Is the olfactory receptor a metalloprotein?

Abstract

The sense of smell is arguably our most primal faculty and also the least understood. Even our own olfactorily impaired species is capable of detecting Ϸ10,000 distinct scents [ Odorants first bind to an OR, which then undergoes some structural change that triggers the G protein activation and the following cascade of events leading to nerve cell activity. The structural details of ORs, however, remain to be determined. In this paper, we will describe a hypothesis in which metal ions play an important role for odorant recognition. We analyze the predicted structure and consensus sequence of the ORs and propose a metal-binding site in the loop between fourth and fifth helix (4 -5 loop). We have prepared synthetically a pentapeptide that contains this putative binding site and find that it not only has high affinity for binding Cu(II) and Zn(II) ions, but that it also undergoes a dramatic transition to an ␣-helical structure upon metal ion binding. Based on these observations, we propose a ''shuttlecock'' mechanism for the possible structural change in ORs upon odorant binding. This mechanism involves membrane penetration of the 4 -5 loop after residue charge neutralization by metal ion binding.olfaction ͉ G protein-coupled receptors ͉ transmembrane protein I norganic chemists know as a rule of thumb that if a volatile compound is a good ligand for metal ion coordination complexes, it probably smells strongly; this observation has lead to recent advances in artificial olfaction (1). The only notable exceptions to this rule are CO and NO, which are produced endogenously as neural messengers (2) and, therefore, elicit no olfactory response. In general, the human olfactory system is extremely sensitive to amines and thiols (good ligands for metal ions) but not to alcohols (which are only weak ligands; ref.3), as shown in Fig. 1. For example, one can smell methylthiol at less than 1 ppb, methylamine at 18 ppb, but methanol only above 100 ppm, and methane is undetected even at 10 6 ppm.Odorants needs to bind to an olfactory receptor (OR) to trigger the cascade of events that finally enables us to smell: methylthiol is bound by some OR Ͼ1 million times stronger than the OR that responds to methanol, and methylamine is bound Ͼ100,000 times stronger than methanol. Differences in hydrogen bonding or van der Waals interactions are insufficient to account for this range, but differences in Lewis basicity to metal ions can. This range of affinities is critical to the pattern recognition processes proposed for neural computation (4).Thus, the most natural explanation to account for this odorant affinity difference is coordination to a metal ion bound in an OR. Cu(II) or Zn(II) are particularly likely candidates, because they have strong amine-and thiol-binding property and are frequently found in metalloproteins. More than two decades ago, Crabtree (7) prophetically speculated that Cu(I) might be found in ORs, because of the high olfactory sensitivity to amines and thiols.Consistent with the metalloprotein hypothesis, the ORs ...

Experimental evaluation of the generalized vibrational theory of G protein-coupled receptor activation

“…All four compounds were assayed for functional activity, using calcium flux assays ( The process of protein agonism/antagonism involves, at minimum, two steps: binding of the ligand to the active site of the protein and the activation of the protein. It should be noted that these actions may happen in concert as proposed in the hand-inglove/multiconformation (5-8) models or as two individual steps (11,60,67,80,90). In Table 1, we present the results of binding displacement assays comparing the relative binding affinity of both (R)-DOI and (R)-d6-DOI at several serotonin GPCRs.…”

“…As the electron traverses the active site, it may undertake several paths: (i) elastic tunneling, where no energy is lost or gained by the electron; (ii) inelastic tunneling (IET), where the electron may donate or accept a quantum of energy during transfer; and (iii) subsequently higher ordered inelastic processes (61)(62)(63). The hypothesized presence of a possible metal cofactor site-acting to assist either in binding or in a later activation step-at ORs, GPCRs, and non-GPCR chemokine receptors is supported by altered behavioral response (64)(65)(66), physiological response (66)(67)(68), theory (69,70), and in vitro observations (70)(71)(72)(73)(74)(75)(76)(77)(78)(79). 3.…”

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