生物化学课件：3 nucleotides synthsis

1、Biosynthesis and Degradation of Nucleotides,Contents,De novo purine nucleotide synthesis De novo pyrimidine nucleotide synthesis Nucleoside monophoshates are converted to nucleoside triphosphates Ribonucleotides are the precursors of deoxy-ribonucleotides Degradation of purine and pyrimidine The sal

2、vage pathways for purine and pyrimidine Many chemotherapeutic agents target enzymes in the nucleotide biosynthetic pathways,Two types of pathway lead to nucleotides,De novo synthesis of nucleotides begins with their metabolic precursors: amino acids, ribose 5-phosphate, NH3, and CO2. Salvage pathway

3、s recycle the free bases and nucleosides released from nucleic acid breakdown.,Overview,The purine ring is assembled on ribose phosphate to make AMP and GMP; the pyrimidine ring is first synthesized as orotate, which is then attached to ribose phosphate before being converted to UTP and CTP (dTMP is

4、 made from dUMP). The deoxyribonucleotides (dNDPs) are synthesized by reduction of ribonucleotides (NDPs).,The free bases are not intermediates during the de novo synthesis,The origin of the atoms in the purine and pyrimidine rings,Radioisotope tracer experiments revealed the origin of the atoms in

5、the purine and pyrimidine rings Buchanan and Greenberg did this by feeding a variety of isotopically labeled compounds to pigeons (1940s). The atoms of the purine rings were found to be derived from formate, CO2, Gly, Asp, and Gln. The atoms of the pyrimidine rings were found to be derived from Asp,

6、 Gln and HCO3-.,Radioisotope tracer experiments revealed the origins of the ring atoms of purines,Gln amide,HCO3-,C,The atoms of the pyrimidine rings are derived from HCO3-, Gln and Asp.,1. De novo purine nucleotide synthesis begins with PRPP,PRPP is synthesized from ribose 5-P in a Reaction catalyz

7、ed by PRPP synthetase.,The bases are not synthesized and then attached to ribose,The purine ring is built up one or a few atoms at a time, attached to ribose throughout the process.,The enzymes of IMP synthesis appear to be organized as large multienzyme complexes,Steps 1, 3, and 5; steps 7 and 8 ;s

8、teps 10 and 11 are catalyzed by a multifunctional protein respectively in some eukaryotic cells. In bacteria, these activities are found on separate proteins, but a large noncovalent complex may exist in these cells.,amidotransferase,The biosynthesis of AMP and GMP is regulated by feedback inhibitio

9、n,Three major feedback mechanism cooperate PRPP synthetase and glutamine-PRPP amidotransferase are both inhibited by the end products IMP, AMP and GMP (AMP and GMP act synergistically). AMP inhibits adenylosuccinate synthetase and GMP inhibits IMP dehydrogenase. AMP and GMP synthesis is balanced by

10、the following mechanism: GTP is needed for AMP synthesis and ATP for GMP synthesis.,The de novo synthesis of AMP and GMP is regulated mainly by sequential feedback Inhibition.,Summary,The first intermediate to have a complete purine ring is IMP (inosinate次黄嘌呤单核苷酸). IMP is converted to AMP by accepti

11、ng an amino group from Asp, and converted to GMP by accepting an amino group from Gln (catalyzed by an amidotransferase). The production of AMP from IMP requires GTP, and the production of GMP from IMP requires ATP.,2. De novo pyrimidine nucleotide synthesis,Pyrimidine(嘧啶) nucleotides are made from

12、aspartate, PRPP, and carbamoyl phosphate The pyrimidine ring is synthesized as orotate, attached to ribose phosphate, and then converted to the common pyrimidine mucleotides used in nucleic acid synthesis (the six-numbered pyrimidine ring is made first and then attached to ribose 5-phosphate).,The p

13、yrimidine ring is synthesized as orotate (乳清酸), attached to ribose phosphate, and then converted to the common pyrimidine mucleotides used in nucleic acid synthesis (the six-numbered pyrimidine ring is made first and then attached to ribose 5-phosphate).,CTP is derived from UTP by accepting an amino

14、 group from Gln or NH4+. In eukaryotic cells, carbamoyl phosphate synthetase II, aspartate transcarbamoylase, and dihydroorotase, are part of a trifunctional proteins called CAD.,Synthesis of carbamoyl phosphate,In animals the carbamoyl phosphate synthesis requires different enzymes and takes place

15、in different cellular compartments in urea synthesis(synthetase I, mitochondra) and in pyrimidine synthesis(synthetase II, cytosol). In bacteria, a single enzyme supplies carbomoyl phosphate for the synthesis of arginine and pyrimidines,Pyrimidine nucleotide biosynthesis is regulated by feedback inh

16、ibition,Aspartate transcarbamoylase (ATCase) is inhibited by CTP, the end product of the de novo pyrimidine nucleotide biosynthesis pathway. CTP binds to the regulatory subunits, they undergo a changes in conformation. This change is transmitted to the catalytic subunits. ATP is able to prevent the

17、changes induced by CTP.,3. Nucleoside monophosphates are converted to nucleoside triphosphaates,Nucleoside monophosphate kinases ATP +NMP ADP + NDP specific for the base, nonspecific for the sugar,Nucleoside dimonophosphate kinases NTPD +NDPA NDPD + NTPA not specific for the base or the sugar NTPD i

18、s almost invariably ATP,4. Deoxyribonucleotides are derived from ribonucleotides at the NDP level,This occurs by direct reduction at the 2-carbon. Ribonucleotide reductase catalyzes all such conversions. The electrons are provided by NADPH and transmitted to the ribonucleotide reductase through thio

19、redoxin(硫氧还蛋白)or glutaredoxin (谷氧还蛋白).,dTMP is derived from dCDP and dUMP,dTMP is derived from dUMP via a methylation reaction using N5, N10-methylene H4 folate as donors of both one-carbon unit and electrons.,dUMP is formed from dUTP in a reaction catalyzed by dUTPase (keeping dUTP at a low level t

20、o prevent its incorporation into DNA). dUMP is then converted to dTMP by the catalysis of thymidylate synthase(合酶）, with a methylene group transferred from and reduced by N5, N10-methylene H4 folate (being donors of both one-carbon unit and electrons). The reactions catalyzed by ribonucleotide reduc

21、tase and thymidylate synthase are probably key for the transition from an RNA world to one in which DNA stores genetic information.,5. Degradation of purine and pyrimidine,Uric acid is the excreted end product of purine catabolism in humans and many other animals. The pathways for degradation of pyr

22、imidine generally lead to NH4+ production and thus to urea synthesis.,(尿囊素）,(尿囊酸),(黄嘌呤),次黄嘌呤，6-羟基嘌呤,Uric acid can be further converted to allantoin, allantoate, urea or NH4+ in various animals. The deficiency of adenosine deaminase causes the severe immunodeficiency disease in humans (accumulated ad

23、enosine is converted to dATP, which inhibits the formation of other dNDPs by ribonucleotide reductase). Overproduction of uric acid causes gout (痛风). Allopurinol, an inhibitor of xanthine oxidase, is used to treat gout.,Allopurinol was designed to be a competitive inhibitor of xanthine oxidase to tr

24、eat gout by Elion and Hitchings, who shared the Nobel Prize in 1988 for their discoveries of important principles for drug treatment,Degradation of pyrimidine produces urea,The degradation of thymine can produce succinyl-CoA. The degradation of uracil and cytidine produces malonyl-CoA, which is one

25、precursor for fatty acid biosynthesis. To a limited extent, catabolism of pyrimidine nucleotides contributes to the energy metabolism of the cell.,Degradation products of pyrimdines can enter the citric acid cycle,Succinyl-CoA,Fourth lecture,Radioisotope tracer experiments revealed the origins of th

26、e ring atoms of purines,Gln amide,HCO3-,C,The atoms of the pyrimidine rings are derived from HCO3-, Gln and Asp.,6. Purine and pyrimidine bases are recycled by salvage pathway,Adenine phosphoribosyltransferase catalyzes the synthesis of AMP from adenine and PRPP. Hypoxanthine-guanine phosphoribosylt

27、ransferase (HGPRT) catalyzes the synthesis of GMP and IMP. The lack of HGPRT will cause Lesch-Nyhan syndrome(自毁容貌综合症). Pyrimidine bases are recycled in a similar way in microorganisms, but pyrimidine bases does not seem to be salvaged in significant amounts in mammals.,The purine bases can be conver

28、ted to purine nucleotides through the salvage pathways,7.Many chemotherapeutic agents target enzymes in the nucleotide biosynthetic pathways,Analogs of Gln, like azaserine (重氮丝氨酸) and acivicin (阿西维辛) , inhibits many amidotransferases used in nucleotide (and amino acid) biosynthesis. Fluorouracil, af

29、ter being converted to FdUMP by the salvage pathway, can inhibit the thymidylate synthase, thus inhibit the dTMP synthesis. Methotrexate(氨甲碟呤）, a folate analog, inhibits the dihydrofolate reductase, thus the dTMP synthesis.,Azaserine(重氮丝氨酸) and acivicin (阿西维辛), inhibitors of amidotransferases. These

30、 analogs of glutamine interfere in a number of amino acid and nucleotides biosynthetic pathways,Purine nucleotides are synthesized from PRPP, Gln, Gly, N10-formyl H4 folate, Gln, HCO3-, Asp through the de novo pathway. Pyrimidine nucleotides are synthesized using HCO3-, Gln, Asp, and PRPP. De novo s

31、ynthesis of nucleotides are regulated via feedback inhibition. Deoxyribonucleotides are derived from ribonucleotides at the NDP level.,Summary,The dTMP molecule is derived from dUMP by thymidylate synthase, an enzyme using N5, N10-methylene-tetrahydrofolate as the donor of both one-carbon unit and electrons. Degradation of purines and pyrimidines produces ur