G protein is one of a number of guanosine triphosphate (GTP)-binding, regulatory proteins that serve as membrane-bound transducers of chemically and physically coded information; they are intermediaries in transmembrane signaling pathways that consist of three proteins: receptor, G protein, and effector. The G protein becomes activated upon binding GTP. The latter is subsequently slow hydrolyzed to GDP. When the hydrolysis is complete, the regulatory effect of the G protein is terminated and it is then available for reactivation by binding GTP.

a repressor protein

Regulate gene transcription.

Yes, it does.

No. Binding energy differs from element to element,