I read in a textbook that ATP is made from ribose and not deoxyribose. Originally, I thought that the pentose sugar didn’t have functional importance.
Is there a functional reason why the ATP used in metabolism has to be made from ribose?
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$\begingroup$ I have edited your question, in particular changing the reference to ATP being used in respiration to being used in metabolism, and making it clear that your concern is the lack of a functional role of the sugar ring in ATP hydrolysis. If this is not what you meant, do revert the question, clarifying. $\endgroup$– DavidCommented Feb 15, 2017 at 15:20
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$\begingroup$ Can I ask for a clarification of your question. Are you asking why dATP does not function in contemporary energy metabolism, whereas rATP does? (i.e. why the enzymes will not use it). Or are you asking why organisms evolved to use rATP rather than dATP? $\endgroup$– DavidCommented Feb 15, 2017 at 16:57
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$\begingroup$ Mainly why rATP is used in metabolism and dATP is not, I assume it may be to do with the fact dATP is used in DNA but I was wondering if there was a specific biochemical reason $\endgroup$– user6817585Commented Feb 15, 2017 at 16:58
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1 Answer
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Short answer:
The pentose ring does not participate chemically in the energy transfer reactions involving hydrolysis of ATP to ADP.
Enzymes that catalyse these reactions would tend to be specific for ATP (rather than dATP) because they bind the whole molecule and this would most likely include the 2'-OH of the ribose ring. dATP would therefore bind less well or not at all. (There may also be an advantage for evolving such specifity so that dATP could not compete for the active sites.)
The reason that evolution produced ATP as the energy transfer molecule rather than dATP is purely a matter for speculation. One might argue that it is an extra energy expense to reduce the ribose ring, although I prefer a more mundane answer. That is that if one assume RNA genomes (and enzymes) predated DNA genomes evolutionarily, then molecules involved in energy metabolism that contain ribose — not just ATP, but NAD and coenzyme A — arose at that time. When the genomes switched to DNA there was no selective advantage in changing ATP (and NAD) as well, and such an advantage would have been required as the enzyme systems that used these small molecules would by then have been already established.