Biomedical subjects
Publications and source records attributed to C E Schafmeister.
It is now possible to design small proteins capable of folding into compact structures with well-packed cores. Given the present state of knowledge of protein folding and design, it is possible to extract a set of engineering guidelines that may assist in future de novo protein design.
A 108 amino acid protein was designed and constructed from a reduced alphabet of seven amino acids. The 2.9 A resolution X-ray crystal structure confirms that the protein is a four helix bundle, as it was designed to be. Hydrogen/deuterium exchange experiments reveal buried amide protons with protection factors in excess of 1 x 10(6) in the range characteristic of well protected protons in functional folded proteins (10(3)-10(8)) rather than protons in rapid exchange (0-10(2)). The protein is monomeric at 1 mM, the concentration at which the exchange experiments were undertaken, indicating that the exchange factors are due to a unique stable tertiary structure fold, and not due to any higher order quaternary structure. Thermodynamic analysis provides an estimate of the free energy of folding of -9.3 kcal mole-1 at 25 degrees C, consistent with the free energy of folding derived from the protection factors of the most protected protons, indicating that global unfolding is required for exchange of the most protected protons.
Thymidylate synthase (TS) methylates only dUMP, not dCMP. The crystal structure of TS.dCMP shows sCMP 4-NH2 excluded from the space between Asn-229 and His-199 by the hydrogen bonding and steric properties and Asn-229. Consequently, 6-C of dCMP is over 4 A from the active site sulfhydryl. The Asn-229 side chain is prevented from flipping 180 degrees to and orientation the could hydrogen bond to dCMP by a hydrogen bond network between conserved residues. Thus, the specific binding of dUMP by TS results from occlusion of competing substrates by steric and electronic effects of residues in the active site cavity. When Asn-229 is replaced by a cysteine, the Cys-229 S gamma rotates out of the active site, and the mutant enzyme binds both dCMP and dUMP tightly but does not methylate dCMP. Thus simply admitting dCMP into the dUMP binding site of TS is not sufficient for methylation of dCMP. Structures of nucleotide complexes of TS N229D provide a reasonable explanation for the preferential methylation of dCMP instead of dUMP by this mutant. In TS N229D.dCMP, Asp-229 forms hydrogen bonds to 3-N and 40NH2 of dCMP. Neither the Asp-229 carboxyl moiety nor ordered water appears to hydrogen bond to 4-O of dUMP. Hydrogen bonds to 4-O (or 4-NH2) have been proposed to stabilize reaction intermediates. If their absence in TS N229D.dUMP persists in the ternary complex, it could explain the 10(4)-fold decrease in kcat/Km for dUMP.
A 24-amino acid peptide designed to solubilize integral membrane proteins has been synthesized. The design was for an amphipathic alpha helix with a "flat" hydrophobic surface that would interact with a transmembrane protein as a detergent. When mixed with peptide, 85 percent of bacteriorhodopsin and 60 percent of rhodopsin remained in solution over a period of 2 days in their native forms. The crystal structure of peptide alone showed it to form an antiparallel four-helix bundle in which monomers interact, flat surface to flat surface, as predicted.