Nucleotide Metabolism

What are the characteristics that make every nucleotide specific for its function?
what makes GTP, UTP or ATP have their own functions in metabolism, while chemically they only differ in side chains (side groups)?
guanine, uracil and adenine are different size and shape so the answer may be in the active sites of the kinases that use them rather than the energy molecule. not 100% sure though
statistics on cancer and information for Easy Pha-max Pro-Can

Metabolism Masterclass