Oxygen Binding Kinetics and Coordination States of Hemoglobins from Early Land Plants

Bryophytes, comprising mosses (<i>Bryophyta</i>), hornworts (<i>Anthocerotophyta</i>), and liverworts (<i>Marchantiophyta</i>), represent early diverging lineages in land plant evolution. Each of these phyla contains hemoglobin genes whose functional properties remain largely unexplored. Here, we report phylogenetic analysis that confirms that bryophyte globins form a distinct monophyletic group equally distant from both class 1 and class 2 nonsymbiotic hemoglobins of vascular plants. Spectroscopic characterization revealed that all three representative bryophyte hemoglobins exhibit predominant hexacoordination in the ferrous deoxy state. This predominantly hexacoordinate structure and the auto-oxidation rates, which are similar to other nonsymbiotic hemoglobins, are inconsistent with efficient oxygen transport function. However, kinetic analysis revealed a striking paradox: oxygen binding and dissociation rates closely resembling those of oxygen-transporting leghemoglobins. These findings reveal that bryophyte globins possess the kinetic capacity for rapid oxygen exchange but have structural features, namely, a hexacoordinate heme prosthetic group, that preclude efficient oxygen transport. This suggests that hemoglobins from early land plants served alternative physiological functions and that the inherent oxygen-binding capabilities of the globin fold were later optimized for transport in vascular plant lineages.