Thiol Reactivities in GPCRs Richard Lanzara, Bio Balance, Inc. 18 April 2008 The sensitivity of some G protein-coupled receptors to S-nitrosothiols suggests the presence of at least one or more free thiol groups. The varying reactivities of these free thiols also suggest very complex mechanisms that regulate their redox states and reactivities with other thiol containing molecules. For example, the finding that unlike the P2Y1 receptor, which has 2 essential disulfide bridges in its extracellular domains, the P2Y12 receptor has 2 free cysteines in its extracellular domains (Cys17 and Cys270), both of which are the targets of thiol reagents and the active metabolites of clopidogrel, which forms disulfide bridges with Cys17 and/or Cys270 in the P2Y12 receptor and thereby inactivates the receptor ( Ding Z, Kim S, Dorsam RT, Jin J, Kunapuli SP. Inactivation of the human P2Y12 receptor by thiol reagents requires interaction with both extracellular cysteine residues, Cys17 and Cys270. Blood. 2003;101: 3908-3914 - http://bloodjournal.hematologylibrary.org/cgi/content/full/101/10/3908 ). The presence of disulfide bonds, which could be reduced under the right redox conditions and the possible binding of endogenous ligands to biological receptors suggest just some of the difficulties involved in characterizing cellular receptors in their native and functional states. From our work, we’ve deduced that there must be at least one free thiol group that modulates ligand binding and receptor activation (see - Molecular dynamics of a biophysical model for beta-2-adrenergic and G protein-coupled receptor activation Journal of Molecular Graphics and Modelling 25: 396-409 (2006) and Activation of G Protein-Coupled Receptors Entails Cysteine Modulation of Agonist Binding, J. Molecular Structure (Theochem), 430/1-3: 57-71 (1998) also http://www.bio-balance.com/Ref.htm ). From the complex redox states (thiol-disulfide exchange, sulfenic acid (R-SOH), sulfinic acids (R-SO2H) and sulfonic acids (R-SO3H)), thiol chelation of transition metals (chiefly Zn(2+), Mn(2+) and Cu(2+)) and S-nitrosylation, we begin to see the devilish complexity associated with studying and understanding the reactivities of free thiols in many important cellular and biological systems. Competing interests Bio Balance is a private research organization studying biophysical models of GPCRs.