Nucleotide Metabolism

Amino Acids are precursors to

• immune system signals

• hormones

• membrane lipid constituents

• electron carriers

• nucleotide bases

Salvage Pathways

Activated ribose (PRPP) + base → nucleotide

• preformed bases

• attached to an activated ribose

Nucleoside is a base attached to a

sugar

Nucleotide is a nucleoside with one or more

phosphoryl groups

De Novo Pathway

activated ribose (PRPP) + AAs + ATP + CO2 → nucleotide

Pyrimidine pathways to RNA

UTP → TTP

Pyrimidine pathways to DNA

CTP → dCTP

Purine pathway to RNA

ATP → dATP

Purine pathway to DNA

GTP → dGTP

Purines are synthesized on a ____ molecule in the de novo pathway

ribose

PRPP and glutamine form

5-phosphoribosyl-1-amine in the beta configuration

What catalyzes the reaction where 5-phosphoribosyl-1-amine is formed?

glutamine phosphoribosyl amidotransferase

5-phosphoribosyl-1-amine is converted to ___ ___ in 9 steps

inosine monophosphate (IMP)

IMP formed both ___ and ____

AMP and GMP

IMP is converted into adenylate (AMP) in a pathway that requires

GTP

IMP is also metabolized to guanosine monophosphate (GMP/guanylate) in a pathway that requires

ATP

Two salvage enzymes recycle

purine bases

• APRT

• HGPRT

What catalyzes the formation of adenylate

Adenine phosphoribosyltransferase (APRT)

What catalyzes the formation of inosinate and guanylate?

hypoxanthine-guanine phosphoribosyltransferase (HGRPT)

What end products inhibit glutamine-PRPP amidotransferase?

• IMP

• AMP

• GMP

Excess GMP inhibits formation of xanthylate from inosinate by

IMP dehydrogenase

GTP is needed for ___ synthesis

AMP

ATP is needed for ___ synthesis

GMP

PRPP synthesis is inhibited by ___ and ___

ADP and GDP

Pyrimidines are synthesized from:

• bicarbonate

• aspartate

• ammonia

• glutamine (as a N donor)

What catalyzes the first step 3-step reaction in pyrimidine synthesis to form carbamoyl phosphate

carbamoyl phosphate synthetase II (CPS II)

First Name =

substrate

Last Name =

Rxn

Carbamoyl phosphate + aspartate =

carbamoylaspartate

• rxn catalyzed by aspartate transcarbamolyase (ATCase)

  • regulatory enzyme in pyrimidine synthesis

Carbamoylaspartate is metabolized to

orotate

Difference between orotate and uracil

COO- on orotate ring

orotate reacts with activated ribose in the form of ______ to form orotidylate

5-phosphoribosyl-1-pyrophosphate (PRPP)

What enzyme synthesizes PRPP from ribose 5-phosphate and ATP?

PRPP synthetase

Orotidylate is converted to uridine monophosphate (UMP/uridylate) by

orotidylate decarboxylase

UMP is one of the 2 major pyrimidines found in

RNA

What amino acids make up the purine rings?

• glutamine

• aspartate

• glycine

UMP is converted to UDP and UTP via

kinases

• adds phosphate to subtrates

Phosphatases

• OPPOSITE of kinase

• removes phosphate groups

CTP is formed by the action of ____ ____ on UTP

cytidylate synthetase

Rising CTP levels ___ ATCase

inhibit

Triphosphates CTP is formed from UTP by the replacement of a

carbonyl group with an amino group

• GLUTAMINE provides the amino group

What enzyme is responsible for salvaging thymine during DNA degradation?

thymidine phosphorylase

What enzyme is responsible for generating thymine nucleotides?

thymidine kinase

The 2’ C-OH bond is directly reduced to 2’-H bond and is catalyzed by

ribonucleotide reductase

Explain the production of deoxyribonucleotides from ribonucleotides

1. 2 H atoms are donated by NADPH (e- donor) and carried by proteins thioredoxin or glutaredoxin

2. glutathione pathway OR thioredoxin pathway

3. sulfhydryls in the reductase become oxidized (reductase must be reduced)

4. reduced thioredoxin is generated by e- flow from NADPH

   • CATALYZED BY THIOREDOXIN REDUCTASE

5. electrons flow from NADPH to bound FAD of reductase to

6. disulfide of oxidized thioredoxin to

7. ribonucleotide reductase to

8. ribose unit

Explain the role of thymidylate synthase in the production of dTMP

adds a methyl group to dUMP to form thymidylate

Thymidylate is formed by the methylation of

deoxyuridylate

deoxyuridine diphosphate (dUDP) is converted into

dUMP

Explain the step catalyzed by DHFR (dihydrofolate reductase) enzyme

regenerates tetrahydrofolate from dihydrofolate

Explain the conversion of Guanine, Hypoxanthine, and xanthine to uric acid, and include the enzymes involved.

• dephosphorylation of purine via 5’-nucleotidase

• deamination and hydrolysis of ribose → xanthine

• hypoxanthine and xanthine are oxidized into uric acid by xanthine oxidase

thymine is converted to dihydrothymine via

dihydrouracil dehydrogenase (redox step)

What is the drug that inhibits xanthine oxidase and is an anti-gout agent?

allopurinol

Adenine reacts with PRPP to form the adenine nucleotide AMP and is catalyzed by

adenosine phosphoribosyltransferase

The lack of hypoxanthine-guanine phosphoribosyltransferase leads to

Lesch-Nyhan syndrome with neurological impairment and finger-toe biting behavior

Glutamine analogs

• azaserine, acivicin

• inhibits glutamine amidotransferases

Fluorouracil

• converted by salvage pathway into FdUMP

  • inhibits thymidylate synthase

Methotrexate and aminopterin

inhibits dihydrofolate reductase (competitive inhibitor)

T/F: inosinate is the purine nucleotide that is the precursor of both adenylate and guanylate

true

Which is the product of purine degradation in humans?

uric acid

The ribosyl phosphate moiety needed for the synthesis of orotidylate, inosinate, and guanylate is provided by

5-phosphoribosyl-1-pyrophosphate

In purine biosynthesis, which contributes 2 of the ring nitrogens?

glutamine

Precursors for the biosynthesis of the pyrimidine ring system include:

carbamoyl phosphate and aspartate