I understand that after the translation stage of ribosomal protein synthesis, tRNA molecules are floating in the cytoplasm without attached amino acids until they find the correct aminoacyl tRNA synthetase enzyme which revitalizes them.

However, in this process the enzyme takes an ATP molecule and returns an AMP (not ADP) molecule. It's been my understanding that an ATP's energy is normally harnessed via removing a single phosphate group and leaving an ADP molecule. Processes like respiration and photosynthesis deal with the conversion of ATP to ADP and vice versa. I understand that in removing the extra phosphate group to get an AMP more energy is released, but that also means that more energy has to be used to convert the AMP to ATP.

Does the cell convert the AMP to ADP, which will ultimately become ATP in respiration, or does the cell use some retrofitted respiration/metabolism to revitalize the AMP?

I would expect the first option but it would take energy (most likely from an ATP) and I would expect the amount of energy released from an ATP becoming an ADP to be different from the amount of energy released from an ADP becoming an AMP (so an ATP becoming an ADP would take more or less energy than would be required to make an AMP become an ADP, leading to inefficiencies). The other possibility (that I can see) is that the AMP is just used as a nucleotide in DNA and RNA, but I don't think that the numbers add up there. I've looked around but I can't find an answer.

So how do cells convert AMP to ATP? Also I was told that the ATP to ADP conversion is the general power-conversion. Is that not the case, or is this just some rare occurrence when AMPs are involved in power conversions?

  • $\begingroup$ Are you interested here in plant cells, animal cells, etc? Bare in mind for many cell types, ADP are largely converted to ATP by ATP-synthase, but AMP plays a crucial homeostatic roll, and coupled to adenylate kinase, helps to, accoring to this article, optimize the ATP-synthase process (I believe they're studying plant cells there). $\endgroup$ – CKM Jan 26 '16 at 1:52
  • $\begingroup$ I'd be interested to learn about AMP conversions in both plant AND animal cells, but I'll settle for whichever is more common (as in whichever one that uses AMP molecules more frequently). $\endgroup$ – Jean Valjean Jan 26 '16 at 2:28
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    $\begingroup$ Your statement about AMPs possibly being used in DNA/RNA synthesis is wrong, those are built with nucleotide triphosphates, including ATP, TTP, UTP, CTP, and GTP. Nucleic acid synthesis is similarish to your tRNA question because each triphosphate loses 2 phosphates when added to the growing DNA/RNA chain. If the chain is ever broken up, the nucleotides are released as monophosphates. $\endgroup$ – user137 Jan 26 '16 at 4:15
  • $\begingroup$ @user137 Nucleotides do, in fact, have only one phosphate group. Molecules like ATP and GTP have three. Nucleotides have ONE phosphate group, a ribose sugar, and a nitrogenous base $\endgroup$ – Jean Valjean Jan 26 '16 at 4:32
  • $\begingroup$ @jacob Nucleotide vs Nucleoside is just being pedantic. The point is that three phosphates go in, 2 get thrown out. See this image $\endgroup$ – user137 Jan 26 '16 at 4:39

AMP is first converted to ADP in the reaction

AMP + ATP $\leftrightarrow$ 2 ADP

catalyzed by adenylate kinase. So one phophate group is transfered from ATP to AMP, resulting in two ADP molecules. The ADP formed is then used to synthesize ATP as usual, by the mitochondrial ATP synthase or by the glycolysis enzymes.

For other nucleotides and deoxynucleotides, the monophosphate form NMP are usually phosphorylated to NDP using ATP, catalyzed by various kinases. For example, UMP and CMP is handled by the enzyme CMPK. Note that this is essentially the same type of reaction as adenylate kinase.

Regarding the energetics, the above reaction is only slightly favorable (at equal reactant concentrations) with $\Delta G=-2.6$ kJ/mol. In most cells, ATP concentration is much higher than ADP though, which helps drive the adenylate kinase reaction forward. See this site where you can investigate the energetics more closely.

  • $\begingroup$ Roland, where the ATP is found if it is not synthesized (оxidative phosphorylation needs ADP for synthesis of ATP)? $\endgroup$ – Alexander Bestavashvili Jan 10 at 10:06
  • $\begingroup$ @AlexanderBestavashvili, I'm not sure I understand you question, but there is plenty of ADP, since most reactions that hydrolyze ATP produce ADP. The adenylate kinase reaction just takes care of the more rare case where AMP is produced instead of ADP. $\endgroup$ – Roland Jan 11 at 15:58

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