Carbamates ammonia equivalent reactions

CPSase catalyzes the formation of carbamyl phosphate from glutamine, bicarbonate, and two equivalents of ATP. The biosynthesis involves four partial reactions. GLNase catalyzes the formation of ammonia from glutamine. The remaining three partial reactions are catalyzed by SYNase. Bicarbonate is activated by ATP to form carboxyphosphate, which reacts with ammonia to form carbamate. The ATP-dependent phosphorylation of carbamate results in the production of carbamyl phosphate. 

Fulop and co-workers [124] have reported the synthesis of some new (ami-noalkyl) naphthols and (aminoalkyl) quinolinols 85 in good yields through a one-pot, microwave-assisted three-component modified Mannich reaction of naphthol or quinolinol with two equivalents of an aldehyde using ammonium carbamate or ammonium hydrogen carbonate as solid ammonia sources. It was observed that aliphatic aldehydes did not lead to the formation of the desired (aminoalkyl) quinolinols 85 (Scheme 64). 

During the enzymic synthesis of carbamyl phosphate (34), two molecules of ATP are involved for every molecule of (34) that is synthesized. One molecule of ATP reacts with bicarbonate to form a mixed anhydride of orthophosphoric and carbonic acids, while the second molecule of ATP phosphorylates the carbamate once it is formed. The half-life of the mixed anhydride is short (two minutes or less), but it can be trapped chemically, and moreover, 0 is transferred from bicarbonate to orthophosphate during this reaction. P P -Diadenosine 5 -polypentaphosphate is an inhibitor of the enzyme from E. coli, while the equivalent diadenosine pyro- and polyhexa-phosphates are not. It has been suggested that the two molecules of ATP and the bicarbonate bind at the active site of the enzyme as shown in (35). Once the enzyme-bound mixed anhydride has been formed, this reacts with glutamine or ammonia to generate the enzyme-bound carbamate, which is finally phosphorylated by the second molecule of ATP (Scheme 10). 

The hypothesis states that the proximity to the GLN domain determines the reaction catalyzed by the CPS subdomain. The GLN domain is in intimate contact with CPS.A but there are no direct interactions with CPS.B. According to this interpretation, CPS.A catalyzes the bicarbonate dependent ATPase reaction because of this proximity to the source of ammonia, while CPS.B, which is far from the GLN domain, catalyzes the phosphorylation of carbamate. To test the idea that the GLN domain directs the function of the CPS domain, the mutagenesis experiments previously conducted with E. coli CPSase (55) were repeated with the same results. However, if the GLN domain is removed and a disabling mutation is introduced into CPS.A, rather than selectively abolishing the ATPase activity, both the ATPase and ATP synthetase reactions were inhibited by approximately 50%. These results suggest that, in the absence of the GLN domain, CPS.A and CPS.B of the intact . coli CPSase can catalyze both ATP dependent partial reactions. However, when the CPS subunit is associated with the GLN domain, CPS.A is specialized for the activation of bicarbonate while CPS.B phosphorylates carbamate. Thus, we have the interesting situation in which the role of two functionally equivalent domains is determined by their juxtaposition relative to a third domain in the complex. 

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