Purine nucleotide cycle

The purine nucleotide cycle is a metabolic pathway in which ammonia and fumarate are generated from aspartate and inosine monophosphate (IMP) in order to regulate the levels of adenine nucleotides, as well as to facilitate the liberation of ammonia from amino acids. This pathway was first described by John Lowenstein, who outlined its importance in processes including amino acid catabolism and regulation of flux through glycolysis and the Krebs cycle. The purine nucleotide cycle is a metabolic pathway in which ammonia and fumarate are generated from aspartate and inosine monophosphate (IMP) in order to regulate the levels of adenine nucleotides, as well as to facilitate the liberation of ammonia from amino acids. This pathway was first described by John Lowenstein, who outlined its importance in processes including amino acid catabolism and regulation of flux through glycolysis and the Krebs cycle. The cycle is composed of three enzyme-catalysed reactions. The first stage is the deamination of the purine nucleotide Adenosine monophosphate (AMP) to form inosine monophosphate (IMP), catalysed by the enzyme AMP deaminase: The second stage is the formation of adenylosuccinate from IMP and the amino acid aspartate, which is coupled to the energetically favourable hydrolysis of GTP, and catalysed by the enzyme adenylosuccinate synthetase: Finally, Adenylosuccinate is cleaved by the enzyme adenylosuccinate lyase to release fumarate and regenerate the starting material of AMP: