Leghemoglobins

Leghemoglobins are plant hemoglobins produced in the nitrogen-fixing root nodules of legume plants. They are small heme proteins, typically about 14–17 kDa, encoded by leghemoglobin genes and consisting of a single polypeptide around 140–170 amino acids in length. Like other globins, they contain a heme iron center and share structural features with non-symbiotic plant and animal hemoglobins, including a proximal histidine coordinating the heme iron.

The principal biological role of leghemoglobins is to regulate oxygen availability within the nodule. Nitrogenase, the enzyme that fixes atmospheric nitrogen, is extremely oxygen sensitive, so leghemoglobins help maintain a low free O2 concentration while delivering sufficient oxygen to the bacteroids for respiration. They bind oxygen with high affinity, yielding a very low p50 (in the low torr range under physiological conditions), thereby buffering oxygen levels and supporting efficient nitrogen fixation.

Leghemoglobins are expressed at high levels specifically in nodules and are among the most abundant nodular proteins. They are produced in response to nodule development and the establishment of the legume–rhizobium symbiosis. Structurally, leghemoglobins resemble other plant and animal globins and can display hexacoordinate deoxy forms, becoming pentacoordinate upon O2 binding, which contributes to their high oxygen affinity.

Their study has provided key insights into how plants manage oxygen in symbiotic nodules and has informed understanding of plant–microbe interactions and the evolution of legume nitrogen fixation.