1.70 ANGSTROM RESOLUTION STRUCTURE OF MYOGLOBIN FROM YELLOWFIN TUNA -AN EXAMPLE OF A MYOGLOBIN LACKING THE D-HELIX

The crystal structure of metmyoglobin from yellowfin tuna (Thunnus alb acares) has been determined by molecular replacement methods and refin ed to a conventional R factor of 0.177 for all observed reflections in the range of 6.0-1.70 angstrom resolution. Like other myoglobins for which a high-resolution structure is available, the polypeptide chain is organized into several helices that cooperate to form a hydrophobic pocket into which the heme prosthetic group is non-covalently bound; however, the D helix observed in other myoglobins is absent in myoglob in from yellowfin tuna and has been replaced with a random coil. As we ll, the A helix has a pronounced kink due to the presence of Pro16. Th e differences in structure between this and sperm whale myoglobin can be correlated with their reported dioxygen affinity and dissociation. The structure is in agreement with reported fluorescence data which sh ow an increased Trp14...heme distance in yellowfin tuna compared to sp erm whale myoglobin.