In support of Nitric Oxide Dioxygenase function: Algal Hemoglobins and their Reduction partners
The ubiquity of hemoglobins as a superfamily to life has enthused the field with renewed vigor. Reactions like oxygen binding and nitric oxide (NO) dioxygenation appear to be characteristic to the hemoglobin superfamily, as revealed from investigation of recombinant globins, irrespective of whether they are associated to any particular function like oxygen transport/storage, sensing, electron transport, protection against hypoxia and other possibilities. NO dioxygenase reaction, common in vitro, however, was limited by lack of report of specific enzymes that can convert ferric hemoglobin, formed during reaction of oxy hemoglobin with NO, into ferrous hemoglobin – the species that reacts with NO. Absence of a known cognate reductase would prevent reduction of ferric species of hemoglobin to ferrous form and the oxidation-reduction cycle would be incomplete for NO related function to be fruitful. Assignment of NO dioxygenase activity as a physiological function requires the design of experiments that address reduction mechanisms. We used Chlamydomonas reinhardtii as model system since we have identified 12 globins and 3 putative genes that can potentially function as reductase of ferric hemoglobin. Organism database annotated these reductases as dihydrolipoamide dehydrogenase, cytochrome b5 reductase and monodehydroascorbate reductase. So far, we have characterized 3 hemoglobins and 3 putative cognate reductases using biochemical and biophysical methods. Spectroscopic studies reveal that Chlamydomonas contains both pentacoordinate and hexacoordinate hemoglobins. The enzymes were found to contain flavin domain and could reduce ferric Chlamydomonas hemoglobins in vitro to their functional ferrous state. The interactions between hemoglobins and these reductases might support NO scavenging/detoxification function of globins with potential implications in biotechnology.