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 O(2) binding ( approximately 250 muM(-1) s(-1)), moderately high rates of O(2) dissociation ( approximately 5-15 s(-1)), and high oxygen affinity (K(d) or P(50) approximately 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) approximately 2), moderate rates of O(2) binding ( approximately 25 muM(-1) s(-1)), very small rates of O(2) dissociation ( approximately 0.16 s(-1)), and remarkably high O(2) affinities (P(50) approximately 2 nM), suggesting a function involving O(2) and nitric oxide (NO) scavenging. The Class 2 Hbs exhibit strong hexacoordination (K(HisE7) approximately 100), low rates of O(2) binding ( approximately 1 muM(-1) s(-1)), moderately low O(2) dissociation rate constants ( approximately 1 s(-1)), and moderate, Mb-like O(2) affinities (P(50) approximately 340 nM), perhaps suggesting a sensing role for sustained low, micromolar levels of oxygen.
Department of Biochemistry, Iowa State University, Ames, IA 50011, USA.
Benoit J
BJ
Smagghe
Julie A
JA
Hoy
Ryan
R
Percifield
Suman
S
Kundu
Mark S
MS
Hargrove
Gautam
G
Sarath
Jean-Louis
JL
Hilbert
Richard A
RA
Watts
Elizabeth S
ES
Dennis
W James
WJ
Peacock
Sylvia
S
Dewilde
Luc
L
Moens
George C
GC
Blouin
John S
JS
Olson
Cyril A
CA
Appleby
D001665:Binding Sites; D001667:Binding, Competitive; D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D007874:Leghemoglobin / Q000737:chemistry* / Q000145:classification / Q000378:metabolism; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010944:Plants / Q000737:chemistry* / Q000235:genetics; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012196:Review Literature as Topic; D017550:Spectroscopy, Fourier Transform Infrared
D007874:Leghemoglobin; D010940:Plant Proteins; D011994:Recombinant Proteins; D006418:Heme; D010100:Oxygen