F.R. Salemme

Competitor analogs for defined T cell antigens: Peptides incorporating a putative binding motif and polyproline or polyglycine spacers

We describe a new approach for modeling antigenic peptides recognized by T cells. Peptide A24 170-182 can compete with other antigenic peptides that are recognized by H-2kd-restricted cytolytic T cells, presumably by binding to the Kd molecule. By comparing substituted A24 peptides as competitors in a functional competition assay, the A24 residues Tyr-171, Thr-178, and Leu-179 were identified as possible contact residues for Kd. A highly active competitor peptide analog was synthesized in which Tyr was separated from the Thr-Leu pair by a pentaproline spacer. The choice of proline allowed the prediction of a probable conformation for the analog when bound to the Kd molecule. The simplest conformation of the A24 peptide that allows the same spacing and orientation of the motif as in the analog would be a nearly extended polypeptide chain incorporating a single 3(10) helical turn or similar structural kink.

An independent crystallographic refinement of porcine phospholipase A2 at 2.4 A resolution.

Porcine phospholipase A2 (Mr = 13,980), trigonal, P3(1)21, a = b = 69.4, c = 70.4 A, one molecule per asymmetric unit, lambda (Cu K alpha) = 1.54 A. Model incorporating 975 protein atoms and eight solvent molecules refined by restrained least-squares fit to a residual R = 0.21 for 6382 reflections from 5 to 2.4 A resolution.

Crystallographic refinement of bovine pro-phospholipase A2 at 1.6 Å resolution

Bovine pro-phospholipase A2 (Mr = 14,520), trigonal, P3(1)21, a = b = 46.5, c = 102.0 A, one molecule per asymmetric unit, lambda (Cu K alpha) = 1.54 A. The model incorporating 895 protein atoms, two molecules of 2-methyl-2,4-pentanediol, and 60 solvent water molecules, was refined by restrained least squares to a residual R = 0.194 for 14,667 reflections from 5 to 1.6 A resolution.

Cooperative ligand reorientations in cytochrome c3: a molecular dynamics simulation

Molecular dynamics simulations of a tetraheme cytochrome c3 were performed to investigate dynamic aspects of the motion of the axial heme iron ligands. It was found that persistent transitions between alternate axial imidazole orientations of the histidine incorporated in the CXXCH heme binding sequence occurred via correlated motions. The correlated motions involved virtually all of the atoms comprising the polypeptide backbone of the heme binding sequence as well as the histidine imidazole side-chain.

Molecular Switching Transitions In Biological Macromolecules

Coupled conformational transitions potentially provide the basis for controlled switching processes in biological macro-molecules. The present work examines results of molecular dynamics simulations of heme proteins. Of particular interest are conformational transitions in aromatic amino acid side-chains or heme iron ligands that may electronically couple or alter the physical state of the heme prosthetic groups.