Activation of several receptor types is followed by their binding to a G-protein. Prior to transmission of the agonist signal, the G-protein which had affinity for guanosine 5-diphosphate (GDP) binds guanosine 5-triphosphate (GTP) instead. Because evidence exists that several agonists groups activate their receptors by reduction, we evaluated whether the nucleotide associated with G-proteins could enhance electron flow. Using a model system of ferrous iron and ferric cytochrome c, it was determined that substitution of GTP for GDP led to an enhanced reduction of ferric cytochrome c. These results support the concept that cellular activation by certain receptors may involve reductive activation with the participation of GTP and G-proteins. We speculate that GTP, when bound to G-protein, can facilitate electron transfer perhaps from the receptor or the G-protein to the catalytic subunit of the adenylate cyclase enzyme.
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Acknowledgements -- This work was supported in part by NIH grant R01-AI-25625. The authors would like to thank Diane Konzen and Marie Starks for assistance in preparation of this manuscript, and John Eaton for helpful comments.
- Electron transfer
- Free radicals
- Reductive activation