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

## TL;DR

This study explores the oxygen-binding properties of hemoglobins in early land plants and finds they are structurally and functionally distinct from transport hemoglobins.

## Contribution

The paper reveals a paradox in bryophyte hemoglobins: they have fast oxygen kinetics but structural features that hinder transport.

## Key findings

- Bryophyte hemoglobins form a distinct monophyletic group, equally distant from class 1 and class 2 plant hemoglobins.
- These hemoglobins are predominantly hexacoordinate in the ferrous deoxy state, which limits oxygen transport efficiency.
- Despite structural limitations, their oxygen binding and dissociation rates resemble those of leghemoglobins, suggesting rapid oxygen exchange.

## Abstract

Bryophytes, comprising mosses (Bryophyta), hornworts (Anthocerotophyta), and
liverworts (Marchantiophyta), 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.