ATP

From Palaeos

ATP (adenosine triphosphate) is the common currency of chemical energy in most cells. It is more or less accurate to say that the whole object of metabolizing food, of whatever kind, is to recover energy to make ATP. Specifically, the energy from the breakdown of food is used to add a third phosphate group onto adenosine diphosphate (ADP) to make the triphosphate. The energy stored in ATP is then used to drive various reactions by cleaving the third phosphate back off (hydrolysis of ATP). This job is done by enzymes (ATPases) which couple the ATP hydrolysis with some other reaction which absorbs energy. Chemically, ATP is built from exactly the same chemical unit that supplies the adenosine or "A" monomers to RNA. Now, stop fidgeting and pause a moment to reflect on the evolutionary implications of all this: precisely the same molecule, phosphorylated adenosine, is at the core of (a) most information exchange (b) the vast majority of food metabolism reactions, and (c) most synthetic reactions, in every known organism. Why? No one knows the answer to this question. This is not the sort of molecule you'd expect to form spontaneously -- or at least not under the kinds of conditions found today. There are two relatively high-probablility implications. (1) We may be missing a lot of hidden evolution -- lost diversity -- which occurred between the first living organism and the last common ancestor of all organisms alive today. (2) Life may have gotten started at the interface between radically different chemical environments. Notice that the adenosine base in ATP is strongly reduced, non-polar, compact, and basic. The phosphate end is highly oxidized, ionic, has an extended shape, and is quite acidic. The ribose ring in between is intermediate in all four respects. It's hard to see how this molecule could form without either a sophisticated biochemical system already in place (implication "a"), the close juxtaposition of wildly different chemical environments (implication "b"), or both.

Credits

ATW? Transferred CKT 060921