M.G. Marinus Page 4 Note that like any other biochemical pathway, there is control by feedback inhibition. IMP feedback inhibits the first enzyme in the pathway. Purine Uptake Purines are obtained from food and from the breakdown of RNA and DNA and a portion of the latter is recycled (”salvaged”) to make nucleotides. This involves two enzymes: HGPRTase(hypoxanthine-guanine phosphoribosyltransferase) and APRTase (adenine phosphoribosyltransferase). The general reaction that they catalyze is shown below.

Nomenclature Base Base Nucleoside Nucleotide Formula X=H X=ribose or X=ribose or deoxyribose deoxyribose phosphates N N N N NH 2 X adenine adenosine AMP ade AR adenylic acid A adenosine monophosphate ADP, ATP deoxyadenosine dAMP, dADP, dATP AdR guanine guanosine GMP NH N O NH 2 gua GR guanylic acid G guanosine monophosphate N N X GDP, GTP deoxyguanosine dGMP, dGDP, dGTP GdR hypoxanthine inosine IMP N O N NH 2 X N O N O H X N N CH 3 O O H X inosinic acid HN N N N O X cytosine cytidine CMP cyt CR cytidylic acid C cytidine monophosphate CDP, CTP deoxycytidine dCMP, dCDP, dCTP CdR uracil uridine UMP ura UR uridylic acid U uridine monophosphate UDP, UTP deoxyuridine dUDP, dUTP UdR thymine thymidine TMP thy TdR thymidylic acid T thymidine monophosphate TDP, TTP….. Nucleotide reduction At this point in the saga, the synthesis of ATP, GTP, CTP and UTP has been described. Thus all the building blocks necessary for the synthesis of RNA are ready. To make DNA, however, the ribonucleotides need to be reduced. The general reaction is shown below and catalyzed by ribonucleoside diphosphate reductase (RR) using NADPH. RR NTP ? NDP ? dNDP ? dNTP The products of the reaction are dATP, dGTP, dCTP and dUTP. The dUTP can be used efficiently by DNA polymerase and to prevent this, it is converted to dUMP by deoxyuridine triphosphatase. Hydroyurea is a potent inhibitor of RR. Its structure is shown below. H 2 N C NHOH O Note that it has no structural similarity to the enzyme’s substrates. Will the synthesis of RNA or DNA be affected first after addition of hydroxyurea to cells? Hydroxyurea will inhibit RR from any source. Used in combination for the treatment of leukemias. Resistance mechanisms include increased RR activity via gene amplification. Formation of TMP The dUMP is converted to TMP by the action of thymidylate synthase (TS) as shown below. This reaction requires THF.C1 which is converted to DHF (dihydrofolic acid). N O N O H O HOH — O 3 PO N O N O H O HOH — O 3 PO CH 3 THF.C1 DHF TS dUMP TMP Following the synthesis of TMP, it is kinased to TTP and now all the building blocks are present to make DNA.

Download Medical Biochemistry Nucleotide Metabolism.Pdf