Inactive Tutor answered • 10/14/21
This was explained to me in a graduate biochemistry class, and what I was told is that it is because ATP contains relatively UNstable (not highly unstable, but has a low activation energy), high energy bonds. This allows enzymatic hydrolysis of ATP to occur fairly easily, and releases a large amount of energy. In terms of ATP v. GTP I don't have an answer, but energetically this seems to be a reasonable ontological answer.
Also- I wouldn't attribute energy storage to ATP, at least long-term I would say things like neutral lipids and carbohydrates do that, but ATP is more of a mechanism of energy allocation and converts that stored energy into something that's broadly useable by cellular processes.
