If a modified form of GTP that cannot be enzymatically converted to GDP were added to a culture of cells, the likely result would be __________.

• A. the inactivation of ligand-gated ion channels
• B. the inactivation of G-protein-coupled signaling pathways
• C. the inhibition of pathways stimulated by tyrosine-kinase receptors
• D. receptor tyrosine kinases would be stimulated by the additional phosphate groups present in the modified GTP
• E. that the activated G proteins would remain locked in the "on" position, transmitting signal even in the absence of a signaling molecule

Answer: (E)

Verified step by step guidance

1. Understand the role of GTP in cell signaling: GTP (guanosine triphosphate) is a molecule that binds to G-proteins, activating them. When GTP is bound, the G-protein is in an 'on' state, capable of transmitting signals inside the cell.
2. Recognize the normal process of GTP hydrolysis: In typical cellular signaling, GTP is hydrolyzed to GDP (guanosine diphosphate), which inactivates the G-protein, turning the signal 'off'.
3. Consider the effect of a non-hydrolyzable GTP analog: If a modified form of GTP that cannot be converted to GDP is introduced, the G-protein cannot be turned 'off' because the hydrolysis step is blocked.
4. Analyze the impact on G-protein-coupled signaling pathways: Since the G-protein remains in the 'on' state, it continuously transmits signals, even without the presence of a signaling molecule, leading to prolonged activation of downstream pathways.
5. Conclude the outcome: The continuous activation of G-proteins would result in the persistent transmission of signals, potentially leading to cellular responses that are independent of external signals, which could disrupt normal cellular functions.