HIGH RESOLUTION STRUCTURES OF LIGANDED CANINE AND FELINE HEMOGLOBIN REVEAL AN ALTERNATIVE QUATERNARY CONFORMATION. Victoria L. Robinson, Timothy C. Mueser, Paul H. Rogers, Arthur Arnone, Dept. of Biochemistry, Univ. of Iowa, Iowa City, IA 52242

In addition to the well known T quaternary structure of deoxyhemoglobin and R quaternary structure of liganded hemoglobin, it was recently shown that liganded human hemoglobin can exist in yet another quaternary structure, the R2 structure (Silva et al. (1992) J. Biol. Chem. 267, 17248). In higher vertebrate hemoglobins, the residues that have the greatest influence on the quaternary structure of the [[alpha]]₂[[beta]]₂ hemoglobin tetramer - the residues at the dynamic [[alpha]]₁[[beta]]₂ interface - are almost completely conserved. Thus a quaternary structure observed by crystallographic methods for one vertebrate hemoglobin should be accessible to all other vertebrate hemoglobins with the same [[alpha]]₁[[beta]]₂ interface residues. It may be possible, therefore, to crystallize each quaternary structure from more than one vertebrate hemoglobin, and in this way determine the full range of quaternary structures that are energetically accessible to the hemoglobin tetramer.

High quality (2.0Å resolution) crystals of canine and feline hemoglobin were grown under low-salt conditions at pH 8.5 and 5.8, respectively. The subunit tertiary structures of these two hemoglobins are essentially identical to those previously observed in all other liganded hemoglobin structures, and the quaternary structure of liganded dog hemoglobin exactly matches the R structure previously observed in human and equine hemoglobin. However, the quaternary structure of liganded feline hemoglobin is neither R-like nor R2-like in nature but is midway between the two. Moreover, this quaternary structure of liganded feline hemoglobin is identical to one found in liganded bovine hemoglobin (Mueser et al., in preparation). Analysis of all the hemoglobin structures determined to date suggests that only one quaternary structure is energetically accessible to deoxygenated hemoglobin, whereas at least three structures are accessible to fully liganded hemoglobin.

This work was supported by NIH grant HL-51084, by a predoctoral fellowship to V. L. R. from the University of Iowa Center for Biocatalysis and Bioprocessing, and by a NIH postdoctoral fellowship to T. C. M. Present address for T. C. M.: NIH/NIAMS, Bldg. 6, 9000 Rockville Pike Bethesda, MD 20892.