Dr. Aga Syed SameerCSIR Lecturer (Demonstrator)

Department of Biochemistry,

Medical College,

Sher-I-Kashmir Institute of Medical Sciences,

Bemina, Srinagar, Kashmir, 190010. India.

Nucleic Acid MetabolismPyrimidine Metabolism

IMP Pathway

AMP and GMP Pathway

• Important for those cells which are not capable of de novo synthesis of purine nucleotides

• Brain

• RBCs

• Leukocytes

Salvage Pathway

Salvage Pathway

Hypoxanthine

Guanine

Adenine

PRPP

IMP

GMP

AMP

PPi

HGPRTase

AGPRTase

Kinases for Interconversion

AMP

+

GMP

ATP

2ADP

GDP + ADP

Adenylate Kinase

Guanylate Kinase

Pyrimidine Biosynthesis

• UMP (Uridine Monophosphate)• Precursor to

• TMP and CTP

• Utilizes (Substrates)• One Aspartate

• γ-NH3 of Glutamine

• CO2 is recycled (Used in 1st step and released back in last)

• Water molecule

• NAD+

• ATP’s (Two : Till UMP Synthesis)

• Methyl group of THF + GTP {dTMP Synthesis}

• NH3 of Glutamine + ATP (2 Equivalent) {CTP Synthesis}

2 ATP + HCO3- + Glutamine + H2O

CO

O PO3-2

NH2

Carbamoyl Phosphate

NH2

C

N

H

CH

CH2

C

COOO

HO

O

Carbamoyl Aspartate

HN

C

N

H

CH

CH2

C

COOO

O

Dihydroorotate

HN

C

N

H

C

CH

C

COOO

O

Orotate

HN

C

N

C

CH

C

COOO

O

HH

CH2

OH OH

H H

OO

2-O3P

Orotidine-5'-monophosphate

(OMP)

HN

C

N

CH

CH

C

O

O

HH

CH2

OH OH

H H

OO

2-O3P

Uridine Monophosphate

(UMP)

2 ADP +

Glutamate +

Pi

Carbamoyl

Phosphate

Synthetase II

Aspartate

Transcarbamoylase

(ATCase)

Aspartate

Pi

H2O

Dihydroorotase

Quinone

Reduced

QuinoneDihydroorotate

Dehydrogenase

PRPP PPi

Orotate Phosphoribosyl

Transferase

CO2

OMP

Decarboxylase

Pyrimidine Biosynthesis

Enzymes and Reactions• 2 condensation Rxns: form carbamoyl aspartate and

dihydroorotate (intramolecular)

• Dihydroorotate dehydrogenase (FMN) is an intra-mitochondrial enzyme; oxidizing power comes from ubiquinone reduction

• Attachment of base to ribose ring is catalyzed by OPRT; PRPP provides ribose-5-P.

• OMP Decarboxylase enhances the rate of decarboxylation by a factor of 2x1023

• Channeling: • Enzymes 1, 2, and 3 are on same chain;

• Enzymes 5 and 6 are on same chain

CTP and UTP synthesis• UMP is then converted UDP &

UTP

• The conversion takes place by the action of Nucleoside Mono/Di Phosphate Kinases.

• CTP formed from UTP via CTP Synthetase driven by ATP hydrolysis • Glutamine provides amide

nitrogen

Conversion of Ribonucleotidesto Deoxyribonucleotides• Ribonucleotide reductase

• NADP

• Thioredoxin reductase

Regulation of RNR’s

• ATP activates

• dATP Inhibits

dTMP synthesis• dUMP formed; produces dTMP via Thymidylate

Synthetase• N5,N10 Methylene THF provides methyl group

• THF is first reduced and then methylated

dTMP synthesis

dTMP synthesis

Regulation• Differs between bacteria and animals

• Bacteria – regulation at ATCase rxn

• Animals – regulation at carbamoyl phosphate synthetaseII

• UDP and UTP inhibit enzyme;

• ATP and PRPP activate it

• Also UMP and CMP competitively inhibit OMP Decarboxylase

Degradation of Pyrimidines• CMP and UMP degraded to bases similarly to purines

• Dephosphorylation

• Deamination

• Glycosidic bond cleavage

• Uracil reduced in liver, forming β-Alanine

• Is then converted to malonyl-CoA used in fatty acid synthesis for energy metabolism

• dTMP is degraded to β-Amino Isobutyrate

• Is then converted to methyl malonyl-CoA used in fatty acid synthesis for energy metabolism