REPTILE HEME PROTEIN-STRUCTURE - X-RAY CRYSTALLOGRAPHIC STUDY OF THE AQUO-MET AND CYANO-MET DERIVATIVES OF THE LOGGERHEAD SEA-TURTLE (CARETTA-CARETTA) MYOGLOBIN AT 2.0 ANGSTROM RESOLUTION

The X-ray crystal structures of the aquo-met and cyano-met derivatives of the loggerhead sea turtle (Caretta caretta) myoglobin have been de termined at 2.0 Angstrom resolution (R = 0.182, and 0.178, respectivel y). The results here reported, representing the first reptile globin s olved by X-ray crystallography, have been analyzed in parallel with da ta for related monomeric hemoproteins, and indicate a strong overall s tructural similarity between the loggerhead sea turtle and mammalian m yoglobins, reflected by the 63% amino acid identity of their primary s tructures. The root-mean-square deviation between the entire polypepti de backbones of loggerhead sea turtle and sperm whale myoglobins, afte r structure superposition, is 0.83 Angstrom. Upon cyanide binding to t he protein distal site, the iron-bound water molecule present in the a quo-met form is displaced by the incoming ligand. Cyanide is oriented towards the inner part of the heme distal site forming a Fe-C-N angle of 133 degrees.