Thymidine triphosphate synthesis in senescent WI38 …
Much of what is known about pathways of dNTP synthesis in bacterial and animal cells came from investigations of the biosynthesis of unusual modified nucleotides in infection by certain bacteriophage (11). T-even bacteriophages of E. coli contain 5-hydroxymethylcytosine completely substituted for cytosine; many hydroxymethyldeoxycytidylate (HM-dCMP) residues in phage DNA are further modified by glycosidic links through the hydroxymethyl group to one or two glucose residues. These modifications occur through the action of phage-encoded enzymes that catalyze reactions comparable to those in cellular metabolism (Fig. 4). For example, the hydroxymethylation reaction is carried out by an enzyme, dCMP hydroxymethylase, that transfers a single-carbon group from methylenetetrahydrofolate to C-5 of dCMP, much as thymidylate synthase modifies C-5 of the pyrimidine dUMP (12); in fact, T-even phages encode a thymidylate synthase that displays significant amino acid sequence homology with dCMP hydroxymethylase. Not shown in Figure 4 is the involvement of glucosylation in the transfer of glucose to hydroxymethyl groups of HM-dCMP residues after their incorporation into DNA.
intracellularly to the dTTP triphosphate form, ..
Figure 3. dCMP deaminase as a branch point between dCTP and dTTP synthesis. Allosteric modifiers of dCMP deaminase are shown, as is the regulation of salvage synthetic pathways. Enzyme 1, ribonucleoside diphosphate reductase; 2, nucleoside diphosphate kinase; 3, dCMP kinase (possibly); 4, dUTPase; 5, dCMP deaminase; 6, dCMP kinase; 7, thymidylate synthase; 8, thymidylate kinase; 9, deoxycytidine kinase; 10, thymidine kinase.
Thymidylate synthase catalyzes the transfer of a single-carbon functional group from 5,10-methylenetetrahydrofolate to position 5 of the pyrimidine ring in deoxyuridine monophosphate, yielding thymidine monophosphate (Fig. 2). dTMP is phosphorylated to dTDP by , and conversion to dTTP involves nucleoside diphosphate kinase.
Thymidine triphosphate synthesis in senescent WI38 cells
Figure 1. Pathways of dNTP biosynthesis de novo. Although these pathways are widespread, they are not universal, as indicated in the text. Enzyme 1, ribonucleoside diphosphate reductase; 2, nucleoside diphosphate kinase; 3, dCMP kinase (probably); 4, dCMP deaminase; 5, dUTPase; 6, thymidylate synthase; 7, thymidylate kinase. CH2 = THF is 5,10-methylenetetrahydrofolate, and DHF is dihydrofolate.
THYMIDINE TRIPHOSPHATE SYNTHESIS IN TETRAHYMENA…
It is useful to begin our survey of deoxyribonucleotide biosynthesis by considering the processes through which the chemical differences between DNA nucleotides and RNA nucleotides arise. The methyl group of thymine, which distinguishes it from uracil, arises through the transfer of a single-carbon functional group to a uracil nucleotide, catalyzed by thymidylate synthase. The presence of 2-deoxyribose as the sugar in DNA nucleotides rather than the ribose found in RNA comes about through reduction of the ribose sugar on a ribonucleotide substrate (see Ribonucleotide Reductases). Some aerobic bacteria and all anaerobic microorganisms studied carry out this reduction at the ribonucleoside triphosphate (rNTP) level. In all other organisms studied, however, the substrates for ribonucleotide reductase are the ribonucleoside 5′ -diphosphates (rNDPs). Whether a particular reductase acts on rNDPs or rNTPs, a single enzyme reduces all four ribonucleotide substrates. Accordingly, the enzyme interacts with allosteric modifiers to ensure that the four DNA precursors are produced at rates commensurate with the base composition of the organism’s genome. In this article, we will describe the predominant pathways to dNTPs that begin with reduction of rNDPs to deoxyribonucleoside diphosphates (dNDPs). These pathways are summarized in Figure 1.
Thymine and thymidine synthesis - [PDF Document]
MacFarlane AJ, Anderson DD, Flodby P, et al. (2011) Nuclear localization of the thymidylate synthesis pathway is required to prevent uracil accumulation in DNA. Journal of Biological Chemistry 286: 44015–44022.
28/09/2004 · Herpes Simplex Thymidine Kinase ..
Field MS, Kamynina E, Watkins D, et al. (2015) Human mutations in methylenetetrahydrofolate dehydrogenase 1 impair nuclear de novo thymidylate biosynthesis. Proceedings of the National Academy of Sciences of the USA 112: 400–405.