CRYSTAL-STRUCTURE OF ASIAN ELEPHANT (ELEPHAS-MAXIMUS) CYANO-METMYOGLOBIN AT 1.78-ANGSTROM RESOLUTION - PHE(29)(B10) ACCOUNTS FOR ITS UNUSUAL LIGAND-BINDING PROPERTIES

The crystal structure of Asian elephant cyano-metmyoglobin which has a glutamine instead of the usual distal site histidine has been determined to high resolution. In addition to this replacement, the substitution of a conserved leucine residue in position 29(B10) at the distal side by a phenylalanine was unambiguously identified based on the available electron density. The suspicion, that there were errors in the original sequence which has caused some confusion, is thus confirmed. Comparison with other myoglobin structures in various ligated forms reveals an essentially unchanged tertiary structure in elephant myoglobin despite the two amino acid substitutions in the heme pocket. Our current structural model shows that the N epsilon 2 atom of Gln(64)(E7) has moved with respect to the corresponding nitrogen position of His(64)(E7) in the CO complex of sperm whale myoglobin. The newly assigned residue Phe(29)(B10) penetrates into the distal side of the heme pocket approaching the ligand within van der Waals distance and causing a much more crowded heme pocket compared to other myoglobins. Kinetic properties of Asian elephant myoglobin, wild type, and recombinant sperm whale myoglobins are discussed in relation to the structural consequences of the two amino acid substitutions H64Q and L29F.