Purine Synthesis

Major Purines

• Adenine

• Guanine

• Uric Acid

Sources of atoms in Purine ring

N1 = Asparate

C2, C8 = 10-Formyl tetrahydrofolate (THF)

N3, N9 = Glutamine

C6 = CO2

C4, C5, N7 = Glycine

Sources of atoms in Purine ring

Asparate, 10-Formyl tetrahydrofolate (THF), GlutaMINE, CO2, Glycine

PRPP Synthesis reaction

Ribose 5-phosphate + ATP → 5-Phospho-alpha-D-ribosyl 1-pyrophosphate (PRPP)

PRPP (Ribose phosphate pyrophosphokinase) Synthesis Enzyme

PRPP Synthetase

• ATP → AMP

What does PRPP stand for?

5-Phosphoribosyl-1-pyrophosphate

What is PRPP Synthetase inhibited by?

Purine ribonucleotide

What pathways is PRPP a precursor for?

1. De novo Purine Synthesis (IMP → AMP/GMP)

2. Purine Salvage Pathway

3. Pyrimidine Synthesis

• Ribose donor for all nucleotide pathways

What is IIMP or inosinate?

Inosine 5’-monophosphate

• initial product of nucleotide pathway

What is the base of IMP?

Hypoxanthine

What is the first step of IMP synthesis?

PRPP + L-Glutamine + H2O → PRA + L-Glutamate + PPi

What is PRA

First product of IMP synthesis = 5'-phosphoribosylamine

What enzyme catalyzes the first step of IMP synthesis?

Amidophosphoribosyl transferase

What activated amidophosphoribosyl transferase?

PRPP

What inhibits amidophosphoribosyl transferase?

• AMP

• GMP

• IMP

How many steps does IMP synthesis have?

10 step pathway

What can IMP be converted into?

AMP or GMP (seperate pathways)

What is step 1 of AMP Synthesis?

IMP + Aspartate → Adenylosuccinate

What enzyme is needed to convert IMP + Aspartate → Adenylosuccinate

Adenylosuccinate Synthetase

• 1 GTP used

What is step 2 of AMP Synthesis?

Adenylosuccinate → Fumerate + AMP

What enzyme is needed to catalyze Adenylosuccinate → Fumerate + AMP?

Adenylosuccinate Lyase

What is step 1 of GMP Synthesis?

IMP + H2O → Xanthosine Monophosphate (XMP)

What enzyme is used to catalyze IMP + H2O → Xanthosine Monophosphate (XMP)?

IMP dehydrogenase

• uses 1 NAD+ → NADH

What is step 2 of GMP synthesis?

XMP + Glutamine + H2O → GMP + Glutamate

What enzyme is used to catalyze XMP + Glutamine → GMP + Glutamate

GMP Synthetase

• uses 1 ATP

What enzyme catalyzes PRPP → 5-Phosphoribosyl amine

Glutamine-PRPP amindotransferase

What inhibits Glutamine-PRPP amindotransferase?

• AMP

• GMP

• IMP

What inhibits Adenylosuccinate Synthetase?

AMP

What inhibit IMP dehydrogenase?

• XMP

• GMP

What are salvage pathways (of Purine and Pyrimidines)?

Recycling of intact bases to make new ones to save energy

How are the bases salvages?

They are directly converted to 5’ - mononucleotides

AMP and GMP

Adenosine and Guanosine

Inosine

Hypoxanthine and Guanine

90% Salvage Pathway

10% Excretion Pathway

AMP ← IMP

GMP

Xanthine Oxidase

Xanthine Oxidase Inhibitor?

Allopurinol

Amidotransferase aka Glutamine-PRPP amidotransferase

What happens if Glutamine-PRPP amidotransferase/ Amidotransferase OR 6-merceptopurine is inhibited?

PRPP buildup

decreased IMP, AMP, GMP

6-mercaptopurine

• also inhibits Purine synthesis by acting like IMP

Adenosine Deaminase

Purine Nucleoside Phosphorylase (PNP)

Xanthine

Uric Acid

Adenosine Deaminase Deficiency (ADA)

HGPRT Deficiency aka Lesch Nyhan Syndrome

(Hypoxanthine-Guanine Phosphoribosyltransferase)

What are some effects of adenosine?

• increased blood flow

• dcreased blood pressure (BP)

• muscle relaxation

• lowers ATP use

Is Adenosine a harmone, a neurotrandmitter, or both?

BOTH

What effect does caffeine binding to adenosine receptors have?

1. increases blood pressure (BP)

2. Blocks adenosine binding

What is the role of Purine-nucleoside phosphorylase (PNP)?

• seperates free purine base from the ribose (or deoxyribose)

Reaction involving PNP (Purine-nucleoside phosphorylase)

(Deoxy)nucleoside + Pi → Base + (Deoxy)-α-D-ribose-1-phosphate (aka ribose sugar with one phosphate attached)

What happens when uric acid is overproduced?

Gout is formed

• insoluble Sodium Urate crystallizes

What deficiency causes gout?

• HGPRT deficiency or defective purine biosynthesis regulation

What is the treatment for gout?

Allopurinol (Xanthine Oxidase inhibitor)

What prevents high uric acid levels?

Allopurinol

What is AMP converted to in intense exercise?

AMP → IMP + NH3

What enzyme converts AMP to IMP?

AMP deaminase

What happens when AMP levels decreases?

Pushes for increased ATP production

Net reaction for 1 Purine Nucleotide Cycle

Asparate + GTP + H2O → Fumerate + GDP + Pi + NH3

Purine Nucleotide Cycle Step 1

AMP + H2O → IMP + NH3

Purine Nucleotide Cycle Step 2

IMP + GTP + Asparate → Adenylosuccinate + GDP + Pi

Purine Nucleotide Cycle Step 3

Adenylosuccinate → Fumerate + AMP

• then cycle continues

Azathioprine

• purine synthesis inhibitor

• Also decreases the proliferation of rapidly dividing cells

• Immunosuppressant drug used in organ transplantation and autoimmune diseases (Arthritis)

Mycophenolate mofetil

• Another immunosuppressant drug = prevents organ transplant rejection

• inhibits purine synthesis by blocking inositol monophosphate dehydrogenase

Methotrexate

• indirectly inhibits purine synthesis by blocking folic acid metabolism

  • inhibits the enzyme dihydrofolate reductase (DHFR = Folate Cycle)

Allopurinol

• inhibits the enzyme xanthine oxireductase = decresing uric acid levels

• treatment for gout

Cholchicine

• reduces inflammation

• no effect on uric acid synthesis

• Reduces macrophage ingestion of uric acid crystals.

Probenecid and sulfinpyrazone

• increase uric acid excretion

Hypoxanthine

Xanthine

Guanine

Guanase

Uric Acid