Cysteine sulfonic acid (Cys-SO3H; cysteic acid) is an oxidative post-translational modification of cysteine, resulting from further oxidation from cysteine sulfinic acid (Cys- SO2H). Cysteine sulfonic acid is considered an irreversible post-translational modification, which serves as a biomarker of oxidative stress that has resulted in oxidative damage to proteins. Cysteine sulfonic acid is anionic, as a sulfonate (Cys-SO –; cysteate), in the ionization state that
3
is almost exclusively present at physiological pH (pKa ~ –2). In order to understand protein
structural changes that can occur upon oxidation to cysteine sulfonic acid, we analyzed its conformational preferences, using experimental methods, bioinformatics, and DFT-based computational analysis. Cysteine sulfonic acid was incorporated into model peptides for α-helix and polyproline II helix (PPII). Within peptides, oxidation of cysteine to the sulfonic acid proceeds rapidly and efficiently at room temperature in solution with methyltrioxorhenium (MeReO3) and H2O2. Peptides containing cysteine sulfonic acid were also generated on solid phase using trityl-protected cysteine and oxidation with MeReO3 and H2O2. Using methoxybenzyl (Mob)-protected cysteine, solid-phase oxidation with MeReO3 and H2O2 generated the Mob sulfone precursor to Cys-SO – within fully synthesized peptides. These two
solid-phase methods allow the synthesis of peptides containing either Cys-SO – or Cys-SO – in a 32
practical manner, with no solution-phase synthesis required. Cys-SO – had low PPII propensity 3
for PPII propagation, despite promoting a relatively compact conformation in φ. In contrast, in a PPII initiation model system, Cys-SO – promoted PPII relative to neutral Cys, with PPII
initiation similar to Cys thiolate but less than Cys-SO – or Ala. In an α-helix model system, Cys- 2
SO – promoted α-helix near the N-terminus, due to favorable helix dipole interactions and 3
favorable α-helix capping via a sulfonate-amide side chain-main chain hydrogen bond. Across
all peptides, the sulfonate side chain was significantly less ordered than that of the sulfinate.
Analysis of Cys-SO – in the PDB revealed a very strong propensity for local (i/i or i/i+1) side 3
chain-main chain sulfonate-amide hydrogen bonds for Cys-SO –, with > 80% of Cys-SO – 33
residues exhibiting these interactions. DFT calculations conducted to explore these conformational preferences indicated that side chain-main chain hydrogen bonds of the sulfonate with the intraresidue amide and/or with the i+1 amide were favorable. However, hydrogen bonds to water or to amides, as well as interactions with oxophilic metals, were weaker for the sulfonate than the sulfinate, due to lower charge density on the oxygens in the sulfonate.
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Glycine in Water Favors the Polyproline II State
Conformational preferences of amino acid residues in water are determined by the backbone and side-chain properties. Alanine is known for its high polyproline II (pPII) propensity. The question of relative contributions of the backbone and side chain to the conformational preferences of alanine and other amino acid residues in water is not fully resolved. Because glycine lacks a heavy-atom side chain, glycine-based peptides can be used to examine to which extent the backbone properties affect the conformational space. Here, we use published spectroscopic data for the central glycine residue of cationic triglycine in water to demonstrate that its conformational space is dominated by the pPII state. We assess three commonly used molecular dynamics (MD) force fields with respect to their ability to capture the conformational preferences of the central glycine residue in triglycine. We show that pPII is the mesostate that enables the functional backbone groups of the central residue to form the most hydrogen bonds with water. Our results indicate that the pPII propensity of the central glycine in GGG is comparable to that of alanine in GAG, implying that the water-backbone hydrogen bonding is responsible for the high pPII content of these residues.
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- Award ID(s):
- 1817650
- NSF-PAR ID:
- 10251353
- Date Published:
- Journal Name:
- Biomolecules
- Volume:
- 10
- Issue:
- 8
- ISSN:
- 2218-273X
- Page Range / eLocation ID:
- 1121
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/biom10081121
