Nucleotide Metabolism Biochemistry by Lippincott pp

Transcription

1 Nucleotide Metabolism Biochemistry by Lippincott pp

2 Metabolism: CONCEPT Ø Metabolism is the totality of an organism s chemical reactions. Ø A metabolic pathway begins with a specific molecule and ends with a product. Ø Each step is catalyzed by a specific enzyme. The processes of metabolism can degrade OR synthesize compounds. The process of degradation is called catabolism and that of synthesis is called anabolism A Enzyme 1 B Enzyme 2 C Enzyme 3 D Reaction 1 Reaction 2 Reaction 3 starting molecule Product

3 Why study nucleotides? Nucleotides are the building blocks of DNA and RNA. Nucleotides serve as carriers of activated intermediates in the synthesis of some carbohydrates, lipids, and proteins. Nucleotides are structural components of coenzymes, including coenzyme A (CoA), FAD, NAD+, and NADP+. Nucleotides (ATP and GTP) act as "energy currency" in the cell. The nucleotides cyclic AMP (camp) and cgmp are important regulatory (controlling) compounds for many intermediary metabolism pathways.

4 NUCLEOTIDE STRUCTURE Purines Pyrimidines Nucleotides consist of: a nitrogenous base, a pentose monosaccharide (sugar with 5 carbons), and one, two, or three phosphate groups. The nitrogen-containing bases belong to two families of compounds: the purines and the pyrimidines.

5 Nucleosides and Nucleotides The addition of a pentose sugar to a base through a glycosidic bond produces a nucleoside. If the sugar is ribose, a ribonucleoside is produced; if the sugar is 2-deoxyribose, a deoxyribonucleoside is produced DNA has deoxyribose sugar in them and RNA has ribose sugar Addition of phosphate group to nucleoside produces nucleotide The first phosphate group is attached to 5 -OH of the pentose by a ester bond. If one phosphate group is attached to the 5 -carbon of the pentose the structure is a nucleoside monophosphate like adenosine monophosphate (AMP) If a second or third phosphate is added to the nucleoside, a nucleoside diphosphate (ADP) or nucleoside triphosphate (ATP). The second and third phosphates are each connected to the nucleoside monophosphate by a high energy bond.

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7 DNA & RNA Nucleotides The DNA and RNA also differ in the nitrogenous bases present Both DNA & RNA contain: The same purine bases: adenine (A) and guanine (G). The pyrimidine cytosine (C). Purines DNA contains thymine (T) whereas RNA contains uracil (U). T and U differ only by one methyl group present on T but absent on U. So DNA have A,G,C,T and RNA has A,G,C,U.

8 Naming Nucleotides R N A D N A

9 Purine Synthesis De Novo Pathway: The atoms of the purine ring are contributed by a number of compounds including amino acids, carbon dioxide and N10- formyltetrahydrofolate. The ring is made in liver by reactions that add the donated carbons and nitrogens to a preformed ribose5-phosphate. Salvage Pathway: Purines also obtained by the degradation of nucleic acids and nucleotides from Normal turnover of cellular nucleic acids Diet (meat especially organs like liver or kidneys and tomatoes)

10 de novo Purine Synthesis The atoms of the purine ring are contributed by aspartic acid, glycine, and glutamine, CO 2, and derivatives of tetrahydrofolate. First, but not committed step of purine synthesis: 5-phosphoribosyll-pyro-phosphate (PRPP) formation. The enzyme is PRPP synthetase The committed step in forming purines: Synthesis of 5'- phosphoribosylamine (PRA). The enzyme is amido-phosphoribosyl transferase. Highly regulated by feedback inhibition.

11 de novo Purines Synthesis Purines PRPP SYNTHETASE The next 9 steps in purine synthesis lead to synthesis of inosine monophosphate [IMP]. This pathway requires four ATP molecules as an energy source.

12 AMP & GMP Formation A two-step, energy-requiring pathway: AMP synthesis requires GTP, GMP synthesis requires ATP. The first reaction in each pathway is inhibited by the end product of that pathway. If AMP and GMP are present in adequate amounts, the de novo pathway of purine synthesis is turned off at an earlier step. Inhibited by AMP, GMP, IMP

13 SALVAGE PATHWAY Preformed purines can be derived from turnover of cellular nucleic acids, or from the diet. They are packaged into mature nucleotides by the "salvage pathway" for purines. Two enzymes are involved in the salvage pathway, which utilize PRPP as the source of ribose 5-phosphate group. The enzymes are adenine phosphoribosyltransferase (APRT) which act on adenine whereas hypoxanthine-guanine phosphoribosyltransferase (HGPRT) acting of hypoxanthine and guanine. q A deficiency of HGPRT causes the Lesch-Nyhan syndrome. This results in decreased utilization of purines in making nucleotides by salvage pathway thus increases synthesis of purines nucleotides by de novo synthesis. The decreased utilization and increased synthesis of purines results in increased degradation of purines and the production of large amounts of uric acid causing hyperuricemia frequently resulting in formation of uric acid stones in kidneys and crystals in joints and soft tissues. Inaddition syndrome is characterized by mental disorders.

14 Can be utilized in salvage pathway also ie why formation of PRPP is not committed to de novo synthesis PRPP SYNTHETASE

15 Regulation of Purine Synthesis Purine synthesis: tightly regulated at the level of individual nucleotides. Coordinate the relative amounts of ATP and GTP. Exacted at late and earlier levels Increase in adenine nucleotides: stimulate conversion of IMP to GMP Increase in guanylate stimulate conversion of IMP to AMP. Negative feedback inhibition at earlier stages, leading to reduced IMP concentrations Distant Intermediate Immediate