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 O-2 binding (similar to 250 mu M-1 s(-1)), moderately high rates of O-2 dissociation (similar to 5-15 s(-1)), and high oxygen affinity (K-d or P-50 approximate to 50 nM). These properties both facilitate O-2 diffusion to respiring N-2 fixing bacteria and reduce O-2 tension in the root nodules of legumes. The Class 1 plant Hbs show weak hexacoordination (K-HisE7 approximate to 2), moderate rates of O-2 binding (similar to 25 mu M-1 s(-1)), very small rates of O-2 dissociation (similar to 0.16 s(-1)), and remarkably high O-2 affinities (P-50 approximate to 2 nM), suggesting a function involving O-2 and nitric oxide (NO) scavenging. The Class 2 Hbs exhibit strong hexacoordination (K-HisE7 approximate to 100), low rates of O-2 binding (similar to 1 mu M-1 s(-1)), moderately low O-2 dissociation rate constants (similar to 1 s(-1)), and moderate, Mb-like O-2 affinities (P-50 approximate to 340 nM), perhaps suggesting a sensing role for sustained low, micromolar levels of oxygen. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 1083-1096, 2009.