Lecture 23: Nucleotide Metabolism

What is the difference between a Nucleoside and Nucleotide?

• Nucleoside consists of a purine or pyrimidine base linked to sugar

• A nucleotide contains a base, ribose, and a phosphate ester

How can Nucleotides be synthesized?

 de novo—from simple precursors

How can pyrimidines be synthesized?

• Bicarbonate

• Aspartate

• Ammonia

• With glutamine as N donor

Step 1 of de novo sythesis of pyrimidines?

 A three-step reaction to form carbamoyl phosphate, catalyzed by carbamoyl phosphate synthetase II (CPS II)

CPSI

Catalyzes the first step in the Urea cycle: coupling of free ammonia with bicarbonate

Step 2 of de novo synthesis of pyrimidines?

Carbamoyl phosphate reacts with aspartate to form carbamoylaspartate by aspartate transcarbamolyase

The key regulatory enzyme in pyrimidine synthesis?

Aspartate transcarbamolyases

Does urea cycle need aspartate?

Yes, Aspartate condenses with citrulline to form argininosuccinate, a crucial intermediate in the cycle. 

Step 3 of de novo pyrimidine synthesis?

 Carbamoylaspartate is then metabolized to orotate

Step 4 of de novo pyrimidine synthesis?

Orotate reacts with activated ribose to form orotidylate

Step 5 of de novo pyrimidine synthesis?

Uridine monophosphate (UMP) is generated by the decarboxylation of orotidylate by orotidylate decarboxylase.

How is CTP generated from UMP?

(1) UMP kinase;

(2) Nucleoside diphosphate kinases;

(3) Amination

What do Salvage pathways use?

Preformed pyrimidine bases, recovered from nucleic acid breakdown, to

synthesize nucleotides

de novo synthesis of Purines

• Synthesized on PRPP

What is the commited step of purine synthesis catalyzed by?

• Glutamine phosphoribosyl amidotransferase

• Uses PRPP & glutamine to form 5-phosphoribosyl-1-amine

IMP

• Requires 9 steps to be formed from 5-phosphoribosyl-1-amine

• Converted into adenylate (AMP) in a pathway requiring GTP

• metabolized to guanosine monophosphate (GMP ) in a

  pathway that requires ATP

What are the bases derived from multiple amino acids in Purine synthesis?

1 glycine (2C,1N)

2 THF (2C)

1 CO2 (1C=O)

2 Glutamine (2N)

1 Aspartate (1N)

How is AMP and GMP synthesis?

AMP synthesis requires GTP and needs aspartate.

GMP synthesis requires ATP and needs glutamine

How is Deoxyribonucleotides synthesized?

• From ribonucleoside diphosphates

• By ribonucleotide reductase, which acts on all four ribonucleotides using NADPH

How is Pyrimidine biosynthesis is regulated?

Aspartate transcarbamoylase

At what steps are Purine biosynthesis is regulated?

1. AMP inhibits the formation of adenylosuccinate, a precursor to AMP. GMP inhibits the formation of xanthylate, a precursor to GMP

2. GTP stimulates the synthesis of AMP, while ATP stimulates the synthesis of GMP