Nonsymbiotic hemoglobins are broadly present across evolution; however, the function of these proteins is unknown. Cultured maize cells have been transformed to constitutively express a barley
hemoglobin gene in either the sense (HB+) or antisense (HB−) orientation.
Hemoglobin protein in the
transformed cell lines was correspondingly higher or lower than in
wild type cells under normal atmospheric conditions. Limiting
oxygen availability, by placing the cells in a
nitrogen atmosphere for 12 hours, had little effect on the energy status of cells constitutively expressing
hemoglobin, but had a pronounced effect on both
wild type and HB− cells, where ATP levels declined by 27% and 61% respectively.
Energy charge was relatively unaffected by the treatment in HB+ and
wild type cells, but was reduced from 0.91 to 0.73 in HB− cells suggesting that the latter were incapable of maintaining their energy status under the
low oxygen regime. Similar results were observed with P. aeruginosa cells transformed with an Hb
expression vector. It is suggested that nonsymbiotic hemoglobins act to maintain the energy status of cells in
low oxygen environments and that they accomplish this effect by promoting glycolytic flux through NADH oxidation, resulting in increased substrate level
phosphorylation. Nonsymbiotic hemoglobins are likely ancestors of an early form of
hemoglobin that sequestered
oxygen in
low oxygen environments, providing a source of
oxygen to oxidize NADH to provide ATP for
cell growth and development. This in turn suggests that cells containing increased levels of Hb
protein will survive longer under low
oxygen tension or
high energy demand.