Nucleotides, Folic Acid, Diseases and Meds Flashcards
Role of Nucleotides
Nucleotides are critically important cellular constituents. They serve as the energy “currency” of the cell, as “second messengers” in signal transduction cascades and as building blocks for DNA and RNA.
Purine Bases
- purine
- adenine
- guanine
- hypoxanthine
- xanthine
[…], a product of the pentose phosphate pathway is the major precursor for purine biosynthesis.
Ribose-5-phosphate, a product of the pentose phosphate pathway is the major precursor for purine biosynthesis
The first purine nucleotide (base + sugar + phosphate) produced is […].
The first purine nucleotide (base + sugar + phosphate) produced is inosinic acid (IMP).
Synthesis of 5-phosphoribosyl-1-pyrophosphate (PRPP) is catalyzed by […]. PRPP is used in the synthesis of both purine and pyrimidine nucleotides by providing the ribose sugar and the alpha-phosphate. The subsequent reaction in purine biosynthesis is catalyzed by[…]. The final reaction requires […] with […] as co-factor.
Synthesis of 5-phosphoribosyl-1-pyrophosphate (PRPP) is catalyzed by PRPP synthetase. PRPP is used in the synthesis of both purine and pyrimidine nucleotides by providing the ribose sugar and the alpha-phosphate. The subsequent reaction in purine biosynthesis is catalyzed by PRPP amidotransferase. The final reaction requires N10-formyl THF with amino acids Gly + Gln + Asp.
Purine Synthesis
- Adenosine monophosphate (AMP) and guanine monophosphate (GMP) are synthesized from inosine monophosphate via separate pathways.
- Synthesis of GMP requires nitrogen from gln and is inhibited by drugs, mycophenolate and ribavirin, which decrease activity of IMP dehydrogenase.
- AMP acquires nitrogen from asp.
- The monophosphate products are phosphorylated to the diphosphate and triphosphate forms.
- The end result of the purine synthesis pathway is the production of the purine ribonucleotides.
- Formation of purine deoxyribonucleotides is catalyzed by ribonucleotide reductase.
Allosteric inhibition of purine biosynthesis; […] and […] inhibit PRPP Synthetase and PRPP aminotransferase. In addition to this allosteric inhibition, […] allosterically stimulates formation of […] and […] allosterically stimulates the formation of […].
Allosteric inhibition of purine biosynthesis; GMP and AMP inhibit PRPP Synthetase and PRPP aminotransferase. In addition to this allosteric inhibition, ATP allosterically stimulates formation of GMP and GTP allosterically stimulates the formation of AMP.
Purine Nucleotide Degradation
- Degradation of purine nucleotides commences with the removal of phosphate from the nucleotide form, yielding a nucleoside (base + sugar).
- Adenosine is first converted to inosine by adenosine deaminase.
- The sugar group is removed by purine nucleoside phosphorylase to produce the purine bases guanine and hypoxanthine, as well as ribose-1- phosphate.
- The base products are converted to xanthine, which is oxidized by xanthine oxidase to uric acid. (Note that xanthine oxidase also converts hypoxanthine to xanthine.)
Uric Acid
- Uric acid, the purine degradative product, is a weak acid with a 5.8 pK [pH at which it is 50% ionized].
- Urate, the ionized form, is more water-soluble than is the protonated form.
-When urine is at a pH of 6.8, the molecule is 90% ionized and is 10-times more soluble than when urine is at pH 4.8 with only 10% ionized.
•Since the pH of urine is normally below 5.8, overproduction of uric acid can lead to formation of stones in the urinary collection system.
Purine Salvage Pathway
- Besides de novo synthesis, purine nucleotides also can be formed directly from the purine bases via a salvage pathway.
- The primary enzyme of the salvage pathway is hypoxanthineguanine phosphoribosyl transferase.
- HGPRT attaches PRPP, derived from the PRPP synthetase reaction, either to hypoxanthine to regenerate IMP or to guanine to restore GMP.
- Thus the salvage pathway prevents the irreversible destruction of hypoxanthine, guanine and adenine. Instead these purine bases can be reutilized.
- Because of the high-energy demand of the de novo synthesis pathway, the salvage pathway is an energy saving process.
- This pathway is also important for the salvage of dietary nucleotides. By salvaging the purine bases, production of uric acid is kept low, which is necessary for preventing gout.
- The salvage pathway is a critical source of purine nucleotides.
Disorders Asociated with Defects of Enzymes in the Purine Metabolic Pathway
•Hyperuricemia
-Type I Glycogen Storage Disease —> Gout
•Gout
*primary
*secondary
•Lesch-Nyhan Syndrome
Hyperuricemia - Type I Glycogen Storage Disease
- A defect of glucose-6-phosphatase in this disease increases the conversion of glucose-6-phosphate to ribose-5-phosphate via the pentose phosphate pathway.
- This in turn elevates production of purines through saturation of PRPP synthetase.
- Additionally, lactic acidosis associated with this disease can lower the pH of urine and thereby diminish excretion of uric acid as well.
Hyperuricemia - Gout
- Hyperuricemia may lead to gout, which describes clinically the physiological consequences associated with the excessive accumulation of uric acid in body fluids.
- The most common symptom of gout is arthritic pain in joints, which is caused by the deposition of urate crystals in cartilage surrounding the joint.
- Urate crystals may also develop into kidney stones.
- Gout occurs more commonly in men than in women, and even more rarely in premenopausal women. The overall prevalence of gout is about 1.4% of the population with an incidence of perhaps 7% in men over age 65.
Hyperuricemia - Gout Primary
- Primary gout is an inherited disorder
- Three different defects of PRPP synthetase have been identified.
- A superactive variant of this enzyme is associated with an increased Vmax
- An increased affinity (low Km) for ribose-5-phosphate thus leading to overproduction of PRPP.
- The loss of feedback inhibition of this enzyme by purine nucleotides. Consequently, when purine nucleotides reach an excessive concentration, there is no signal for shutting off their further production.
•Moderate defects of HGPRT that allow for at least 50% of normal activity lead to gout caused by overproduction of uric acid because of the inability to salvage the purine bases from complete degradation.
Hyperuricemia - Gout Primary
Hyperuricemia - Gout Primary
Hyperuricemia - Gout Secondary
•secondary gout is a secondary consequence of a variety of disorders, such as leukemia
Treatment of Gout
•Acute
-anti-inflammatory medications (e.g., nonsteroidal anti-inflammatory drugs [indomethacin; naproxen]; colchicine; corticosteroids [prednisone])
•Long Term Management
- There are a variety of clinical interventions, including diet and medications, that are recommended in the long-term management of gout.
- Adequate fluid intake, weight reduction, dietary changes, and reduction in alcohol consumption are general approaches to increasing excretion or decreasing production of uric acid.
- In addition, patients with gout are advised to avoid consuming animal products rich in nucleic acids, especially the organ meats.
•First is the need, during an acute attack, to curtail the acute inflammation of joints affected by gouty arthritis. Thereafter, the long-term management of the disease focuses on preventing future attacks and diminishing the extent of gouty tophi crystal deposits.
Pharmacological Treatment of Gout I
•allopurinol
- a competitive inhibitor of xanthine oxidase
- Hypoxanthine and xanthine are excreted during allopurinol therapy since xanthine oxidase uses both of these purines as substrates
- purine nucleotide synthesis is lowered during allopurinol treatment.
- Reducing the formation of uric acid by allopurinol treatment relieves the symptoms and decreases the possibility that uric acid kidney stones will form.
•febuxostat
- also a xanthine oxidase inhibitor, appears to be more effective than allopurinol in preventing acute attacks and reducing the size of the tophi deposits
- because this drug is not metabolized by kidney, it may be a better choice of drugs in patients with kidney disease
•probenecid and sulfinpyrazone
- lower the blood concentration of uric acid by increasing its excretion through inhibition of uric acid reabsorption
- taken with large amount of fluids to increase rapid excretion of uric acid and thereby lessen the risk of renal stones
***azathioprine and 6 mercaptopurine are antineoplastic drugs used block the synthesis of nucleotides, affecting the synthesis of both RNA and DNA leading to decreased proliferation of cancer cells***
Hyperuricemia - Lesch Nyhan Syndrome
- Considerable overproduction of uric acid occurs in individuals diagnosed with Lesch-Nyhan syndrome due to a severe defect in the gene encoding for HGPRT.
- The amount of HGPRT activity in these patients is <5%.
- The disease is X-linked recessive and thus generally limited to males.
- Lesch-Nyhan syndrome has a very early age of onset.
- The syndrome, in its most severe form is characterized by extremely aggressive behavior, which generally leads to self-mutilation, and by mental retardation.
- The severity of the consequences of the syndrome supports the important role for the salvage pathway.
- Most likely the enzyme has an essential role in non-hepatic tissues where de novo synthesis of purines occurs at a very slow rate. Therefore, non-hepatic tissues depend on circulating purine bases or nucleosides derived from the liver.
-Non-hepatic tissues are thought to take up the circulating purines and, through the action of HGPRT, form nucleotides.
- Though allopurinol is available for treatment, the overproduction of uric acid is so severe that there is little benefit from its use.
- Patients usually do not survive beyond their early teens, though there are exceptions.
Pyrimidine Synthesis
- The pyrimidines, like the purines, are synthesized in a multi-step pathway.
- The first step of the pathway is catalyzed by carbamoyl phosphate synthetase II, a cytoplasmic enzyme. This enzyme uses glutamine as a nitrogen donor.
-The product of this reaction, carbamoyl phosphate, is identical chemically to the carbamoyl phosphate made in the mitochondria by carbamoyl phosphate synthetase I as part of the urea cycle.
•Pyrimidine biosynthesis continues with the addition of aspartate, via aspartate transcarbamoylase, to provide the remainder of the ring elements.
-Aspartate transcarbamoylase is highly regulated in bacteria but not in humans.
•The final product of pyrimidine synthesis is uridine monophosphate (UMP). This can be phosphorylated to the triphosphate form (UTP), which in turn is converted into cytidine triphosphate (CTP).
-Both of these pyrimidines along with ATP and GTP are required for synthesis of RNA.
•Formation of pyrimidine deoxyribonucleotides is catalyzed by ribonucleotide reductase.
Regulation of pyrimidine biosynthesis occurs at […]. This enzyme is feedback inhibited by […]. When […] is in excess, the enzyme is stimulated. To maintain a balance of purine and pyrimidine nucleotides, elevated amounts of purine nucleotides will also activate carbamoyl phosphate synthetase II.
Regulation of pyrimidine biosynthesis occurs at carbamoyl phosphate synthetase II. This enzyme is feedback inhibited by uridine nucleotides. When PRPP is in excess, the enzyme is stimulated. To maintain a balance of purine and pyrimidine nucleotides, elevated amounts of purine nucleotides will also activate carbamoyl phosphate synthetase II.
[…] is the rate determining and committed step of the purine synthetic pathway
PRPP amidotransferase is the rate determining and committed step of the purine synthetic pathway
Disorders Associated with Defects of Enzymes in the Pyrimidine Metabolic Pathways
•Orotic Aciduria
-Orotic aciduria, an inherited autosomal recessive disorder of pyrimidine metabolism, is caused by a deficiency of the enzyme complex that includes both the orotate phosphoribosyl and orotidylic acid decarboxylase activities (Figure 8). As a result of this defect, orotic acid cannot be metabolized to UMP. Instead orotic acid accumulates in cells, and the body excretes it in large amounts. The pyrimidine pathway is diminished thereby limiting the de novo synthesis of uridine and cytidine nucleotides. Because of the nucleotide pool imbalance (purines favored over pyrimidines), children exhibit a failure to thrive. The disease is treated by administration of oral uridine that can be converted to UMP to bypass the enzymatic defect.
