Phosphorylation of Leghemoglobin at S45 is Most Effective to Disrupt the Molecular Environment of Its Oxygen Binding Pocket

In leguminous plants, nitrogenase that catalyzes anaerobic symbiotic nitrogen fixation is protected by the sequestration of O2 by Leghemoglobin (LegH). The modulation of the oxygen binding capacity of Hemoglobin (Hb) by different post-translational modifications is well studied; whereas similar studies on LegH’s O2 binding are not yet benchmarked. Our results show that in vitro serine phosphorylation of recombinant LegH from Lotus japonicus, a model legume, by a homologous kinase caused a reduction in its oxygen consumption as determined by Clark type electrode. Although mass spectrometry revealed a few phosphorylated serine residues in the LegH, molecular modeling study showed that particularly S45 is the most critical one, along with S55, however the latter with lesser impact on its molecular environment responsible for oxygen consumption. Separate S45D and S55D mutants of recombinant LegH also corroborated the results obtained from molecular modeling study. Thus, this work lays groundwork for further investigation of structural and functional role of serine phosphorylation as one of the mechanisms by which oxygen consumption by LegH may possibly be regulated during nodulation.