Purines and Pyrimidines Flashcards

Purines and Pyrimidines

• Purines are nitrogenous bases with a fused six-membered and five-membered ring structure, while pyrimidines have a single six-membered ring.

Nucleosides vs Nucleotides

• A nucleoside forms when a sugar (ribose or 2-deoxyribose) is added to a nitrogen base.
• Purine nucleosides end in -osine; pyrimidine nucleosides end in -idine.
• Nucleotides are formed by adding one or more phosphates to the sugar of a nucleoside.

Inosine

• Inosine is a nucleoside created when hypoxanthine bonds to a ribose ring via a β-N9-glycosidic bond.
• It is essential in tRNA for decoding genetic information through wobble base pairing.

Uridine

• Uridine consists of uracil attached to a ribose ring via a β-N1-glycosidic bond and is present in RNA but not in DNA.
• Deoxyuridine derivatives serve as antiviral agents, mimicking uracil for incorporation into DNA.

Roles of Nucleotides

• Serve as information carriers in DNA and RNA.
• Act as universal energy sources, particularly ATP (30 kJ/mol).
• Function as second messengers (cGMP, cAMP) and assist in enzyme activity as coenzymes.

Cell Cycle and Synthesis

• Purine nucleotides are synthesized in two pathways: de novo and salvage.
• De novo biosynthesis is closely tied to the cell cycle, starting from basic materials like amino acids and bicarbonate.

Locations of Synthesis

• De novo synthesis of purines predominantly occurs in the liver, whereas non-hepatic tissues have limited synthesis capacity.
• Pyrimidine synthesis is carried out in various tissues throughout the body.

Pathway Summaries

• First step in purine synthesis involves PRPP while prydimidine synthesis concludes with UMP formation.
• Purine degradation results in uric acid and ammonia; pyrimidine metabolism yields CO2, NH4+, β-alanine, and β-aminoisobutyrate.

Control of Purine Synthesis

• Regulated in two phases: total synthesis control at amidotransferase step and maintaining ATP/GTP balance.
• ATP stimulates GTP synthesis and vice versa for balance and energy provision.

Control of Pyrimidine Synthesis

• Governed mainly by cytoplasmic carbamoyl phosphate synthetase II (CPS II).
• UTP competitively inhibits CPS II while PRPP activates the enzyme.

Purinosomes

• Purinosomes are cellular structures formed by enzymes in the de novo purine biosynthetic pathway, organizing into punctate bodies for metabolic efficiency.

Uric Acid and Metabolism

• Uric acid is the primary end product of purine metabolism from the liver, muscles, and intestines.
• Allantoin and related compounds can result from nonenzymatic reactions in certain tissues.

Metabolic Disorders in Purine Metabolism

• Disorders can involve deficiencies or superactivities in key enzymes such as PRPP synthase and adenylosuccinate lyase.
• Adenosine deaminase deficiencies and xanthine oxidase deficiencies lead to various metabolic disturbances.

Metabolic Disorders in Pyrimidine Metabolism

• Dihydropyrimidine dehydrogenase deficiency affects the degradation of thymine and uracil, leading to conditions like neurotrophic keratitis.

Test your knowledge on purines and pyrimidines with these flashcards. This quiz provides essential definitions and distinctions such as the difference between nucleosides and nucleotides. Perfect for students of biochemistry and molecular biology.