Nucleotides and Purine Synthesis

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Questions and Answers

Which nitrogenous bases are classified as purines?

Adenine and Thymine

Thymine and Cytosine

Cytosine and Uracil

Adenine and Guanine (correct)

Which of the following correctly describes the structure of pyrimidines?

Single six-membered ring (correct)

Single five-membered ring

Two-ring structure

Complex multi-ring structure

In DNA, which base does Adenine pair with?

Uracil

Thymine (correct)

Cytosine

Guanine

What roles do purines and pyrimidines play in cellular processes?

Encoding genetic information Signup and view all the answers

Which pathway is involved in the synthesis of nucleotides?

De novo synthesis and salvage pathways Signup and view all the answers

Which pair of bases is correctly matched with its complement in RNA?

Adenine pairs with Uracil Signup and view all the answers

What is the significance of purine and pyrimidine metabolism in cells?

DNA replication and cell division Signup and view all the answers

Which nitrogenous base is found only in RNA?

Uracil Signup and view all the answers

What is required as a cofactor for the synthesis of Thymidine Monophosphate (TMP) from dUMP?

Tetrahydrofolate Signup and view all the answers

Which enzyme is primarily involved in the degradation of Uracil?

Dihydropyrimidine dehydrogenase Signup and view all the answers

What is the end product of β-alanine metabolism?

Malonyl-CoA Signup and view all the answers

Which condition is characterized by hyperuricemia due to excessive purine degradation?

Lesch-Nyhan Syndrome Signup and view all the answers

What is the effect of Methotrexate on cellular processes?

It inhibits purine and pyrimidine synthesis. Signup and view all the answers

Which of the following symptoms is NOT associated with Gout?

Megaloblastic anemia Signup and view all the answers

Accumulation of orotic acid is primarily linked to which disorder?

Orotic Aciduria Signup and view all the answers

Which molecule is primarily synthesized from dUMP?

Thymidine Monophosphate Signup and view all the answers

What is the starting molecule for the de novo synthesis of purines?

Ribose-5-phosphate Signup and view all the answers

Which enzyme catalyzes the conversion of PRPP to 5-phosphoribosylamine?

Glutamine-PRPP amidotransferase Signup and view all the answers

What is the critical branch point in purine synthesis?

IMP Signup and view all the answers

How is AMP synthesized from IMP?

Through an intermediate called adenylosuccinate Signup and view all the answers

What initiates the de novo pyrimidine synthesis process?

Synthesis of carbamoyl phosphate Signup and view all the answers

Which compound is NOT a precursor in the de novo synthesis of purines?

Ornithine Signup and view all the answers

What regulates the purine synthesis pathway?

Feedback inhibition and energy levels Signup and view all the answers

What enzyme is involved in the conversion of xanthine to uric acid?

Xanthine oxidase Signup and view all the answers

Which enzyme is responsible for converting UMP to CTP?

CTP synthetase Signup and view all the answers

What is the significance of inosine in purine metabolism?

It acts as a precursor for both AMP and GMP Signup and view all the answers

Which product forms from the hydrolysis of AMP?

Adenosine Signup and view all the answers

What are the products of inosine in purine degradation?

Xanthine and uric acid Signup and view all the answers

What is the primary function of purine nucleotide synthesis?

Supply nucleotides for DNA and RNA synthesis Signup and view all the answers

What occurs when there is excessive accumulation of purine nucleotides?

Induction of feedback inhibition Signup and view all the answers

Signup and view all the answers

Study Notes

Nucleotide Structure & Classification

Nucleotides are crucial for all life, made of a nitrogenous base (purine or pyrimidine), a pentose sugar (ribose or deoxyribose), and one or more phosphate groups.
Purines are adenine (A) and guanine (G), with a double-ring structure.
Pyrimidines are cytosine (C), thymine (T), and uracil (U), with a single-ring structure. Thymine is only in DNA, uracil is only in RNA.

Purine Synthesis (De Novo)

De novo synthesis builds complex molecules from simpler ones, unlike recycling existing ones.
Advantages include creating essential molecules, supporting cell processes, and adaptable nutrient utilization in cellular functions.
The process starts with ribose-5-phosphate from the pentose phosphate pathway (PPP), converted to phosphoribosyl pyrophosphate (PRPP) by PRPP synthetase.
A key step is converting PRPP to 5-phosphoribosylamine, catalyzed by glutamine-PRPP amidotransferase (rate-limiting step).
Successive steps add atoms, forming inosine monophosphate (IMP).
IMP is a branch point, leading to either adenosine monophosphate (AMP) or guanosine monophosphate (GMP).
AMP formation involves adenylosuccinate intermediate, GTP is used during the process.
GMP synthesis uses IMP dehydrogenase and GMP synthetase, requiring NAD+.
Purine synthesis is regulated by feedback inhibition from AMP and GMP.

Purine Degradation

AMP is deaminated to inosine, hydrolyzed to hypoxanthine, and then oxidized to xanthine by xanthine oxidase.
GMP is dephosphorylated to guanosine, then converted to guanine; also goes through a xanthine oxidase conversion to xanthine.
Xanthine is converted to uric acid by xanthine oxidase. Uric acid is excreted.
High uric acid can cause gout.

Pyrimidine Synthesis (De Novo)

Pyrimidine synthesis begins with carbamoyl phosphate formed from glutamine and carbon dioxide (using carbamoyl phosphate synthetase II) in the cytosol.
Carbamoyl phosphate is transferred to aspartate for dihydroorotate formation.
Dihydroorotate is oxidized to orotate.
Orotate combines with PRPP to form orotidine monophosphate (OMP).
OMP is converted to uridine monophosphate (UMP) by decarboxylation.
UMP is phosphorylated to UTP, then converted to CTP (cytidine triphosphate) by CTP synthetase.
Thymidine monophosphate (TMP) is made from dUMP (a derivative of UMP) using thymidylate synthase and tetrahydrofolate.

Pyrimidine Degradation

Uracil and thymine are broken down to β-alanine and β-aminoisobutyric acid.
Products of these degradation processes are eventually excreted.

Clinical Significance

Purine disorders: Gout (excessive uric acid), Lesch-Nyhan syndrome (HGPRT deficiency).
Pyrimidine disorders: Orotic aciduria (deficient pyrimidine synthesis enzymes).
Cancer treatment: Methotrexate inhibits folate metabolism, impacting purine and pyrimidine synthesis, used to slow cancer cell division.

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Description

This quiz covers the structure and classification of nucleotides, focusing on the differences between purines and pyrimidines. Additionally, it explores the de novo synthesis of purines, detailing the biochemical pathways and key steps involved. Test your knowledge on these fundamental aspects of molecular biology.