De novo purine 2 (24)

What is the next portion of the de novo pathway?

Base formation

Step #3 of purine pathway

Addition of glycine (ring)

Step #4 of purine pathway

C from N¹⁰-formyl THF (folate)

Step #5 of purine pathway

N from glutamine sidechainS

Step #6 of purine pathway

Cyclization

Step #7 of purine pathway

Addition of HCO₃^-

Step #8 of purine pathway

N from aspartate backbone

How is that addition performed?

In 2 steps:

1. formation of adduct

2. release of fumarate

What key intermediate is formed after that addition?

AICAR

Why is AICAR structurally interesting?

It is an AMP analog = can stimulate AMP-dependent protein kinase (AMPK)

Step #9 of purine pathway

2nd C from N¹⁰-formyl THF (folate)

Step #10 of purine pathway

2nd ring closure

What other key intermediate is formed after that step?

Inosine monophosphate or IMP or inosinate

Why is it a key molecule?

It is the branch point for AMP and GMP synthesis

What substrates does AMP synthesis require?

Aspartate backbone + GTP

What substrates does GMP synthesis require?

Glutamine sidechain + ATP via XMP (NAD⁺)

What kind of kinases are used for diphosphate formation?

Base specific kinases + non-specific for ribose/deoxyribose

Then, give an example of DNA AMP becoming a diphosphate

(d)AMP + atp → (d)ADP + adp

What is the enzyme used for triphosphate formation?

Nucleoside diphosphate kinase

What is intersting ab the regulation of these reactions?

They run close ot thermodynamic equilibrium (ΔG~0)

Compare RNA and DNA intracellular amounts

7 [RNA] = [DNA]

Compare the NTP and dNTP intracellular amounts

7 [NTP] = [dNTP]

Compare the NTP and NDP intracellular amounts

7 [NTP] = [NDP]

Compare the NDP and NMP intracellular amounts

7 [NDP] = [NMP]

Discuss levels of regulation in the purine pathway

1. Activation by PRPP

2. Feedback inhibition by nucleotides (mono, di, tri)

What does that mean pharmacologically?

It is the main target when there’s an excess (gout)

What is THF?

Essential vitamin that also works as a cofactor

Discuss the carbon supplying capacities of THF

1. Carbons in 3 different oxidation states

2. Carbons in 5 different chemical forms

How do antibiotics work?

Sulfonamides have a similar structure to p-Aminobenzoic acid. This inhibits folate synthesis in bacteria while humans get it from food.

What are the 5 groups carried by THF?

From reduced to oxidized:

1. Methyl

2. Methylene

3. Formyl

4. Formimino

5. Methenyl

What do folate supplements reduce?

Risk for neural tube defects