaction product is metLb, a further Lb form that has been detected in vivo. Since a specific reductase is active in nodules, which reduces metLb, reactive nitrogen species could contribute to the recycling of these inactive forms to regenerate deoxyLb, the oxygen-binding form of Lb