The Purine Nucleotide Cycle is a

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), cell. The reactants, products, and Metabolic intermediate, intermediates of an enzymatic reaction are known as metabolites, which are ...

protein metabolism Protein metabolism denotes the various biochemical processes responsible for the synthesis of proteins and amino acids (anabolism), and the breakdown of proteins by catabolism. The steps of protein synthesis include transcription, translation, an ...

requiring the amino acids

aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. The L-isomer of aspartic acid is one of the 22 proteinogenic amino acids, i.e., the building blocks of protein ...

and

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

. The cycle is used to regulate the levels of adenine nucleotides, in which

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

and fumarate are generated. AMP converts into IMP and the byproduct ammonia. IMP converts to S-AMP ( adenylosuccinate), which then converts to AMP and the byproduct fumarate. The fumarate goes on to produce ATP (energy) via

oxidative phosphorylation Oxidative phosphorylation(UK , US : or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order ...

as it enters the

Krebs cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of biochemical reactions that release the energy stored in nutrients through acetyl-CoA oxidation. The e ...

and then the

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. Lowenstein first described this pathway and outlined its importance in processes including amino acid catabolism and regulation of flux through

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

and the

. AMP is produced after strenuous muscle contraction when the ATP reservoir is low (ADP > ATP) by the adenylate kinase (myokinase) reaction. AMP is also produced from adenine and adenosine directly; however, AMP can be produced through less direct metabolic pathways, such as

de novo synthesis In chemistry, ''de novo'' synthesis () is the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to recycling after partial degradation. For example, nucleotides are not needed in the diet as they can ...

of IMP or through salvage pathways of

guanine Guanine () (symbol G or Gua) is one of the four main nucleotide bases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside ...

(a purine) and any of the purine nucleotides and nucleosides. IMP is synthesized de novo from

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

through the

pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt or HMP shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (five-carbon sugars) as well as ribose 5-ph ...

which produces ribose 5-P, which then converts to PRPP that with the amino acids glycine, glutamine, and aspartate (''see Purine metabolism'') can be further converted into IMP.

Reactions

The cycle comprises three enzyme-catalysed reactions. The first stage is the deamination of the purine

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

adenosine monophosphate Adenosine monophosphate (AMP), also known as 5'-adenylic acid, is a nucleotide. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine. It is an ester of phosphoric acid and the nucleoside adenosine. As a substituent it t ...

(AMP) to form inosine monophosphate (IMP), catalysed by the enzyme AMP deaminase: :AMP + + → IMP + The second stage is the formation of adenylosuccinate from IMP and the amino acid

, which is coupled to the energetically favourable hydrolysis of GTP, and catalysed by the enzyme adenylosuccinate synthetase: :Aspartate + IMP + GTP → Adenylosuccinate + GDP + 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 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

The purine nucleotide cycle occurs in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

(intracellular fluid) of the sarcoplasm of

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

, and in the

myocyte A muscle cell, also known as a myocyte, is a mature contractile Cell (biology), cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal muscle, skeletal, smooth muscle, smooth, and Cardiac muscle, cardiac ...

ic compartment of the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

cardiac The heart is a muscular organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrients to the tissu ...

and

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

. The cycle occurs when ATP reservoirs run low (ADP > ATP), such as strenuous exercise, fasting or starvation. Proteins catabolize into amino acids, and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s are precursors for purines, nucleotides and nucleosides which are used in the purine nucleotide cycle. The amino acid

is used to neutralize the ammonia produced when AMP is converted into IMP. Another amino acid,

, is used along with IMP to produce S-AMP in the cycle. Skeletal muscle contains amino acids for use in catabolism, known as the free amino acid pool; however, inadequate carbohydrate supply and/or strenuous exercise requires protein catabolism to sustain the free amino acids. When the phosphagen system (ATP-PCr) has been depleted of

phosphocreatine Phosphocreatine, also known as creatine phosphate (CP) or PCr (Pcr), is a phosphorylation, phosphorylated form of creatine that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle, myocardium and the brain to recyc ...

(creatine phosphate), the purine nucleotide cycle also helps to sustain the myokinase reaction by reducing accumulation of AMP produced after muscle contraction in the below reaction. During muscle contraction: : + ATP → + ADP + ( assisted, utilization of ATP for

muscle contraction Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in musc ...

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or ...

) : + ADP + CP → ATP + Creatine ( assisted, catalyzed by

creatine kinase Creatine kinase (CK), also known as creatine phosphokinase (CPK) or phosphocreatine kinase, is an enzyme () expressed by various tissues and cell types. CK catalyses the conversion of creatine and uses adenosine triphosphate (ATP) to create phos ...

, ATP is used again in the above reaction for continued muscle contraction) : 2 ADP → ATP + AMP (catalyzed by adenylate kinase/myokinase when CP is depleted, ATP is again used for muscle contraction) Muscle at rest: : ATP + Creatine → + ADP + CP ( assisted, catalyzed by

) : ADP + → ATP (during

anaerobic glycolysis Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available. This occurs in health as in exercising and in disease as in sepsis and hemorrhagic shock. providing energy for a period ranging fr ...

and

) AMP can dephosphorylate to adenosine and

diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

out of the cell; the purine nucleotide cycle may therefore also reduce the loss of adenosine from the cell since nucleosides permeate cell membranes, whereas nucleotides do not.

Consequences

Aspartate and glutamate synthesis

Interaction between Branched-Chain Amino Acids and the Purine Nucleotide Cycle in muscles

Fumarate, produced from the purine nucleotide cycle, is an intermediate of

TCA cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of chemical reaction, biochemical reactions that release the energy stored in nutrients through acetyl-Co ...

and enters the

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

by converting into malate and utilizing the malate shuttle where it is converted into oxaloacetic acid (OAA). During exercise, OAA either enters into

or converts into aspartate in the mitochondria. As the purine nucleotide cycle produces ammonia ''(see below in ammonia synthesis)'', skeletal muscle needs to synthesize glutamate in a way that does not further increase ammonia, and as such the use of

glutaminase Glutaminase (, ''glutaminase I'', ''L-glutaminase'', ''glutamine aminohydrolase'') is an amidohydrolase enzyme that generates glutamate from glutamine. Glutaminase has tissue-specific isoenzymes. Glutaminase has an important role in glial cell ...

to produce glutamate from glutamine would not be ideal. Also, plasma glutamine (released from the kidneys) requires

active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...

into the muscle cell (consuming ATP). Consequently, during exercise when the ATP reservoir is low (ADP>ATP), glutamate is produced from branch-chained amino acids (BCAAs) and α-ketoglutarate, as well as from alanine and α-ketoglutarate. Glutamate is then used to produce aspartate. The aspartate enters the purine nucleotide cycle, where it is used to convert IMP into S-AMP. :BCAAs + α-Ketoglutarate ⇌ Glutamate + Branch-chain keto acids (BCKAs) (catalyzed by Branched-chain aminotransferases (BCAT)) :Alanine + α-Ketoglutarate ⇌ Pyruvate + Glutamate (catalyzed by

alanine transaminase Alanine aminotransferase (ALT or ALAT), formerly alanine transaminase (ALT), and even earlier referred to as serum glutamate-pyruvate transaminase (GPT) or serum glutamic-pyruvic transaminase (SGPT), is a transaminase enzyme () that was first c ...

) : :Oxaloacetic acid + Glutamate ⇌ α-Ketoglutarate + Aspartate (catalyzed by aspartate aminotransferase) When skeletal muscle is at rest (ADPα-Ketoglutarate + Aspartate ⇌ Oxaloacetic acid + Glutamate (catalyzed by aspartate aminotransferase)

Ammonia and glutamine synthesis

During exercise when the ATP reservoir is low (ADP>ATP), the purine nucleotide cycle produces ammonia () when it converts AMP into IMP. (With the exception of AMP deaminase deficiency, where ammonia is produced during exercise when adenosine, from AMP, is converted into inosine). During rest (ADPAMP deaminase in skeletal muscle) :Adenosine + → Inosine + (catalyzed by adenosine deaminase in skeletal muscle, blood, liver) Ammonia is toxic, disrupts cell function, and permeates cell membranes. Ammonia becomes ammonium () depending on the pH of the cell or plasma. Ammonium is relatively non-toxic and does not readily permeate cell membranes.

+ ⇌

Ammonia () diffuses into the blood, circulating to the liver to be neutralized by the

urea cycle The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic. The urea cycle converts highl ...

. (N.b.

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

is not the same as

uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the Chemical formula, formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the meta ...

, though both are end products of the purine nucleotide cycle, from ammonia and nucleotides respectively.) When the skeletal muscles are at rest (ADPglutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...

Reactions

Occurrence

Consequences

Aspartate and glutamate synthesis

Ammonia and glutamine synthesis

Pathology

AMP deaminase deficiency (MADD)

Glycogenoses (GSDs)

See also

References