Purine nucleotide cycle

The Purine Nucleotide Cycle is a metabolic pathway in which fumarate is generated from aspartate in order to increase the concentration of Krebs cycle intermediates.^[1] The pathway was first described by John Lowenstein, who demonstrated its role in increasing the rate of oxidative phosphorylation in skeletal muscle.^[2]

Outline

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:

AMP + H₂O → IMP + NH₄⁺

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:

Aspartate + IMP + GTP → Adenylosuccinate + GDP + P_i

Finally, Adenylosuccinate is cleaved by the enzyme adenylosuccinate lyase to release fumarate and regenerate the starting material of AMP:

Adenylosuccinate → AMP + Fumarate

A recent study conducted by Sridharan et al. (AJP Cell Physiology, 2008, 295:C29-C37) showed that activation of HIF-1α allows cardiomyocytes to sustain mitochondrial membrane potential during anoxic stress by utilizing fumarate produced by adenylosuccinate lyase as an alternate terminal electron acceptor in place of oxygen. This mechanism should help provide protection in the ischemic heart.

Occurrence

This cycle occurs in Skeletal muscle myocyte's cytosolic compartment. This reaction helps to dispose AMP produced after following reaction.

ATP → ADP + Pi (Utilisation of ATP for Muscle contraction)

2 ADP → ATP + AMP (Catalysed by Adenylyl kinase/Myokinase)

Purine nucleotide cycle occurs during strenuous exercise, fasting or starvation when ATP reservoirs run low.

Consequences

1) Synthesis of fumarate

Fumarate is an intermediate of TCA cycle and enters the mitochondria by converting into malate and utilising the malate shuttle where it is converted into Oxaloacetic acid (OAA). OAA either enters into TCA cycle or converts into aspartate in the mitochondria. Aspartate can re-enter purine nucleotide cycle.

Oxaloacetic acid + Glutamate ↔ α-Ketoglutarate + Aspartate ( Catalysed by Aspartate Aminotransferase)

2) Synthesis of ammonia (Ammonia genesis)

The glutamate produced by OAA as above gains an NH₃ to become a Glutamine and enters the circulation to reach kidneys. In kidneys, glutamine is deaminated twice to form glutamate and then α Ketoglutarate. These NH₃ molecules neutralise the organic acids (lactic acid and ketone bodies) produced in the muscles.

References

1. Salway, J. G., Metabolism at a glance (3rd edition), Blackwell Publishing Ltd., Oxford, 2004, pp. 56-57
2. Voet, D., Voet, J. G., Biochemistry (3rd Edition), John Wiley & Sons, Inc., 2004, pp. 1094-1095