De Novo Synthesis of Purines

Introduction to Purine Synthesis

• The discussion focuses on the de novo synthesis of purines, their regulation, and various analogues used in this process.

• Understanding purine synthesis is crucial for grasping the etiology of various disorders and mechanisms of action for certain drugs.

Dietary Nucleic Acids and Their Degradation

• Dietary nucleic acids (DNA/RNA) are degraded into mononucleotides by enzymes present in intestinal and pancreatic secretions.

• Mononucleotides are further broken down into nucleosides and bases (purines/pyrimidines), with purines oxidized to uric acid and pyrimidines to carbon dioxide and ammonia.

Biosynthesis Pathways of Purine Nucleotides

• All living organisms can synthesize purine nucleotides, primarily adenosine monophosphate (AMP) and guanosine monophosphate (GMP).

• There are two main pathways for synthesizing purine nucleotides: the de novo synthesis pathway and the salvage pathway.

De Novo Synthesis Process

• In de novo synthesis, the purine ring is formed from various precursors assembled on ribose 5-phosphate derived from the hexose monophosphate shunt pathway.

• Key contributors include glycine (C4, C5, N7), aspartate (N1), glutamine (N9), one-carbon groups from tetrahydrofolate, and carbon dioxide for C6.

Initial Steps in Purine Nucleotide Synthesis

• The first step involves synthesizing phosphoribosyl pyrophosphate (PRPP) from ribose 5-phosphate via ATP attachment at C1.

• PRPP serves as a critical intermediate for both purine and pyrimidine nucleotide synthesis.

Rate-Limiting Step in Purine Synthesis

• The conversion of PRPP to 5-phosphoribosylamine is catalyzed by glutamine PRPP amidotransferase; this step is rate-limiting in de novo synthesis.

• This reaction transfers an amide group from glutamine to PRPP, contributing to the formation of N9 in the purine structure.

Further Conversions Leading to Ring Closure

• Following initial reactions, 5-phosphoribosylamine converts into glycinamide ribonucleotide through additional enzymatic actions involving ATP consumption.

• One-carbon units are transferred during subsequent steps leading towards forming more complex structures like formylglycinamidine ribonucleotide.

Final Steps Towards Complete Purine Structure

• The final stages involve multiple transformations including ring closure facilitated by specific enzymes that utilize ATP again.

Synthesis of Purine Nucleotides

Overview of Purine Synthesis Pathway

• The synthesis begins with the conversion of carbon dioxide into imidazole ribonucleotide, which is then transformed into carboxy amino imidazole ribonucleotide through enzymatic action.

• Following this, a cyclic group is removed from the cycle as ethoxide, leading to further transformations involving carboxamide ribonucleotide and its derivatives.

• A one-carbon group is transferred from tetrahydrofolate to carboxamide ribonucleotide, facilitating the synthesis of formal amino imidazole carboxamide ribonucleotide via specific enzymes.

• The second ring closure occurs in the pathway, resulting in the formation of C2 up to purine nucleotides with assistance from inosine monophosphate synthase enzyme.

• This process culminates in the production of inosine monophosphate (IMP), marking it as the first purine nucleotide synthesized in de novo pathways.

Conversion Processes

Synthesis of Adenosine Monophosphate (AMP)

• The amino group from aspartate transfers to IMP, converting it into adenyl succinate through an enzyme called adenyl succinate synthetase while utilizing ATP for energy.

• Fumarate is subsequently removed from adenyl succinate, yielding adenosine monophosphate (AMP), showcasing how AMP is synthesized from IMP.

Synthesis of Guanosine Monophosphate (GMP)

• The transformation continues with IMP being converted into xanthosine monophosphate (XMP), facilitated by IMP dehydrogenase which also converts NAD+ into NADH + H+ during this reaction.

• An amide group from glutamine transfers to XMP, resulting in guanosine monophosphate (GMP). This step also requires ATP for energy input.

Regulatory Mechanisms

Key Regulatory Steps

• The primary regulatory step involves glutamine PRPP amidotransferase enzyme which is feedback inhibited by AMP, ADP, and GMP. This inhibition regulates de novo synthesis effectively.

• Another regulatory point occurs at the conversion stages between IMP and both AMP and GMP; high levels of either nucleotide inhibit respective enzymes involved in their conversions.

Inhibition by Analogues

• Various analogues act as inhibitors within these pathways; for instance, 6-Mercaptopurine inhibits conversion processes involving IMP and AMP.

• Additional analogues replace components like ribose with alternatives such as arabinose to disrupt normal nucleotide synthesis.

Understanding Purine and Pyrimidine Biosynthesis

Mechanisms of Inhibition in Nucleotide Synthesis

• Follette acts as an antagonist by inhibiting the transfer of one carbon group, which prevents the synthesis of C2 and C8 in the current ring structure.

• The inhibition also affects the synthesis of EN9 due to glutamine's role in producing N3 and N9 within the current ring.

Biological Precursors in Nucleotide Formation

• I know Sonique acid is identified as a biological precursor for uracil and thymine, linking it to adenylate acid and guanylate acid synthesis.

• The first pyrimidine nucleotide synthesized is converted into IDNO sign monophosphate, highlighting its importance in nucleotide metabolism.

Key Questions on Purine Biosynthesis

• A question posed regarding purine biosynthesis indicates that it requires vitamin B12; however, this statement is incorrect as it primarily relies on ribose 5-phosphate from the pentose phosphate pathway.

• Clarification on dietary sources reveals that purines and pyrimidines are non-essential nutrients derived from essential fatty acids and amino acids.

Contributions of Amino Acids to Purine Structure