Pathways

A group of heterocyclic aromatic organic compound, pyrimidines are similar in structure to benzene and pyridine and count the nucleic acids cytosine, thymine, and uracil as structural derivatives. The following pathway illustrates a many pyrimidine-associated processes such as nucleotide biosynthesis, degradation, and salvage. This pathway depicts a number of pyrimidine-related processes such as nucleotide biosynthesis, degradation, and salvage. For pyrimidine nucleotide biosynthesis, carbamoyl phosphate derived from the action of carbamoyl phosphate synthetase II (CPS-II) on glutamine and bicarbonate is converted into carbamoyl aspartate by aspartate transcarbamoylase, ATCase. Dihydroorotic acid is subsequently generated by the action of carbamoyl aspartate dehydrogenase on carbamoyl aspartate. Dihydroorotate dehydrogenase then converts dihydroorotic acid to orotic acid. From this point, orotate phosphoribosyltransferase incorporates phosphoribosyl pyrophosphate into (PRPP) to produce orotidine monophosphate. Orotidine-5’-phosphate carboxylase subsequently converts orotidine monophosphate into uridine monophosphate (UMP). UMP is further phosphorylated twice to form UTP; the first instance by uridylate kinase and the second instance by ubiquitous nucleoside diphosphate kinase. UTP moves into the CTP synthesis pathway with the action of CTP synthase which aminates the molecule. The uridine nucleotides are also feedstock for the de novo thymine nucleotides synthesis pathway. DeoxyUMP which is derived from UDP or CDP metabolism is transformed by the action of thymidylate synthase into deoxyTMP of which the methyl group is sourced from N5,N10-methylene THF. THF is subsequently regenerated from DHF via dihydrofolate reductase (DHFR) which is essential for the continuation of thymidylate synthase activity. Serine hydroxymethyl transferase then acts on THF to regenerate N5,N10-THF. Pyrimidine synthesis is a comparatively simpler process than purine synthesis due to a couple of factors; pyrimidine ring structure is assembled as a free base rather being derived from PRPP and there is no branch in the pyrimidine synthesis pathway as opposed to the purine synthesis pathway. For thymidine, the action of thymidine kinase on it (or alternatively deoxyuridine) plays an important role in what is referred to as the salvage pathway to dTTP synthesis. However to form dTMP, the action of thymine phosphorylase and thymidine kinase is required. For deoxycytidine, deoxycytidine kinase is required (deoxycytidine also acts on deoxyadenosine and deoxyguanosine). For uracil, UMP can be formed by the action of uridine phosphorylase and uridine kinase on uracil. Pyrimidine catabolism ultimately results in the formation of the waste products of urea, H2O, and CO2. The product of cytosine breakdown, uracil, can be broken down to N-carbamoyl-β-alanine which can be catabolized into β-alanine. The product of thymine breakdown is β-aminoisobutyrate. The transamination of α-ketoglutarate to glutamate requires both of these breakdown products (β-alanine and β-aminoisobutyrate) to act as amine group donors. The products of this transamination can move through a further reaction that produces malonyl-CoA or methylmalonyl-CoA, a precursor for succinyl-CoA which is used in the Krebs cycle.

A group of heterocyclic aromatic organic compound, pyrimidines are similar in structure to benzene and pyridine and count the nucleic acids cytosine, thymine, and uracil as structural derivatives. The following pathway illustrates a many pyrimidine-associated processes such as nucleotide biosynthesis, degradation, and salvage. This pathway depicts a number of pyrimidine-related processes such as nucleotide biosynthesis, degradation, and salvage. For pyrimidine nucleotide biosynthesis, carbamoyl phosphate derived from the action of carbamoyl phosphate synthetase II (CPS-II) on glutamine and bicarbonate is converted into carbamoyl aspartate by aspartate transcarbamoylase, ATCase. Dihydroorotic acid is subsequently generated by the action of carbamoyl aspartate dehydrogenase on carbamoyl aspartate. Dihydroorotate dehydrogenase then converts dihydroorotic acid to orotic acid. From this point, orotate phosphoribosyltransferase incorporates phosphoribosyl pyrophosphate into (PRPP) to produce orotidine monophosphate. Orotidine-5’-phosphate carboxylase subsequently converts orotidine monophosphate into uridine monophosphate (UMP). UMP is further phosphorylated twice to form UTP; the first instance by uridylate kinase and the second instance by ubiquitous nucleoside diphosphate kinase. UTP moves into the CTP synthesis pathway with the action of CTP synthase which aminates the molecule. The uridine nucleotides are also feedstock for the de novo thymine nucleotides synthesis pathway. DeoxyUMP which is derived from UDP or CDP metabolism is transformed by the action of thymidylate synthase into deoxyTMP of which the methyl group is sourced from N5,N10-methylene THF. THF is subsequently regenerated from DHF via dihydrofolate reductase (DHFR) which is essential for the continuation of thymidylate synthase activity. Serine hydroxymethyl transferase then acts on THF to regenerate N5,N10-THF. Pyrimidine synthesis is a comparatively simpler process than purine synthesis due to a couple of factors; pyrimidine ring structure is assembled as a free base rather being derived from PRPP and there is no branch in the pyrimidine synthesis pathway as opposed to the purine synthesis pathway. For thymidine, the action of thymidine kinase on it (or alternatively deoxyuridine) plays an important role in what is referred to as the salvage pathway to dTTP synthesis. However to form dTMP, the action of thymine phosphorylase and thymidine kinase is required. For deoxycytidine, deoxycytidine kinase is required (deoxycytidine also acts on deoxyadenosine and deoxyguanosine). For uracil, UMP can be formed by the action of uridine phosphorylase and uridine kinase on uracil. Pyrimidine catabolism ultimately results in the formation of the waste products of urea, H2O, and CO2. The product of cytosine breakdown, uracil, can be broken down to N-carbamoyl-β-alanine which can be catabolized into β-alanine. The product of thymine breakdown is β-aminoisobutyrate. The transamination of α-ketoglutarate to glutamate requires both of these breakdown products (β-alanine and β-aminoisobutyrate) to act as amine group donors. The products of this transamination can move through a further reaction that produces malonyl-CoA or methylmalonyl-CoA, a precursor for succinyl-CoA which is used in the Krebs cycle.

A group of heterocyclic aromatic organic compound, pyrimidines are similar in structure to benzene and pyridine and count the nucleic acids cytosine, thymine, and uracil as structural derivatives. The following pathway illustrates a many pyrimidine-associated processes such as nucleotide biosynthesis, degradation, and salvage. This pathway depicts a number of pyrimidine-related processes such as nucleotide biosynthesis, degradation, and salvage. For pyrimidine nucleotide biosynthesis, carbamoyl phosphate derived from the action of carbamoyl phosphate synthetase II (CPS-II) on glutamine and bicarbonate is converted into carbamoyl aspartate by aspartate transcarbamoylase, ATCase. Dihydroorotic acid is subsequently generated by the action of carbamoyl aspartate dehydrogenase on carbamoyl aspartate. Dihydroorotate dehydrogenase then converts dihydroorotic acid to orotic acid. From this point, orotate phosphoribosyltransferase incorporates phosphoribosyl pyrophosphate into (PRPP) to produce orotidine monophosphate. Orotidine-5’-phosphate carboxylase subsequently converts orotidine monophosphate into uridine monophosphate (UMP). UMP is further phosphorylated twice to form UTP; the first instance by uridylate kinase and the second instance by ubiquitous nucleoside diphosphate kinase. UTP moves into the CTP synthesis pathway with the action of CTP synthase which aminates the molecule. The uridine nucleotides are also feedstock for the de novo thymine nucleotides synthesis pathway. DeoxyUMP which is derived from UDP or CDP metabolism is transformed by the action of thymidylate synthase into deoxyTMP of which the methyl group is sourced from N5,N10-methylene THF. THF is subsequently regenerated from DHF via dihydrofolate reductase (DHFR) which is essential for the continuation of thymidylate synthase activity. Serine hydroxymethyl transferase then acts on THF to regenerate N5,N10-THF. Pyrimidine synthesis is a comparatively simpler process than purine synthesis due to a couple of factors; pyrimidine ring structure is assembled as a free base rather being derived from PRPP and there is no branch in the pyrimidine synthesis pathway as opposed to the purine synthesis pathway. For thymidine, the action of thymidine kinase on it (or alternatively deoxyuridine) plays an important role in what is referred to as the salvage pathway to dTTP synthesis. However to form dTMP, the action of thymine phosphorylase and thymidine kinase is required. For deoxycytidine, deoxycytidine kinase is required (deoxycytidine also acts on deoxyadenosine and deoxyguanosine). For uracil, UMP can be formed by the action of uridine phosphorylase and uridine kinase on uracil. Pyrimidine catabolism ultimately results in the formation of the waste products of urea, H2O, and CO2. The product of cytosine breakdown, uracil, can be broken down to N-carbamoyl-β-alanine which can be catabolized into β-alanine. The product of thymine breakdown is β-aminoisobutyrate. The transamination of α-ketoglutarate to glutamate requires both of these breakdown products (β-alanine and β-aminoisobutyrate) to act as amine group donors. The products of this transamination can move through a further reaction that produces malonyl-CoA or methylmalonyl-CoA, a precursor for succinyl-CoA which is used in the Krebs cycle.

The metabolism of pyrimidines begins with L-glutamine interacting with water molecule and a hydrogen carbonate through an ATP driven carbamoyl phosphate synthetase resulting in a hydrogen ion, an ADP, a phosphate, an L-glutamic acid and a carbamoyl phosphate. The latter compound interacts with an L-aspartic acid through a aspartate transcarbamylase resulting in a phosphate, a hydrogen ion and a N-carbamoyl-L-aspartate. The latter compound interacts with a hydrogen ion through a dihydroorotase resulting in the release of a water molecule and a 4,5-dihydroorotic acid. This compound interacts with an ubiquinone-1 through a dihydroorotate dehydrogenase, type 2 resulting in a release of an ubiquinol-1 and an orotic acid. The orotic acid then interacts with a phosphoribosyl pyrophosphate through a orotate phosphoribosyltransferase resulting in a pyrophosphate and an orotidylic acid. The latter compound then interacts with a hydrogen ion through an orotidine-5 '-phosphate decarboxylase, resulting in an release of carbon dioxide and an Uridine 5' monophosphate. The Uridine 5' monophosphate process to get phosphorylated by an ATP driven UMP kinase resulting in the release of an ADP and an Uridine 5--diphosphate. Uridine 5-diphosphate can be metabolized in multiple ways in order to produce a Deoxyuridine triphosphate. 1.-Uridine 5-diphosphate interacts with a reduced thioredoxin through a ribonucleoside diphosphate reductase 1 resulting in the release of a water molecule and an oxidized thioredoxin and an dUDP. The dUDP is then phosphorylated by an ATP through a nucleoside diphosphate kinase resulting in the release of an ADP and a DeoxyUridine triphosphate. 2.-Uridine 5-diphosphate interacts with a reduced NrdH glutaredoxin-like protein through a Ribonucleoside-diphosphate reductase 1 resulting in a release of a water molecule, an oxidized NrdH glutaredoxin-like protein and a dUDP. The dUDP is then phosphorylated by an ATP through a nucleoside diphosphate kinase resulting in the release of an ADP and a DeoxyUridine triphosphate. 3.-Uridine 5-diphosphate is phosphorylated by an ATP-driven nucleoside diphosphate kinase resulting in an ADP and an Uridinetriphosphate. The latter compound interacts with a reduced flavodoxin through ribonucleoside-triphosphate reductase resulting in the release of an oxidized flavodoxin, a water molecule and a Deoxyuridine triphosphate 4.-Uridine 5-diphosphate is phosphorylated by an ATP-driven nucleoside diphosphate kinase resulting in an ADP and an Uridinetriphosphate The uridine triphosphate interacts with a L-glutamine and a water molecule through an ATP driven CTP synthase resulting in an ADP, a phosphate, a hydrogen ion, an L-glutamic acid and a cytidine triphosphate. The cytidine triphosphate interacts with a reduced flavodoxin through a ribonucleoside-triphosphate reductase resulting in the release of a water molecule, an oxidized flavodoxin and a dCTP. The dCTP interacts with a water molecule and a hydrogen ion through a dCTP deaminase resulting in a release of an ammonium molecule and a Deoxyuridine triphosphate. 5.-Uridine 5-diphosphate is phosphorylated by an ATP-driven nucleoside diphosphate kinase resulting in an ADP and an Uridinetriphosphate The uridine triphosphate interacts with a L-glutamine and a water molecule through an ATP driven CTP synthase resulting in an ADP, a phosphate, a hydrogen ion, an L-glutamic acid and a cytidine triphosphate. The cytidine triphosphate then interacts spontaneously with a water molecule resulting in the release of a phosphate, a hydrogen ion and a CDP. The CDP then interacts with a reduced NrdH glutaredoxin-like protein through a ribonucleoside-diphosphate reductase 2 resulting in the release of a water molecule, an oxidized NrdH glutaredoxin-like protein and a dCDP. The dCDP is then phosphorylated through an ATP driven nucleoside diphosphate kinase resulting in an ADP and a dCTP. The dCTP interacts with a water molecule and a hydrogen ion through a dCTP deaminase resulting in a release of an ammonium molecule and a Deoxyuridine triphosphate. 6.-Uridine 5-diphosphate is phosphorylated by an ATP-driven nucleoside diphosphate kinase resulting in an ADP and an Uridinetriphosphate The uridine triphosphate interacts with a L-glutamine and a water molecule through an ATP driven CTP synthase resulting in an ADP, a phosphate, a hydrogen ion, an L-glutamic acid and a cytidine triphosphate. The cytidine triphosphate then interacts spontaneously with a water molecule resulting in the release of a phosphate, a hydrogen ion and a CDP. The CDP interacts with a reduced thioredoxin through a ribonucleoside diphosphate reductase 1 resulting in a release of a water molecule, an oxidized thioredoxin and a dCDP. The dCDP is then phosphorylated through an ATP driven nucleoside diphosphate kinase resulting in an ADP and a dCTP. The dCTP interacts with a water molecule and a hydrogen ion through a dCTP deaminase resulting in a release of an ammonium molecule and a Deoxyuridine triphosphate. The deoxyuridine triphosphate then interacts with a water molecule through a nucleoside triphosphate pyrophosphohydrolase resulting in a release of a hydrogen ion, a phosphate and a dUMP. The dUMP then interacts with a methenyltetrahydrofolate through a thymidylate synthase resulting in a dihydrofolic acid and a 5-thymidylic acid. Then 5-thymidylic acid is then phosphorylated through a nucleoside diphosphate kinase resulting in the release of an ADP and thymidine 5'-triphosphate.

Cytidine and uridine are transported through their corresponding nucleoside hydrogen symporters. Once cytidine is incorporated into the cytosol, it is deaminated through a reaction with water and a hydrogen ion through a cytidine deaminase resulting in the release of ammonium and uridine. Uridine is then lyased by a phosphate through a uridine phosphorylase resulting in the release of a uracil and an alpha-D-ribose-1-phosphate. This compound is then transformed into an isomer D-ribose 5-phosphate through an alpha-D-ribose 1,5-phosphomutase. This compound is then incorporated into the pentose phosphate pathway.