Question 1: What is the initial step in the synthesis of both purine and pyrimidine nucleotides?
Answer: The initial step in the synthesis of both purine and pyrimidine nucleotides is the synthesis of PRPP (5-phosphoribosyl-1-pyrophosphate) from D-ribose-5’-P.
Question 2: What is the significance of Mycophenolic acid in purine nucleotide biosynthesis?
Answer: Mycophenolic acid is an inhibitor of IMP dehydrogenase. It is used as an immunosuppressant drug to prevent graft rejection by depriving rapidly dividing B and T lymphocytes of nucleotides needed for DNA division.
Question 3: What is the rate-limiting step of purine synthesis?
Answer: The rate-limiting step of purine synthesis is the conversion of PRPP to 5-phosphoribosylamine, a reaction catalyzed by the enzyme glutamine PRPP amidotransferase.
Question 4: What distinguishes pyrimidine synthesis from purine synthesis?
Answer: Unlike purine synthesis, where the purine ring is built on ribose-5’-P, in pyrimidine synthesis, the pyrimidine ring is formed first, and ribose 5-phosphate is incorporated later.
Question 5: What is Orotic aciduria?
Answer: Orotic aciduria is a rare autosomal recessive disorder characterized by the excessive excretion of Orotic acid in urine due to a deficiency in one or both of the enzymes that convert it to UMP.
Question 6: What are the clinical manifestations of Orotic aciduria?
Answer: Orotic aciduria typically appears in the first year of life and is characterized by growth failure, developmental retardation, megaloblastic anemia, and increased urinary excretion of orotic acid.
Question 7: How is Orotic aciduria treated?
Answer: Uridine treatment is effective for treating Orotic aciduria. Uridine can be easily converted into UMP, allowing UTP, CTP, and TMP to be synthesized and feedback inhibit further orotic acid production.
Question 8: What is the biochemical defect in Lesch-Nyhan syndrome?
Answer: Lesch-Nyhan syndrome is caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT).
Question 9: What causes hyperuricemia in Lesch-Nyhan syndrome?
Answer: The lack of HGPRT prevents purine bases from being salvaged, leading to their degradation and excretion as uric acid. The synthetic rate for purines also increases to compensate for purines lost due to the failure of the salvage process, further contributing to uric acid overproduction.
Question 10: What is gout, and what causes it?
Answer: Gout is a metabolic disease resulting from an increased body pool of urate and hyperuricemia. It is characterized by episodic acute and chronic arthritis due to the deposition of monosodium urate crystals in joints and connective tissues.
Question 11: What are the substances required for the de novo synthesis of purine nucleotides?
Answer: The de novo synthesis of purine nucleotides requires the following substances:
○ PRPP (5-phosphoribosyl-1-pyrophosphate), which is the starting material
○ Various enzymes such as synthases, transferases, carboxylases, and hydroxylases
○ Energy provided by ATP
○ Amino acids and their derivatives, specifically glycine, aspartic acid, and glutamine
○ Carbon dioxide (CO2) from bicarbonate (HCO3–)
○ Coenzymes and cofactors, N⁵,N¹⁰-methylenetetrahydrofolate: Provides one-carbon groups.
NAD⁺ or NADP⁺: Used indirectly in related processes, such as ribose-5-phosphate generation via the pentose phosphate pathway.
Question 12: How is the de novo synthesis of purine nucleotides regulated?
Answer: The de novo synthesis of purine nucleotides is regulated by feedback inhibition at multiple steps:
○ PRPP synthetase, which catalyzes the formation of PRPP, is allosterically inhibited by PRPP itself and several purine nucleotides, including AMP, GMP, ADP, GDP, NAD, and FAD.
○ Glutamine PRPP amidotransferase, the enzyme responsible for the rate-limiting step, is feedback inhibited by AMP and GMP.
○ The balance between adenine and guanine nucleotide concentrations is maintained by the regulation of adenylosuccinate synthetase and IMP dehydrogenase, respectively.
Question 13: What are the key substances needed for the de novo synthesis of pyrimidine nucleotides?
Answer: The de novo synthesis of pyrimidine nucleotides requires:
○ Carbon dioxide (CO2)
○ PRPP (5-phosphoribosyl-1-pyrophosphate)
○ ATP for energy
○ Amino acids aspartic acid and glutamine
○ Cofactors FAD and Mg++
Question 14: How is pyrimidine nucleotide biosynthesis regulated?
Answer: Pyrimidine nucleotide biosynthesis is regulated through allosteric control and coordinate repression and depression:
○ Carbamoyl phosphate synthetase II, the first enzyme, is inhibited by UTP and purine nucleotides but activated by PRPP.
○ Aspartate transcarbamoylase, the second enzyme, is inhibited by CTP and activated by ATP.
○ The first three and the last two enzymes in the pathway are regulated by coordinate repression and depression, meaning their synthesis is controlled at the gene expression level.
Question 15: What is the biochemical basis for the megaloblastic anemia seen in Orotic aciduria?
Answer: In Orotic aciduria, the deficiency of enzymes involved in UMP synthesis leads to a shortage of UTP, CTP, and TMP. These nucleotides are crucial for nucleic acid synthesis, and their deficiency impairs DNA replication and cell division, particularly affecting rapidly dividing cells like erythrocyte precursors. This impaired erythrocyte formation results in megaloblastic anemia, which is characterized by large, immature red blood cells.
Question 16: What enzyme deficiency causes severe combined immunodeficiency (SCID)?
Answer: A deficiency in the enzyme adenosine deaminase (ADA) causes SCID. This deficiency inhibits ribonucleotide reductase due to excess dATP, impacting DNA synthesis and significantly impairing lymphocyte proliferation, leading to severe immunodeficiency.
Question 17: What is the role of ribonucleotide reductase in nucleotide metabolism, and what is its clinical significance?
Answer: Ribonucleotide reductase catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an essential step in DNA synthesis. The enzyme is clinically significant for several reasons. In adenosine deaminase deficiency, excess dATP inhibits ribonucleotide reductase, hindering DNA synthesis, particularly in rapidly dividing cells like lymphocytes. This inhibition contributes to the reduced immunity seen in affected individuals. Ribonucleotide reductase inhibitors, like hydroxyurea, are used as anticancer drugs because they block deoxyribonucleotide formation, thereby interfering with tumor cell growth.
Question 18: What is the metabolic fate of purine nucleotides in humans, and what are the clinical implications of imbalances in this pathway?
Answer: Purine nucleotides are ultimately degraded to uric acid in humans. However, urate’s low solubility in aqueous environments can lead to the formation of urate crystals if purine base degradation becomes excessive. This overproduction can cause hyperuricemia, defined as a plasma urate concentration exceeding 6.8 mg/dL. Hyperuricemia can result from increased urate production, decreased urate excretion, or a combination of both. One consequence of hyperuricemia is gout, a metabolic disease characterized by the deposition of monosodium urate crystals in joints and connective tissues, leading to inflammation and pain.
Question 19: What are the three major hallmarks of Lesch-Nyhan Syndrome?
Answer: Lesch-Nyhan Syndrome (LNS) is characterized by:
○ Uric acid overproduction (hyperuricemia): This occurs due to the deficiency of HGPRT, which prevents purine salvage and leads to increased purine degradation and excretion as uric acid.
○ Cognitive and behavioral disturbances, including self-mutilation: The exact cause of the behavioral disturbances, particularly the compulsive self-injurious behavior, is not fully understood, but it’s believed to be related to dysfunction in the basal ganglia, a brain region with a high concentration of HGPRT.
○ Neurological dysfunction: This includes hypotonia, developmental delay, speech impairment, irritability, loss of motor control, involuntary movements, and spinal arching. These symptoms are also likely related to impaired purine metabolism and dysfunction in the basal ganglia.
Question 20: How does the regulation of purine biosynthesis ensure a balanced production of adenine and guanine nucleotides?
Answer: A balanced production of adenine and guanine nucleotides is maintained through feedback inhibition and cross-regulation in the IMP metabolic pathway. Specifically, AMP and GMP, the end products of the respective branches, inhibit their own synthesis by acting as feedback inhibitors. For example, AMP inhibits adenylosuccinate synthetase, the enzyme responsible for converting IMP to AMP, while GMP inhibits IMP dehydrogenase, the enzyme that converts IMP to GMP. Additionally, the cross-regulation between the two branches ensures that a deficiency in one purine nucleotide leads to a decrease in its synthesis while promoting the synthesis of the other. This intricate regulatory mechanism helps maintain the appropriate balance of adenine and guanine nucleotides required for various cellular processes.
Question 21: How is a diagnosis of Lesch-Nyhan Syndrome made?
Answer: The diagnosis of LNS is a multi-step process that involves clinical evaluation, biochemical testing, and genetic analysis. Elevated uric acid levels in the blood and urine are early indicators, but they are not specific to LNS. A definitive diagnosis is achieved by measuring HGPRT enzyme activity in blood or tissue samples. Low or absent enzyme activity confirms the deficiency. Furthermore, identifying a molecular genetic mutation in the HGPRT gene provides the most accurate confirmation and allows for carrier detection and prenatal screening in families with a history of LNS.
Question 22: What is the role of phosphoribosyl pyrophosphate (PRPP) in the de novo synthesis of purines?
Answer: Phosphoribosyl pyrophosphate (PRPP) acts as a critical precursor and donor of the ribose-phosphate backbone during the synthesis of purine nucleotides. It provides the ribose sugar onto which the purine ring is built, beginning with the formation of phosphoribosylamine catalyzed by glutamine-PRPP amidotransferase.
Question 23: Which enzyme helps to recycle purines?
Answer: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) catalyzes the conversion of hypoxanthine and guanine to their respective nucleotides (IMP and GMP) using PRPP as a co-substrate. This reaction helps recycle purines, reducing the need for energy-intensive de novo synthesis.
Question 24: How does increased activity of xanthine oxidase contribute to the development of gout?
Answer: Xanthine oxidase catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid, the final product of purine catabolism. Increased activity of xanthine oxidase or excessive purine breakdown leads to elevated uric acid levels, which can crystallize in joints and tissues, causing gout.
Question 25: Why does a deficiency in the enzyme ribonucleotide reductase lead to disruptions in DNA synthesis?
Answer: Ribonucleotide reductase is responsible for converting ribonucleotides (NDPs) to deoxyribonucleotides (dNDPs), which are essential for DNA synthesis. A deficiency in this enzyme disrupts the balanced supply of dNTPs, impairing DNA replication and repair, leading to cell cycle arrest or genomic instability.