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 determi ned to high resolution. In addition to this replacement, the substitut ion of a conserved leucine residue in position 29(B10) at the distal s ide by a phenylalanine was unambiguously identified based on the avail able electron density. The suspicion, that there were errors in the or iginal sequence which has caused some confusion, is thus confirmed. Co mparison with other myoglobin structures in various ligated forms reve als an essentially unchanged tertiary structure in elephant myoglobin despite the two amino acid substitutions in the heme pocket. Our curre nt 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 res idue Phe(29)(B10) penetrates into the distal side of the heme pocket a pproaching the ligand within van der Waals distance and causing a much more crowded heme pocket compared to other myoglobins. Kinetic proper ties of Asian elephant myoglobin, wild type, and recombinant sperm wha le myoglobins are discussed in relation to the structural consequences of the two amino acid substitutions H64Q and L29F.