Photochemical and Structural Studies on Cyclic Peptide Models

The role of S126 in the Staphylococcus equorum MnSOD activity and stability

The dimeric form of manganese superoxide dismutase is instrumental for activity because each of the monomers provides amino acid residues participating in the enzymatic reaction. Hence, preventing dissociation of the dimer would maintain the enzymatic activity in detrimental conditions e.g. high temperature. To prevent dissociation of the dimer, a disulphide (S-S) bond was introduced at the dimer interface. In the wild type structure, S126 interacts with S126 of the other monomer. In the presented work, a mutant was designed with an S126C substitution. The crystal structure of the S126C mutant showed that only 50-70% of monomers formed the S-S bond. This observed imperfect S-S bonding was likely caused by photolytic S-S bond breakage mediated by the neighbouring tryptophan residue. In the wild type, S126 is located facing W163 and forms a water-mediated hydrogen bond with E164; W163 and E164 are crucial in the enzyme's activity. The replacement of S126 by a cysteine residue lowered the activity of the enzyme by ~70%. S126 has never been considered to play a role in the enzyme's activity or stability, thus the finding showed the importance of this residue.

Direct Ultraviolet Laser-Induced Reduction of Disulfide Bonds in Insulin and Vasopressin

Ultraviolet (UV) light has been shown to induce reduction of disulfide bonds in proteins in solution. The photoreduction is proposed to be a result of electron donation from excited Tyr or Trp residues. In this work, a powerful UV femtosecond laser was used to generate photoreduced products, while the hypothesis of Tyr/Trp mediation was studied with spectroscopy and mass spectrometry. With limited irradiation times of 3 min or less at 280 nm, the laser-induced reduction in arginine vasopressin and human insulin led to significant yields of ∼3% stable reduced product. The photogenerated thiols required acidic pH for stabilization, while neutral pH primarily caused scrambling and trisulfide formation. Interestingly, there was no direct evidence that Tyr/Trp mediation was a required criterion for the photoreduction of disulfide bonds. Intermolecular electron transfer remained a possibility for insulin but was ruled out for vasopressin. We propose that an additional mechanism should be increasingly considered in UV light-induced reduction of disulfide bonds in solution, in which a single UV photon is directly absorbed by the disulfide bond.

Characterization of Ultraviolet Photoreactions in Therapeutic Peptides by Femtosecond Laser Catalysis and Mass Spectrometry

Peptides and proteins have diverse ultraviolet (UV) photoreaction pathways that can be activated by the energy of the UV photons absorbed. Simple light sources such as lamps are conventionally used to study these photoreactions in solution. This work provides a proof of concept that femtosecond laser technology can function as a highly potent UV source in rapidly conducting UV photostability studies of peptides. Correspondingly, sufficient quantities of photoproducts were generated in 1 min or less, allowing for identification of known and new photomodifications in the therapeutic peptides somatostatin-14 and arginine vasopressin. Identical photoproducts were also generated with a conventional continuous source. The major modifications included N-formylkynurenine, a cross-link between Trp and Phe, a Tyr product with an NH₃ loss, and disruption of an unstable disulfide bond into a complex mixture of a trisulfide bond and multiple scrambled dimeric products. In conclusion, femtosecond lasers are extremely useful to drive fast UV-induced reactions for high throughput screening of photostability and modifications in amino acid polymers.