Structure of spin-labeled methylmethanethiolsulfonate in solution and bound to TEM-1 beta-lactamase determined by electron nuclear double resonance spectroscopy

Interaction of a beta-sheet breaker peptide with lipid membranes

Aggregation of beta-amyloid peptides into senile plaques has been identified as one of the hallmarks of Alzheimer's disease. An attractive therapeutic strategy for Alzheimer's disease is the inhibition of the soluble beta-amyloid aggregation using synthetic beta-sheet breaker peptides that are capable of binding Abeta but are unable to become part of a beta-sheet structure. As the early stages of the Abeta aggregation process are supposed to occur close to the neuronal membrane, it is strategic to define the beta-sheet breaker peptide positioning with respect to lipid bilayers. In this work, we have focused on the interaction between the beta-sheet breaker peptide acetyl-LPFFD-amide, iAbeta5p, and lipid membranes, studied by ESR spectroscopy, using either peptides alternatively labeled at the C- and at the N-terminus or phospholipids spin-labeled in different positions of the acyl chain. Our results show that iAbeta5p interacts directly with membranes formed by the zwitterionic phospholipid dioleoyl phosphatidylcholine and this interaction is modulated by inclusion of cholesterol in the lipid bilayer formulation, in terms of both peptide partition coefficient and the solubilization site. In particular, cholesterol decreases the peptide partition coefficient between the membrane and the aqueous medium. Moreover, in the absence of cholesterol, iAbeta5p is located between the outer part of the hydrophobic core and the external hydrophilic layer of the membrane, while in the presence of cholesterol it penetrates more deeply into the lipid bilayer.

Membrane charge dependent states of the beta-amyloid fragment Abeta (16-35) with differently charged micelle aggregates

Abeta (16-35) is the hydrophobic central core of beta-amyloid peptide, the main component of plaques found in the brain tissue of Alzheimer's disease patients. Depending on the conditions present, beta-amyloid peptides undergo a conformational transition from random coil or alpha-helical monomers, to highly toxic beta-sheet oligomers and aggregate fibrils. The behavior of beta-amyloid peptide at plasma membrane level has been extensively investigated, and membrane charge has been proved to be a key factor modulating its conformational properties. In the present work we probed the conformational behavior of Abeta (16-35) in response to negative charge modifications of the micelle surface. CD and NMR conformational analyses were performed in negatively charged pure SDS micelles and in zwitterionic DPC micelles "doped" with small amounts of SDS. To analyze the tendency of Abeta (16-35) to interact with these micellar systems, we performed EPR experiments on three spin-labeled analogues of Abeta (16-35), bearing the methyl 3-(2,2,5,5-tetramethyl-1-oxypyrrolinyl) methanethiolsulfonate spin label at the N-terminus, in the middle of the sequence and at the C-terminus, respectively. Our conformational data show that, by varying the negative charge of the membrane, Abeta (16-35) undergoes a conformational transition from a soluble helical-kink-helical structure, to a U-turn shaped conformation that resembles protofibril models.

Multifrequency electron spin resonance spectra of a spin-labeled protein calculated from molecular dynamics simulations

Multifrequency electron spin resonance (ESR) spectra provide a wealth of structural and dynamic information about the local environment of the spin label and, indirectly, about the spin-labeled protein. Relating the features of the observed spectra to the underlying molecular motions and interactions is, however, challenging. To make progress toward a rigorous interpretation of ESR spectra, we perform extensive molecular dynamics (MD) simulations of fully solvated T4 Lysozyme, labeled with the spin label MTSSL at positions 72 and 131. These two sites have been the object of numerous experimental studies and are generally considered as prototypical solvent-exposed sites on the surfaces of alpha-helices. To extend the time window afforded by the MD simulations, stochastic Markov models reflecting the dynamics of the spin label side chains in terms of their rotameric states are constructed from the trajectories. The calculated multifrequency ESR spectra are in very good agreement with experiment for three different magnetic field strengths without adjusting any parameters. During the trajectories, the spin labels interconvert among a fairly large number of conformations and display a propensity to form interactions with protein residues other than their nearest neighbors along the helix. The detailed picture of the spin label emerging from the MD simulations provides useful insight into the molecular origins of the available spectroscopic and crystallographic data.

Chromophoric spin-labeled beta-lactam antibiotics for ENDOR structural characterization of reaction intermediates of class A and class C beta-lactamases

The chromophoric spin-label substrate 6-N-[3-(2,2,5,5-tetramethyl-1-oxypyrrolin-3-yl)-propen-2-oyl]penicillanic acid (SLPPEN) was synthesized by acylation of 6-aminopenicillanic acid with the acid chloride of 3-(2,2,5,5-tetramethyl-1-oxypyrrolinyl)-2-propenoic acid and characterized by physical methods. By application of angle-selected electron nuclear double resonance (ENDOR), we have determined the molecular structure of SLPPEN in solution. SLPPEN exhibited UV absorption properties that allowed accurate monitoring of the kinetics of its enzyme-catalyzed hydrolysis. The maximum value of the (substrate-product) difference extinction coefficient was 2824 M(-1) cm(-1) at 275 nm compared to 670 M(-1) cm(-1) at 232 nm for SLPEN [J. Am. Chem. Soc. 117 (1995) 6739]. For SLPPEN, the steady-state kinetic parameters kcat and kcat/KM, determined under initial velocity conditions, were 637 +/- 36 s(-1) and 13.8 +/- 1.4 x 10(6) M(-1) s(-1), respectively, for hydrolysis catalyzed by TEM-1 beta-lactamase of E. coli, and 0.5 +/- 0.04 s(-1) and 3.9 +/- 0.4 x 10(4) M(-1) s(-1) for hydrolysis catalyzed by the beta-lactamase of Enterobacter cloacae P99. We have also observed "burst kinetics" for the hydrolysis of SLPPEN with P99 beta-lactamase, indicative of formation of an acylenzyme reaction intermediate. In DMSO:H2O (30:70, v:v) cryosolvent mixtures buffered to pH* 7.0, the half-life of the acylenzyme intermediate formed with the P99 enzyme at -5 degrees C was > or = 3 min, suitable for optical characterization. The observation of burst kinetics in the hydrolysis of SLPPEN catalyzed by P99 beta-lactamase suggests that this chromophoric spin-labeled substrate is differentially sensitive to active site interactions underlying the cephalosporinase and penicillinase reactivity of this class C enzyme.