de novo synthesis for purine (nmemonic )\

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de novo purine synthesis mnemonic steps

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Here is a complete, exam-ready breakdown of de novo purine synthesis with mnemonics at every level:

De Novo Purine Synthesis - Complete Mnemonic Guide

The Big Picture

Purines are built on top of ribose-5-phosphate (not made separately and then attached). The end product is IMP (Inosine Monophosphate), which then branches to AMP and GMP.

Site: Primarily the liver (cytosol)

Atom Origin Mnemonic: "GAG CO2 T"

Remember what contributes atoms to the purine ring:

Contributor	Atoms Donated
Glutamine (×2)	N3, N9
Aspartate	N1
Glycine	C4, C5, N7
CO₂	C6
THF (N¹⁰-formyl-THF ×2)	C2, C8

Mnemonic: "Granny Always Gets COugh Twice"

Granny = Glutamine (×2 nitrogens)
Always = Aspartate (N1)
Gets = Glycine (C4, C5, N7)
COugh = CO₂ (C6)
Twice = THF ×2 (C2, C8)

The 10 Steps: PRPP → IMP

Classic step mnemonic:

"Prices Paid Going For Asking, Forget All Funny Stupid Intermediates"

#	Mnemonic Word	Reaction	Key Molecule Added
0	Prices	R5P → PRPP	PRPP synthetase (needs ATP)
1	Paid	PRPP → PRA	Glutamine donates N9 (committed step - GPAT enzyme)
2	Going	PRA → GAR	Glycine added (C4, C5, N7)
3	For	GAR → FGAR	Formyl-THF adds C8
4	Asking	FGAR → FGAM	Glutamine donates N3
5	Forget	FGAM → AIR	Ring closes (ATP used)
6	All	AIR → CAIR	CO₂ added at C6
7	Funny	CAIR → SAICAR	Aspartate added (carries N1)
8	Stupid	SAICAR → AICAR	Fumarate leaves (N1 stays)
9	Intermediates	AICAR → FAICAR	Formyl-THF adds C2
10	(Ring closes)	FAICAR → IMP	Cyclization, water lost

Committed Step - Key Exam Fact

Step 1: PRPP + Glutamine → Phosphoribosylamine (PRA)

Enzyme: Glutamine PRPP Amidotransferase (GPAT)
Inhibited by: AMP, GMP, IMP (negative feedback)
This is the rate-limiting / committed step

IMP → AMP and GMP (Branching)

Mnemonic: "AMP needs GTP, GMP needs ATP" (they use each other's energy - cross-regulation)

Branch	Steps	Energy
IMP → Adenylosuccinate → AMP	Aspartate added, fumarate leaves	Needs GTP
IMP → XMP → GMP	IMP dehydrogenase (IMPDH)	Needs ATP

Drugs here:

Mycophenolate & Ribavirin - inhibit IMP dehydrogenase (block GMP synthesis)
6-MP / Azathioprine - block multiple steps (GPAT and HGPRT salvage)

Regulation Summary Mnemonic: "PRPP is the GAS, purines are the BRAKES"

PRPP concentration - drives synthesis (more PRPP = faster synthesis)
AMP + GMP (+ IMP) - feedback-inhibit GPAT (the committed step)
PRPP synthetase is also inhibited by purine nucleotides

Quick Recall Summary

R5P → PRPP → PRA → ... (9 more steps) ... → IMP → AMP
                                                 ↘ GMP

Energy cost: 5 ATP equivalents to make IMP from R5P

Carbons and nitrogens from:

Glycine (backbone: C4, C5, N7)
Glutamine ×2 (N3, N9)
Aspartate (N1)
CO₂ (C6)
Formyl-THF ×2 (C2, C8)

One-liner Super Mnemonic (for atom origins)

"A Glut of CO₂ from Glycine Fumes Twice" = Aspartate, Glutamine (×2), CO₂, Glycine, Formyl-THF (×2)

Sources: Lippincott's Illustrated Reviews Biochemistry, 8th ed.; Basic Medical Biochemistry: A Clinical Approach, 6th ed.

regulation of purine synthesis

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Here is the complete regulation of de novo purine synthesis, drawn directly from the textbooks:

Regulation of De Novo Purine Synthesis

There are 4 key regulated enzymes - 2 control IMP synthesis, 2 control the AMP/GMP branch point.

Overview Diagram

Regulation of purine synthesis showing feedback inhibition at PRPP synthetase, GPAT, IMP dehydrogenase, and adenylosuccinate synthetase

Level 1: PRPP Synthetase (R5P → PRPP)

PRPP synthetase regulation - activated by Pi, inhibited by purine ribonucleotides

	Regulator	Effect
Activated by	Inorganic phosphate (Pi)	Stimulates synthesis
Inhibited by	ADP, GDP (purine ribonucleotides)	End-product inhibition

Key point: This is an X-linked enzyme. It is not the committed step (PRPP is also used in pyrimidine synthesis and salvage pathways). Both an oxypurine (GDP) and aminopurine (ADP) bind a single allosteric site to inhibit it.

Level 2: Glutamine PRPP Amidotransferase - GPAT (PRPP → PRA) ⭐ COMMITTED STEP

This is the rate-limiting, committed step - the most important regulatory point.

	Regulator	Effect
Inhibited by	AMP + GMP (bind distinct allosteric sites)	Strong feedback inhibition
Also inhibited by	ADP, GDP, ATP, GTP	Less physiologically significant

Mechanism of inhibition: AMP and GMP convert the enzyme from an active monomer (133 kDa) → inactive dimer (270 kDa).

Mnemonic: "GPAT is GAG-ged" - GMP and AMP and Glutamine act at GPAT

Key kinetic trick: Cellular PRPP and glutamine concentrations are normally below their Km for GPAT. So any small rise in PRPP or glutamine causes a proportional increase in synthesis rate - this is how the pathway is "accelerated" when needed.

Level 3: Branch Point Regulation (IMP → AMP or GMP)

This is where cross-regulation creates balance between the two end products:

Mnemonic: "AMP needs GTP, GMP needs ATP" - they require each other's energy

Branch	Enzyme	Inhibited By	Energy Used
IMP → Adenylosuccinate → AMP	Adenylosuccinate synthetase	AMP (feedback)	GTP
IMP → XMP → GMP	IMP dehydrogenase (IMPDH)	GMP (feedback)	ATP

Why this is brilliant:

When ATP is high → more GTP available → drives GMP synthesis → balances the pool
When GTP is high → more ATP available → drives AMP synthesis → balances the pool
This is reciprocal/cross-regulation ensuring neither purine dominates

Complete Regulation Summary Table

Enzyme	Activated By	Inhibited By	Step
PRPP synthetase	Pi	ADP, GDP	R5P → PRPP
GPAT (committed step)	PRPP ↑, Glutamine ↑	AMP, GMP (+ di/triphosphates)	PRPP → PRA
Adenylosuccinate synthetase	GTP (substrate)	AMP	IMP → AMP
IMP dehydrogenase (IMPDH)	-	GMP	IMP → GMP

Clinical Pharmacology: Drugs That Exploit This Regulation

Drug	Target	Use
6-Mercaptopurine (6-MP)	Blocks GPAT + multiple steps (false feedback inhibition)	Leukemia, IBD
Azathioprine	Prodrug of 6-MP	Immunosuppression, IBD
Mycophenolate	Inhibits IMP dehydrogenase (blocks GMP branch)	Transplant rejection
Ribavirin	Inhibits IMP dehydrogenase	Viral infections (HCV)
Methotrexate	Inhibits DHFR → depletes formyl-THF → blocks steps 3 & 9	Cancer, RA
Allopurinol	Inhibits xanthine oxidase (degrades purines)	Gout

One-Line Summary Mnemonic

"PRPP GAS, AMP+GMP BRAKES, Cross-roads at IMP"

PRPP = accelerator (more PRPP = faster synthesis)
AMP + GMP = dual brakes on GPAT (committed step)
IMP = crossroads where AMP inhibits its own branch, GMP inhibits its own branch, but each needs the other's energy to be made

Sources: Basic Medical Biochemistry: A Clinical Approach 6e, p.1420-1421; Lippincott's Illustrated Reviews Biochemistry 8e, p.827-829

gout reasons for precipitating feature

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Excellent content gathered. Here is the complete answer:

Gout: Precipitating Factors - Complete Guide

Understanding WHY Attacks Are Triggered

A gout flare does not simply require hyperuricemia - it needs a sudden change (rise or fall) in urate levels, or local factors that promote crystal formation or crystal shedding from existing deposits. Most people with hyperuricemia never get gout.

The Master Framework: Two Levels of Precipitation

Mnemonic: "CRYSTAL DROPS"

Cold temperature (local) Rapid urate change (up or down) Yield of purines from diet Surgery / trauma Thiazides and other drugs Alcohol Low pH / dehydration

Dehydration (intraarticular) Renal impairment Overproduction states (tumor lysis, haemolysis) Precipitate starters - seed nuclei (cartilage debris, OA) Starting urate-lowering therapy

Level 1: Causes of Hyperuricemia (Root Cause)

Mnemonic: "90% Under, 10% Over"

>90% of gout = underexcretion of uric acid by kidneys
<10% = overproduction (or combined)

A) Underexcretion Causes (RENAL)

Category	Examples
Drugs	Thiazide diuretics, loop diuretics, low-dose aspirin, cyclosporine, pyrazinamide, ethambutol
Renal disease	CKD, chronic renal insufficiency
Metabolic	Hypertension, metabolic syndrome, insulin resistance
Dehydration	Any cause - reduces uric acid clearance
Organic acids compete	Lactic acidosis (alcohol, starvation), ketoacidosis (fasting, DKA) - organic acids compete with urate at renal tubule for excretion

B) Overproduction Causes

Category	Examples
Dietary	Purine-rich foods: red meats, organ meats, shellfish, anchovies, beer
Cell lysis	Tumor lysis syndrome, haemolytic anaemia, psoriasis, myeloproliferative disorders
Enzyme defects	HGPRT deficiency (Lesch-Nyhan), PRPP synthetase overactivity
High fructose intake	High-fructose corn syrup - accelerates purine degradation

Level 2: Local Crystal Formation Promoters

From Rheumatology (2022 Elsevier) - Box 193.3:

Promoter	Mechanism
Low temperature	MSU crystals precipitate preferentially in cooler peripheral joints (1st MTP, ankle)
Low pH	Acidic environment reduces urate solubility
Dehydration	Raises local urate concentration
Seed nuclei	Cartilage debris, chondroitin, hyaluronate from joint trauma or OA
Cation concentration	High Na⁺ promotes MSU crystal nucleation
IgG coat	Immunoglobulins provide stable platform for crystal nucleation
Osteoarthritis	OA cartilage releases chondroitin sulphate, lowers urate solubility; OA sites co-localise with MSU deposits

Level 3: Triggers of the Acute Flare (Precipitating Events)

Mnemonic: "SAT DISH"

Letter	Trigger	Mechanism
S	Surgery / Postoperative	Rapid fluid shifts, fasting, dehydration, tissue trauma
A	Alcohol (especially beer)	Beer = purines + lactic acid (competes at tubule); spirits = dehydration
T	Trauma (local)	Crystal shedding from deposits; seed nuclei released
D	Dietary binge / Fasting	Binge = purine load; fasting = ketoacidosis competes at tubule
I	Intercurrent illness / Infection	Fever, dehydration, systemic inflammation mobilise crystals
S	Starting ULT (allopurinol, febuxostat)	Rapid fall in urate causes crystal remodelling and shedding from tophi
H	Hospitalisation / Severe illness	NPO status, IV fluids, diuretics - all shift urate rapidly

The Paradox of Starting Urate-Lowering Therapy

A key exam point: initiating allopurinol or febuxostat paradoxically triggers acute gout flares in the first weeks-months. This happens because:

Rapid fall in serum urate destabilises existing MSU crystal deposits in tophi
Crystals are shed into the joint space
This triggers the same inflammasome-mediated response as new crystal deposition
Prevention: always co-prescribe colchicine or NSAID prophylaxis for 3-6 months when starting ULT

Pathophysiology of the Flare: The Inflammasome Cascade

Once crystals form or are shed, the sequence is:

MSU crystals in joint
        ↓
Phagocytosed by synovial macrophages
        ↓
NLRP3 Inflammasome activated
        ↓
Caspase-1 activated
        ↓
IL-1β released (+ IL-6, TNF, complement)
        ↓
Neutrophil recruitment → lysosomal enzyme release
        ↓
ACUTE ARTHRITIS (peaks 8-12 hrs, resolves in days-weeks)

Why Night / Early Morning?

Flares characteristically start at night or early morning because:

Nocturnal dehydration concentrates urate in joint fluid
Body temperature drops slightly during sleep (promotes MSU crystallisation)
Cortisol is at its nadir (less natural anti-inflammatory effect)

High-Yield Summary Table

Precipitant	Category	Mechanism
Thiazide diuretics	Drug - underexcretion	Compete with urate at proximal tubule
Alcohol (beer > spirits)	Dietary + metabolic	Purine load + lactic acidosis + dehydration
High-purine diet	Dietary - overproduction	Excess substrate for xanthine oxidase
Dehydration	Metabolic	↑ urate concentration, ↓ renal clearance
Surgery	Systemic event	Fasting + fluid shifts + tissue breakdown
Trauma	Local event	Crystal shedding + seed nuclei
Starting allopurinol	Iatrogenic	Rapid urate fall → crystal remodelling
Tumor lysis / chemo	Overproduction	Massive purine release from cell lysis
Low-dose aspirin	Drug - underexcretion	Blocks urate secretion at tubule
Cyclosporine	Drug - underexcretion	↓ GFR + ↑ tubular reabsorption
Fasting / crash diet	Metabolic	Ketoacidosis competes with urate excretion
Cold weather	Local	↓ temperature → ↓ MSU solubility

Sources: Goldman-Cecil Medicine International Edition; Rheumatology 2-Volume Set (2022, Elsevier); Robbins & Kumar Basic Pathology; ROSEN's Emergency Medicine

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