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Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid ? peptides.

ABSTRACT: Amyloid ? peptide (A?) is causatively associated with Alzheimer's disease (AD), and N-terminally truncated and pyroglutamylated A? peptides (A?pE) exert hypertoxic effect by an unknown mechanism. Recent evidence has identified the prefibrillar oligomers of A?, not the fibrils, as the prevalent cytotoxic species. Structural characterization of A? and A?pE oligomers is therefore important for better understanding of their toxic effect. Here we have used isotope-edited Fourier transform infrared (FTIR) spectroscopy to identify the conformational changes in A?(1-42) and A?pE(3-42) upon aggregation, individually and in 1?:?1 molar combination. During the first two hours of exposure to aqueous buffer, the peptides undergo transition from mostly ?-helical to mostly ?-sheet structure. Data on peptides (13)C,(15)N-labeled at K(16)L(17)V(18) or V(36)G(37)G(38)V(39) allowed construction of structural models for the monomer and early oligomers. The peptide monomer comprises a ?-hairpin that involves residues upstream of the K(16)L(17)V(18) sequence and an N-terminal ?-helix. The oligomers form by non-H-bonding interactions between the ?-strands of neighboring ?-hairpins, in lateral or staggered manner, with the strands running parallel or antiparallel. Relative ?-helical and ?-sheet propensities of A?(1-42) and A?pE(3-42) depend on the ionic strength of the buffer, emphasizing the importance of ionic interactions in A? peptide structure and aggregation. It is inferred that N-terminal modification of A?pE(3-42) affects the helix stability and thereby modulates ?-sheet oligomer formation. The data thus provide new insight into the molecular mechanism of A? oligomerization by emphasizing the role of the N-terminal transient ?-helical structure and by identifying structural constraints for molecular organization of the oligomers.

SUBMITTER: Goldblatt G

PROVIDER: S-EPMC4668202 | biostudies-literature | 2015 Dec

REPOSITORIES: biostudies-literature

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Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid β peptides.

Goldblatt Greg G Matos Jason O JO Gornto Jeremy J Tatulian Suren A SA

Physical chemistry chemical physics : PCCP 20151201 48

Amyloid β peptide (Aβ) is causatively associated with Alzheimer's disease (AD), and N-terminally truncated and pyroglutamylated Aβ peptides (AβpE) exert hypertoxic effect by an unknown mechanism. Recent evidence has identified the prefibrillar oligomers of Aβ, not the fibrils, as the prevalent cytotoxic species. Structural characterization of Aβ and AβpE oligomers is therefore important for better understanding of their toxic effect. Here we have used isotope-edited Fourier transform infrared (F ...[more]

PMID: 26214017

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Project description:Escherichia coli alkaline phosphatase (AP) can hydrolyze a variety of chemically diverse phosphate monoesters while making contacts solely to the transferred phosphoryl group and its incoming and outgoing atoms. Strong interactions between AP and the transferred phosphoryl group are not present in the ground state despite the apparent similarity of the phosphoryl group in the ground and transition states. Such modest ground-state affinity is required to curtail substrate saturation and product inhibition and to allow efficient catalysis. To investigate how AP achieves limited affinity for its ground state, we first compared binding affinities of several related AP ligands. This comparison revealed a paradox: AP has a much stronger affinity for inorganic phosphate (P(i)) than for related compounds that are similar to P(i) geometrically and in overall charge but lack a transferable proton. We postulated that the P(i) proton could play an important role via transfer to the nearby anion, the active site serine nucleophile (Ser102), resulting in the attenuation of electrostatic repulsion between bound P(i) and the Ser102 oxyanion and the binding of P(i) in its trianionic form adjacent to a now neutral Ser residue. To test this model, isotope-edited Fourier transform infrared (FTIR) spectroscopy was used to investigate the ionic structure of AP-bound P(i). The FTIR results indicate that the P(i) trianion is bound and, in conjunction with previous studies of pH-dependent P(i) binding and other results, suggest that P(i) dianion transfers its proton to the Ser102 anion of AP. This internal proton-transfer results in stronger P(i) binding presumably because the additional negative charge on the trianionic P(i) allows stronger electrostatic interactions within the AP active site and because the electrostatic repulsion between bound P(i) and anionic Ser102 is eliminated when the transferred P(i) proton neutralizes Ser102. Indeed, when Ser102 is neutralized the P(i) trianion binds AP with a calculated K(d) of ?290 fM. These results suggest that electrostatic repulsion between Ser102 and negatively charged phosphate ester substrates contributes to catalysis by the preferential destabilization of the reaction's E·S ground state.

Dataset Information

Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid ? peptides.

Publications

Isotope-edited FTIR reveals distinct aggregation and structural behaviors of unmodified and pyroglutamylated amyloid β peptides.

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