Nucleotides
Why have them?

I.  Nucleotide triphosphates (NTPs) and deoxynucleotide triphosphates

    (dNTPs) are activated precursors of RNA and DNA synthesis

II.  ATP, the universal currency of energy.

III.  NTPs  are required in the activation of molecules in synthetic pathways.

IV.  NTPs regulate protein function.

   -through phosphorylation

  -through binding and hydrolysis (G-proteins, Chaperones)

V.  Signal Transduction

  -formation of  cyclic nucleotides

VI.  Synthesis of nicotinamide (NAD), flavin (FAD) and co-enzyme A.

Purine Biosynthesis
the first committed step

-Glutamine-PRPP amidotransferase

Inosine

-the first purine made in purine biosynthesis

-The base of IMP is hypoxanthine.

Conversion of IMP to AMP and GMP

IMP to AMP (: GTP is used for AMP synthesis.)

1) Adenylosuccinate  synthetase

2) Adenylosuccinate lyase

IMP to GMP (ATP is used for GMP synthesis)

1) IMP dehydrogenase 

Formation of AMP and GMP
(What you really to know about the previous slide.)

-IMP is the precursor for both AMP and GMP.

-Reciprocal use of ATP to generate GMP and GTP to generate AMP

serves to balance the relative synthesis the two purines.

-if ATP is higher than GTP more IMP will be converted to GMP

-if GTP is higher than ATP more IMP will be converted to AMP.

Purine Biosynthesis
Regulation

1.  PRPP synthetase

 2.  Glutamine-PRPP amidotransferase

  (the first committed step)

3. AMP inhibits adenylosuccinate synthetase

4.  GMP inhibits IMP dehydrogenase

         equilibrates AMP and GMP synthesis.

Pyrimidine Biosynthesis

-The pyrimidine ring is made from carbamoyl phosphate and aspartate.

-Carbamoyl synthetase II-

generates carbamoyl phosphate in cytosol for pyrimidine biosynthesis.

-Glutamine + 2 ATP + HCO3 → Carbamoyl phosphate + ADP+ Pi + Glutamate

Pyrimidine Biosynthesis
the first committed step

-Transcarbamoylation of aspartate.

Pyrimidine Biosynthesis
ring closure

-Carbamoyl synthetase II, aspartate transcarbamoylase  and dihydroorotase are in the same multifunctional enzyme sometimes called CAD.

-CAD is a trimer with each protein containing an active site for each

reaction.

Pyrimidine Biosynthesis

OROTICACIDURIA I and II

-Oroticaciduria type I:    Defect in the orotate phosphoribosyl transferase

and orotidylate decarboxylase .

 -Oroticaciduria type II: Defect in the orotidylate decarboxylase.

-Patients are anemic.  Orotate accumulates  to high levels.

-These patients are uridine auxotrophs, i.e. uridine is essential.

-Uridine supplementation also decreases orotic acid production, via inhibition of carbamoyl synthetase II.

Pyrimidine Biosynthesis
regulation

-Mammals including man regulate

Carbamoyl synthetase II.

-ATP and PRPP activate

-UTP and CTP inhibit.

Urea Cycle and Pyrimidine Synthesis

-Urea Cycle defects may leadto carbamoyl phosphate accumulation and increased orotate production. 

-Carbamoyl phosphate Synthetase I

Urea Cycle Enzyme

Mitochondrial Enzyme

Uses NH4 + HCO3-

-Carbamoyl phosphate Synthetase II

Pyrimidine Synthetic Enzyme

Cytoplasmic Enzyme

Uses Glutamine + HCO3-