The crystal structure of metmyoglobin from yellowfin tuna (Thunnus albacares) has been determined by molecular replacement methods and refined to a conventional R factor of 0.177 for all observed reflections in the range of 6.0-1.70 A 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 myoglobin from yellowfin tuna and has been replaced with a random coil. As well, the A helix has a pronounced kink due to the presence of Pro16. The 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 show an increased Trp14.heme distance in yellowfin tuna compared to sperm whale myoglobin.
MIROSLAW CYCLER, MARIA PRZYBYLSKA, and RICHARD MACLEOD ELOFSON. Can. J . Chem. 60, 2852Chem. 60, (1982. Benzenediazonium tetrafluoroborate, C6HsN2+.BF,-, crystallizes in space group P2, In with unit cell dimensions n = 17.347(2), b = 8.396(1), c = 5.685(1) A, P = 92.14(1)", Z = 4. The structure was solved by direct phasing methods using the program SHELX 76.The parameters were refined by full-matrix least-squares to a final R = 0.063 for 1346 observed reflections. The bond lengths and angles agree very well with those of Rplmming for benzenediazonium chloride. The C-N and N=N bond lengths are 1.415(3) and 1.083(3) A, respectively, and the bonds of the benzene ring do not show any significant differences as they vary from 1.371 (5)
The molecular structure of ryanodol-p-bromo benzyl ether was solved without any chemical assumptions, using three dimensional data obtained by the heavy atom technique. It was found to be identical with the structure proposed by Dr. K. Wiesner, except that the configuration was reversed at one carbon atom. The refinement of atomic parameters with a least-squares method is still in progress. The R factor is at present 0.12.
The atomic structure of an antibody antigen-binding fragment (Fab) at 2. 45 A resolution shows that polysaccharide antigen conformation and Fab structure dictated by combinatorial diversity and domain association are responsible for the fine specificity of the Brucella-specific antibody, YsT9.1. It discriminates the Brucella abortus A antigen from the nearly identical Brucella melitensis M antigen by forming a groove-type binding site, lined with tyrosine residues, that accommodates the rodlike A antigen but excludes the kinked structure of the M antigen, as envisioned by a model of the antigen built into the combining site. The variable-heavy (V,) and variabielight (V,) domains are derived from genes closely related to two used in previously solved structures, M603 and R19.9, respectively. These genes combine in YsT9.1 to form an antibody of totally different specificity. Comparison of this X-ray structure with a previously built model of the YsT9.1 combining site based on these homologies highlights the importance of V,:V, association as a determinant of specificity and suggests that small changes at the VL:VH interface, unanticipated in modeling, may cause significant modulation of binding-site properties.
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