Review: Correlations between oxygen affinity and sequence classifications of plant hemoglobins

Plants express three phylogenetic classes of hemoglobins (Hb) based on sequence analyses. Class 1 and 2 Hbs are full-length globins with the classical eight helix Mb-like fold, whereas Class 3 plant Hbs resemble the truncated globins found in bacteria. With the exception of the specialized leghemoglobins, the physiological functions of these plant hemoglobins remain unknown. We have reviewed and, in some cases, measured new oxygen binding properties of a large number of Class 1 and 2 plant nonsymbiotic Hbs and leghemoglobins. We found that sequence classification correlates with distinct extents of hexacoordination with the distal histidine and markedly different overall oxygen affinities and association and dissociation rate constants. These results suggest strong selective pressure for the evolution of distinct physiological functions. The leghemoglobins evolved from the Class 2 globins and show no hexacoordination, very high rates of O2 binding (approximately 250 micromolar-1 s-1), moderately high rates of O2 dissociation (approximately 5-15 s-1), and high oxygen affinity (Kd or P50 approximately equal to 50 nM). These properties both facilitate O2 diffusion to respiring N2 fixing bacteria and reduce O2 tension in the root nodules of legumes. The Class 1 plant Hbs show weak hexacoordination (KHisE7 approximately equal to 2), moderate rates of O2 binding (approximately25 micromolar-1 s-1), very small rates of O2 dissociation (approximately 0.16 s-1), and remarkably high O2 affinities (P50 approximately equal to 2 nM), suggesting a function involving O2 and nitric oxide (NO) scavenging. The Class 2 Hbs exhibit strong hexacoordination (KHisE7 approximately equal to 100), low rates of O2 binding (approximately 1 micromolar-1 s-1), moderately low O2 dissociation rate constants (approximately 1 s-1), and moderate, Mb-like O2 affinities (P50 approximately equal to 340 nM), perhaps suggesting a sensing role for sustained low, micromolar levels of oxygen.