19 topics with concise summaries: definitions, key facts, and class questions.
Nucleotides are not only the building blocks of DNA and RNA, but also universal energy sources and metabolic regulators. In this section, we examine in detail the pathways of purine (AMP, GMP) and pyrimidine (UTP, CTP, TMP) nucleotide formation: from the energy-demanding de novo synthesis to efficient salvage pathways. You will learn how key enzymes function (ribonucleotide reductase, xanthine oxidase) and how these molecules are degraded.
In seminars and exams, instructors traditionally focus on clinical biochemistry. Be ready to answer questions about the molecular mechanisms of gout and hyperuricemia, the causes of orotic aciduria, Lesch-Nyhan syndrome, and severe immunodeficiencies associated with purine metabolism enzyme defects.
Special emphasis is placed on the practical significance of the topic: understanding the synthesis of deoxyribo- and thymidylate nucleotides is critical for analyzing the mechanisms of action of antitumor drugs. We will look at how nucleotide synthesis inhibitors halt the division of malignant cells.
Which metabolite serves as the common precursor for the synthesis of both purine nucleotides?
Inosine monophosphate (IMP) serves as the common precursor, at which point the metabolic pathway diverges.
Which reaction is rate-limiting in purine nucleotide synthesis?
The rate-limiting step is the transfer of the glutamine amide group to PRPP with the formation of 5-phosphoribosyl-1-amine. This reaction is catalyzed by PRPP amidotransferase.
Where do the substrates for the salvage synthesis pathways come from?
Substrates (nitrogenous bases and nucleosides) are generated during catabolism—the breakdown of existing nucleic acids. The cell utilizes these degradation products for the resynthesis of nucleotides.
Which enzyme catalyzes the formation of uric acid?
The formation of uric acid is catalyzed by the enzyme xanthine oxidase. It sequentially oxidizes hypoxanthine to xanthine, and then xanthine to uric acid.
What is the sequence of reactions catalyzed by xanthine oxidase?
The enzyme catalyzes two sequential irreversible reactions: first, it oxidizes hypoxanthine to xanthine, and then converts xanthine into the final product—uric acid.
Men are affected 20 times more often than women
This is because the physiological serum urate pool in men is twice as high as in women, meaning the crystallization threshold is reached faster.
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