Question: why gtp instead of atp?
Answer: When GTP instead of ATP is the phosphate donor the pH optimum is 6.5 instead of 7.4. In addition NH4+ inhibits the transfer of phosphate from GTP but not from ATP. ... One is stimulated by cyclic AMP and is specific to ATP while the other is unaffected by cyclic nucleotides and can use either ATP or GTP as phosphate donor.