Gated electron transport in rhodopsin and its relevance to GPCR activation

We identify, by density-functional theory calculations, an electron donor-bridge-acceptor (DBA) complex within the highest resolution X-ray diffraction structures of rhodopsin. The donor is a conserved tryptophan, the acceptor is a zinc ion surrounded by a tryptophan, a histidine and a conserved glutamate. The unusual environment of the zinc ion confers high electron affinity on the zinc site. The bridge is the retinal which can exist either in the neutral aldimine (Schiff base) or aldiminium (protonated) state. When the retinal is unprotonated, no electron transfer occurs. Upon protonation of the aldimine, the DBA complex conducts and a full electron charge is transferred from donor tryptophan to the zinc complex. This gated electron transfer creates the molecular equivalent of a tunnel triode. Since rhodopsin is the ancestor of GPCRs, we discuss the possible relevance of this gated electron transport to other GPCRs, in particular to olfactory receptors which have been proposed to use an electron tunneling mechanism to detect molecular vibrations.