The enigma of the liganded hemoglobin end state: a novel quaternary structure of human carbonmonoxy hemoglobin.
The liganded hemoglobin (Hb) high-salt crystallization condition described by Max Perutz has generated three different crystals of human adult carbonmonoxy hemoglobin (COHbA). The first crystal is isomorphous with the "classical" liganded or R Hb structure. The second crystal reveals a new liganded Hb quaternary structure, RR2, that assumes an intermediate conformation between the R form and another liganded Hb quaternary structure, R2, which was discovered more than a decade ago. Like the R2 structure, the diagnostic R state hydrogen bond between beta2His97 and alpha1Thr38 is missing in the RR2 structure. The third crystal adopts a novel liganded Hb conformation, which we have termed R3, and it shows substantial quaternary structural differences from the R, RR2, and R2 structures. The quaternary structure differences between T and R3 are as large as those between T and R2; however, the T --> R3 and T --> R2 transitions are in different directions as defined by rigid-body screw rotation. Moreover, R3 represents an end state. Compared to all known liganded Hb structures, R3 shows remarkably reduced strain at the alpha-heme, reduced steric contact between the beta-heme ligand and the distal residues, smaller alpha- and beta-clefts, and reduced alpha1-alpha2 and beta1-beta2 iron-iron distances. Together, these unique structural features in R3 should make it the most relaxed and/or greatly enhance its affinity for oxygen compared to the other liganded Hbs. The current Hb structure-function relationships that are now based on T --> R, T -->R --> R2, or T --> R2 --> R transitions may have to be reexamined to take into account the RR2 and R3 liganded structures.
Safo
M K
MK
School of Pharmacy, Department of Medicinal Chemistry, and Institute for Structural Biology and Drug Discovery, Virginia Commonwealth University, Richmond, Virginia 23219, USA. msafo@mail2.vcu.edu
Abraham
D J
DJ
D000328:Adult; D000494:Allosteric Regulation / Q000502:physiology; D002263:Carboxyhemoglobin / Q000737:chemistry* / Q000502:physiology; D003460:Crystallization; D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry* / Q000502:physiology; D006801:Humans; D007501:Iron / Q000737:chemistry; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D020836:Protein Structure, Quaternary
D006454:Hemoglobins; D008024:Ligands; D006418:Heme; D002263:Carboxyhemoglobin; D007501:Iron