This is a real "why" question... even
though we know a lot of detail about the what and
how of this, no one can say for sure "why"
GTP is the favored source. We do know that many of
the proteins involved in protein synthesis have
structures similar to proteins that use GTP in
signal transduction. There may be some energetics
arguments regarding "why GTP" as well, but I am
not personally aware of those arguments. None of
my standard resources here provide reference to
this but if I come across something, I'll pass it
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