Meta-Biol

• Pathways

An array of kinases catalyze the reversible phosphorylation of nucleotide monophosphates to form nucleotide diphosphates and triphosphates.

Nucleoside monophosphate kinases catalyze the reversible phosphorylation of nucleoside and deoxynucleoside 5'-monophosphates to form the corresponding nucleoside 5'-diphosphates. Most appear to have restricted specificities for nucleoside monophosphates, and to use ATP preferentially (Van Rompay et al. 2000; Anderson 1973; Noda 1973). The total number of human enzymes that catalyze these reactions in vivo is not clear. In six cases, a well-defined biochemical activity has been associated with a purified protein, and these are annotated here. However, additional nucleoside monophosphate kinase-like human proteins have been identified in molecular cloning studies whose enzymatic activities are unknown, and several distinctive nucleoside monophosphate kinase activities detected in cell extracts, e.g., a GTP-requiring adenylate kinase activity (Wilson et al. 1976) and one or more guanylate kinase activities (Jamil et al. 1975) have not been unambiguously associated with specific human proteins.

The nucleoside monophosphates against which each of the six well-characterized enzymes is active is shown in the table (Van Rompay et al. 2000). All six efficiently use ATP as a phosphate donor, but have some activity with other nucleoside triphosphates as well in vitro. The high concentrations of ATP relative to other nucleoside triphosphates in vivo makes it the likely major phosphate donor in these reactions under most conditions.

All of these phosphorylation reactions are freely reversible in vitro when carried out with purified enzymes and substrates, having equilibrium constants near 1. In vivo, high ratios of ATP to ADP are likely to favor the forward direction of these reactions, i.e., the conversion of (d)NMP and ATP to (d)NDP and ADP. At the same time, the reversibility of the reactions and the overlapping substrate specificities of the enzymes raises the possibility that this group of reactions can buffer the intracellular nucleotide pool and regulate the relative concentrations of individual nucleotides in the pool: if any one molecule builds up to unusually high levels, multiple routes appear to be open not only to dispose of it but to use it to increase the supply of less abundant nucleotides.

Ribonucleotide reductase catalyzes the synthesis of deoxyribonucleotide diphosphates from ribonucleotide diphosphates.

Nucleotides and their derivatives are used for short-term energy storage (ATP, GTP), for intra- and extra-cellular signaling (cAMP; adenosine), as enzyme cofactors (NAD, FAD), and for the synthesis of DNA and RNA. Most dietary nucleotides are consumed by gut flora; the human body's own supply of these molecules is synthesized de novo. Additional metabolic pathways allow the interconversion of nucleotides, the salvage and reutilization of nucleotides released by degradation of DNA and RNA, the catabolism of excess nucleotides, and the transport of these molecules between the cytosol and the nucleus (Rudolph 1994). These pathways are regulated to control the total size of the intracellular nucleotide pool, to balance the relative amounts of individual nucleotides, and to couple the synthesis of deoxyribonucleotides to the onset of DNA replication (S phase of the cell cycle).

These pathways are also of major clinical interest as they are the means by which nucleotide analogues used as anti-viral and anti-tumor drugs are taken up by cells, activated, and catabolized (Weilin and Nordlund 2010). As well, differences in nucleotide metabolic pathways between humans and aplicomplexan parasites like Plasmodium have been exploited to design drugs to attack the latter (Hyde 2007).

The movement of nucleotides and purine and pyrimidine bases across lipid bilayer membranes, mediated by SLC transporters, is annotated as part of the module "transmembrane transport of small molecules".

Metabolic processes in human cells generate energy through the oxidation of molecules consumed in the diet and mediate the synthesis of diverse essential molecules not taken in the diet as well as the inactivation and elimination of toxic ones generated endogenously or present in the extracellular environment. The processes of energy metabolism can be classified into two groups according to whether they involve carbohydrate-derived or lipid-derived molecules, and within each group it is useful to distinguish processes that mediate the breakdown and oxidation of these molecules to yield energy from ones that mediate their synthesis and storage as internal energy reserves. Synthetic reactions are conveniently grouped by the chemical nature of the end products, such as nucleotides, amino acids and related molecules, and porphyrins. Detoxification reactions (biological oxidations) are likewise conveniently classified by the chemical nature of the toxin.

At the same time, all of these processes are tightly integrated. Intermediates in reactions of energy generation are starting materials for biosyntheses of amino acids and other compounds, broad-specificity oxidoreductase enzymes can be involved in both detoxification reactions and biosyntheses, and hormone-mediated signaling processes function to coordinate the operation of energy-generating and energy-storing reactions and to couple these to other biosynthetic processes.