Biophysical Society Conference | Tahoe 2023

Proton Reactions: From Basic Science to Biomedical Applications

Wednesday Speaker Abstracts

BIOPHYSICS MEETS MEDICINE: PH-DEPENDENT OPIOID PAINKILLERS Christoph Stein 1 ; 1 Freie Universität Berlin, Germany, Berlin, Germany Indiscriminate activation of opioid receptors provides pain relief but also severe central and intestinal side effects. We hypothesized that exploiting pathological (rather than physiological) conformation dynamics of opioid receptor-ligand interactions might yield ligands without adverse actions. By computer simulations at low pH, a hallmark of injured tissue, we designed an agonist that, because of its low acid dissociation constant, selectively activates peripheral mu opioid receptors at the source of pain generation. Unlike the conventional opioid fentanyl, this agonist showed pH-sensitive binding, heterotrimeric guanine nucleotide–binding protein (G protein) subunit dissociation by fluorescence resonance energy transfer, and adenosine 3′,5′ - monophosphate inhibition in vitro. It produced injury-restricted analgesia in rodents with different types of inflammatory pain without exhibiting respiratory depression, sedation, constipation, or addiction potential. 1 Nagoya Institution of Technology, Life Science and Applied Chemistry, Nagoya, Japan Proton transfer reaction belongs to one of the key triggers for the functional expression of membrane proteins. Rod and cone opsins are light-sensitive G protein-coupled receptors (GPCRs) that undergo the cis-trans isomerization of the retinal chromophore in response to light. The isomerization event initiates a conformational change in the opsin protein moiety which propagates the downstream effector signaling. The final step of receptor activation is the deprotonation of the retinal Schiff base, a proton transfer reaction which has been believed to be identical among the cone opsins. Here, we report unexpected proton transfer reaction occurring in the early photoreaction process of primate blue sensitive pigment (MB). By using low temperature UV-visible spectroscopy, we found that the Lumi intermediate of MB formed in transition from BL intermediate shows an absorption maximum in the UV region, indicating the deprotonation of retinal Schiff base. Comparison of light-induced difference FTIR spectra of Batho, BL, and Lumi showed a significant α -helical backbone C=O stretching and protonated carboxylate C=O stretching vibrations only in the Lumi intermediate. The transition from BL to Lumi thus involves dramatic changes in protein environment with proton transfer reaction between Schiff base and the counterion resulting in an absorption maximum in the UV region. On the other hand, no proton transfer reaction was observed in the Lumi intermediate for green pigment (MG); instead, a carboxylic acid deprotonation reaction coupled with large conformational change in protein environment specific to MG was observed in the active intermediate Meta-II. Thus, these results indicate that the photoreaction pathways leading to activation differ among cone opsins that absorb light of difference wavelengths, and that the differences are regulated by proton transfer reactions. PHOTOACTIVATION MECHANISM OF CONE OPSINS REGULATED BY PROTON TRANSFER REACTIONS Kota Katayama 1 ;

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