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Questions and Answers
What is the regulated step in pyrimidine metabolism?
Which enzyme is responsible for catalyzing the formation of orotic acid?
Which of the following regulators activates CPS II?
What is a characteristic of the enzyme UMP synthase?
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What is the primary consequence of a deficiency in UMP synthase activity?
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Which substance is NOT required for both purine and pyrimidine synthesis?
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What can serve as a precursor to acetyl CoA?
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Which enzyme is associated with the inner mitochondrial membrane?
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What is produced as a final end product of pyrimidine nucleotide metabolism?
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What is the first product formed in the pyrimidine synthesis pathway?
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Study Notes
Pyrimidine Synthesis
- Carbamoyl Phosphate Synthesis: The first step in pyrimidine synthesis is the creation of carbamoyl phosphate from glutamine and CO2, catalyzed by carbamoyl phosphate synthetase (CPS) II. This is the regulated step in mammalian cells.
- CPS II Regulation: CPS II is inhibited by UTP (the end product of this pathway), and is activated by PRPP (5-phosphoribosyl 1-pyrophosphate).
- Orotic Acid Synthesis: The second step is the formation of carbamoylaspartate, catalyzed by aspartate transcarbamoylase. The pyrimidine ring is formed through a hydrolytic reaction catalyzed by dihydroorotase. This then produces dihydroorotate, which is oxidized to orotic acid.
- Dihydroorotate Dehydrogenase: The enzyme that converts dihydroorotate to orotic acid is called dihydroorotate dehydrogenase and is found in the inner mitochondrial membrane.
- Cytosolic Enzyme Location: All other enzymes involved in pyrimidine biosynthesis are located in the cytosol.
- Orotidine 5'-monophosphate (OMP) Synthesis: Orotic acid is converted into the nucleotide OMP, the parent pyrimidine mononucleotide, using PRPP as the ribose 5-phosphate donor.
- Uridine Monophosphate (UMP) Synthesis: OMP is then converted to UMP by a bifunctional enzyme called UMP synthase.
Pyrimidine Degradation
- Urea Cycle: Urea is the final product of pyrimidine degradation.
- β-alanine and β-aminoisobutyrate: These are byproducts of pyrimidine degradation and can be used as precursors for acetyl CoA and succinyl CoA, respectively.
- Krebs Cycle: β-alanine and β-aminoisobutyrate play a role in the Krebs cycle.
Orotic Aciduria
- Genetic Defect: Orotic aciduria is a rare genetic disorder caused by a deficiency in the bifunctional enzyme UMP synthase.
- Symptoms: This deficiency leads to an accumulation of orotic acid in the urine, resulting in poor growth and megaloblastic anemia.
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Description
Explore the key steps involved in pyrimidine synthesis, focusing on carbamoyl phosphate synthesis and the role of various enzymes. Understand the regulation of CPS II and the conversion of dihydroorotate to orotic acid. This quiz covers vital concepts in biochemistry and metabolism.
