Manajer Yang Rasional

xiv,248 hlm.; 21 cm

Self-organization of OPV-PEG diblock copolymers in THF/water.

Oligo(phenylenevinylene)-poly(ethyleneglycol) (OPV-PEG) diblock copolymers in tetrahydrofuran (THF) solution at concentrations of 5 to 25 gl self-assemble into rod-like structures with a radius of about 80 {angstrom} for an OPV-PEG diblock copolymer comprising 13 PV and 45 EG monomers. These aggregates consist of a liquid crystalline OPV core and a PEG shell. Addition of about 10% water to the solution induces the formation of a phase of packed rods, as revealed by a sudden and dramatic transition of the scattering pattern. Further addition of water leads to swelling and at about 30% ultimately to disruption of the packed-rod phase

Nascent Hairpins in Proteins: Identifying Turn Loci and Quantitating Turn Contributions to Hairpin Stability

Many factors influence the stability of hairpins that could appear as foldons in partially folded states of proteins; of these, the propensity of certain amino acid sequences to favor conformations that serve to align potential β-strands for antiparallel association is likely the dominant feature. Quantitating turn propensities is viewed as the first step in developing an algorithm for locating nascent hairpins in protein sequences. Such nascent hairpins can serve to accelerate protein folding or, if they represent structural elements that differ from the final folded state, as kinetic traps. We have measured these “turn propensities” for the two most common turn types using a series of model peptide hairpins with four- and six-residue loops connecting the associated β-strands. Loops of four to six residues with specific turn sequences containing only natural l-amino acids and glycine can provide as much as 15 kJ/mol of hairpin stabilization versus loops lacking the defined turn loci. Single-site mutations within some of the optimal connecting loops can have ΔΔ<i>G</i> effects as large as 9–10 kJ/mol on hairpin stability. In contrast to the near universal II′/I′ turns of model hairpins, a number of hairpin-supporting XZZG sequence β-turns with α_R and/or γ_R configurations at the ZZ unit were found. A series of turn replacements (four-residue β-turns replaced by sequences that favor five- and six-residue reversing loops) using identical strands in our model systems have confirmed that several sequences have intrinsic turn propensities that could favor β-strand association in a non-native strand register and thus serve as kinetic traps. These studies also indicate that aryl residues immediately flanking a turn sequence can alter relative turn propensities by as much as 9–11 kJ/mol and will need to be a part of any nascent hairpin recognition algorithm

Human Cyclooxygenase-2 Is a Sequence Homodimer That Functions as a Conformational Heterodimer*

Prostaglandin endoperoxide H synthases 1 and 2, also known as cyclooxygenases (COXs) 1 and 2, convert arachidonic acid (AA) to prostaglandin endoperoxide H2. Prostaglandin endoperoxide H synthases are targets of nonspecific nonsteroidal anti-inflammatory drugs and COX-2-specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer has a peroxidase and a COX active site. We find that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (Ecat) and an allosteric monomer (Eallo). Heme binds tightly only to the peroxidase site of Ecat, whereas substrates, as well as certain inhibitors (e.g. celecoxib), bind the COX site of Ecat. Ecat is regulated by Eallo in a manner dependent on what ligand is bound to Eallo. Substrate and nonsubstrate fatty acids (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the COX site of Eallo. AA can bind to Ecat and Eallo, but the affinity of AA for Eallo is 25 times that for Ecat. Palmitic acid, an efficacious stimulator of human PGHS-2, binds only Eallo in palmitic acid/murine PGHS-2 co-crystals. Nonsubstrate FAs can potentiate or attenuate actions of COX inhibitors depending on the FA and whether the inhibitor binds Ecat or Eallo. Our studies suggest that the concentration and composition of the free FA pool in the environment in which PGHS-2 functions in cells, the FA tone, is a key factor regulating PGHS-2 activity and its responses to COX inhibitors. We suggest that differences in FA tone occurring with different diets will likely affect both base-line prostanoid synthesis and responses to COX inhibitors