Nucleotide MetabolismNucleotide Metabolism

Purine metabolism (Overview)

1.Nomenclature/nucleotide structure

2.Extracellular Hydrolysis of Ingested Nucleic Acid

3. De novo synthesis pathways

4. Re-utilization pathways

5. Metabolic diseases of purine Metabolism (Gout, Lesch-Nyhan, SCID)

Other bases that are not part of nucleic acids but have important metabolic roles include orotate, dihydroorotate, hypoxanthine and xanthine

Nucleosides are bases linked to a ribose or deoxyribose at the sugar's 1' position

Nucleotides, the building blocks of nucleic acids, are nucleoside phosphates, with the phosphodiester bond at the sugar's 5' position.

Ribonucleotides are synthesized first, then deoxyribonucleotide are synthesized from modified ribonucleotides.

Bases are the nitrogenated components of nucleic acids and may be purines (two rings) or pyrimidines (one ring). Adenine and guanine are the purines in nucleic acids Thymine, cytosine and uracil are the pyrimidines in nucleic acids

Roles of Nucleotides in CellularRoles of Nucleotides in Cellular MetabolismMetabolism

The The energyenergy currency in metabolic transactions. currency in metabolic transactions.

The essential chemical links in the response of The essential chemical links in the response of cells to hormones and other cells to hormones and other extracellular extracellular stimuliistimulii..The structural components of an array of The structural components of an array of enzyme enzyme Cofactors Cofactors and metabolic intermediates.and metabolic intermediates.

Biological significance of Biological significance of nucleotide metabolismnucleotide metabolism

Nucleotides make up nucleic acids (DNA and RNA)

Nucleotide triphosphates are the “energy carriers” in cells (primarily ATP)

Many metabolic pathways are regulated by the level of the individual nucleotides◦Example: cAMP regulation of glucose release

Adenine nucleotides are components of many of the coenzymes◦Examples: NAD+, NADP+, FAD, FMN, coenzyme

A

Medical significance of nucleotide Medical significance of nucleotide metabolismmetabolismAnticancer agents:

Rapidly dividing cells biosynthesize lots of purines and pyrimidines, but other cells reuse them. Cancer cells are rapidly dividing, so inhibitor of nucleotide metabolism kill them

Antiviral agents◦ Zidovudine (Retrovir)◦ Lamivudine (Epivir)◦ Valacyclovir (Valtrex)

Structures of nucleotide building Structures of nucleotide building blocks and nucleotidesblocks and nucleotides

N

N N

N

H

N

N1

2

3

4

5

6

PYRIMIDINE

1

2

34

56 7

8

9

PURINE

OHH

OHOH

CH2

H H

BaseOP

O

OH

OH

OHH

HOH

CH2

H H

BaseOP

O

OH

OH

RIBONUCLEOTIDE DEOXYRIBONUCLEOTIDE

Ribonucleotide – phosphate = ribonucleoside

The The PurinePurine and and PyrimidinePyrimidine bases found in nucleotides can bases found in nucleotides can be synthesized be synthesized de novode novo, or can , or can be obtained through be obtained through salvagesalvage pathways that allow the reuse pathways that allow the reuse of the preformed bases of the preformed bases resulting from resulting from normal cell normal cell turnover or from the dietturnover or from the diet..

Structures of Common Purine Bases.

A= 6 amino purineG= 2 amino, 6-oxy purine

H= 6 oxy purineX= 2,6 dioxy purine

Aspartate

Glutamine

(N source)

(N source)

The common mechanistic them for the conversion of A and G is the conversion of a carbonyl oxygen to an amino group

Hypoxanthine is an intermediate for Adenine and Guanine

There are two basic mechanisms to generate purines and pyrimidines

2. SALVAGE PATHWAYS(the reutilization of bases from dietary or catabolic sources)

1. DE NOVO BIOSYNTHETIC PATHWAYS(building the bases from simple building blocks)

The biosynthesis of purine (A and G) begins with the synthesis of the ribose-phosphate

Step 1:Activation of ribose-5-phosphateenzyme: ribose phosphate pyrophosphokinaseproduct: 5-phosphoribosyl--pyrophosphate (PRPP)PRPP is also a precursor in the biosynthesis of pyrimidine nucleotides and the amino acids histidine and tryptophan

Step 2The major regulatory step in purine biosynthesis is the conversion

of PRPP to 5-Phosphoribosyl-1-amine

PRPP

Glutamine Glutamate

PPiAmidophosphoribosyl

transferase

*

Amidophosphoribosyl transferase is an important regulatory enzyme in purine biosynthesis. It is strongly inhibited by the end products IMP, AMP, and GMP.

This type of inhibition is called FEEDBACK INHIBITION.

enzyme: amidophosphoribosyl transferasedisplacement of pyrophosphate group by glutamine amide nitrogenproduct: -5-phosphoribosylamine

Steps 1 and 2 are tightly regulated by feedback inhibition

Several amino acids are utilized in purine biosynthesis

IMP is the precursor for both AMP and GMP, the base is also called hypoxanthine

Step 2: displacement of pyrophosphate group by glutamine amide nitrogenStep 3: -phosphoribosylamine reacts with ATP and glycineStep 4: formylation of free -amino group of GARStep 5: The amide amino group of a second glutamine is transferred to form formylglycinamidine ribotide (FGAM)Step 6: Closing of the imidazole ring or formation of 5-aminoimidazole ribotideStep 7: acquisition of C6 as HCO3

-

Step 8: acquisition of N1 acquired from aspartate in an amide condensation reactionStep 9: Elimination of fumarateStep 10: acquisition of C2 Another formylation reactionStep 11: cyclization or ring closure to form IMP water is eliminated

Purine nucleoside diphosphates and triphosphates: - to be incorporated into DNA and RNA, nucleoside monophosphates (NMP’s) must be converted into nucleoside triphosphates (NTP’s)

- nucleoside monophosphate kinases (adenylate & guanylate kinases)

- nucleoside diphosphate kinase

AMP + ATP 2 ADP

GMP + ATP GDP + ADP

accomplished by separate enzymes

GDP + ATP GTP + ADP

same enzyme acts on all nucleotide di & triphosphatesnucleoside diphosphate kinase is an enzyme which playsa key role in the activation of antiviral nucleosides such as Retrovir/AZT

Last step From IMP to GMP and AMP

Conversion of Hypoxanthine to Adenine/Guanine.

Aspartate

Glutamine

(N source)

(N source)

The common mechanistic theme for the conversion of A and G is the conversion of a carbonyl oxygen to an amino group

Ribose5-phosphate PRPP Phosphoribosyl

amineIMP

AMP

GMP

Inhibited by IMP,AMP, and GMP

Inhibited by AMP

Inhibited by GMP

The regulation of purine biosynthesis is a classic example of negative feedback

Salvage pathways for the re-utilization of purines;

There are 2 salvage enzymes with different specificity;

1. Adenine phosphoribosyl transferase2. Hypoxanthine-guanine phosphoribosyl transferase

PRPP + Adenine Adenylate

PRPP + Guanine Guanylate

A-PRT

HG-PRT

O P

OH

O

O

CH2O

OH OH

PPiO P

OH

O

O

CH2O

OH OH

PPi+Base

(ie Adenine) +

A

Dietary nucleoprotein is degraded by pancreatic enzymes and tissue nucleoprotein by lysosomal enzymes.

The nucleic acids are hydrolyzed randomly by nucleases to yield a mixture of polynucleotides.

polynucleotides are further cleaved by phosphodiesterases (exonucleases) to a mixture of the mononucleotides

The nucleotides are hydrolyzed by nucleotidases to give the nucleosides and Pi.

The nucleosides undergo phosphorolysis with nucleoside phosphorylases to yield the base and ribose 1-P (or deoxyribose 1-P).

The purine and pyrimidine bases released are either degraded or salvaged for reincorporation into nucleotides

Nucleic acid degradation

Purines in humans are degraded to Urate

1. Nucleotides are constantly undergoing turnover!

2. There are many enzymes involved; Nucleotidases Nucleoside phosphorylases Deaminases Xanthine oxidases

3. the final common intermediate in humans is Urate, which is excreted.

4. there are several metabolic disorders resulting from defects in purine catabolism.

N

NN

N

O

H

HNH2

Guanine

H O2

NH4

+

ribose

N

NN

NH

N

Adenosine

2

Adenosinedeaminase

(ADA)

H O2 NH4+

N

NN

ribose

N

Inosine

O

H

purine nucleosidephosphorylase

Pi ribose - 1 - PN

NN

N

Hypoxanthine

O

H

H

xanthineoxidase

N

NN

N

O

H

H

Xanthine

HO

xanthineoxidase

N

NN

N

O

H

H

Uric acid

HO

H

O

N

NN

N

O

H

NH2

Guanosineribose

purine nucleosidephosphorylase

Piribose - 1 - P

Guaninedeaminase

Catabolism of Purine Nucleotides