5-2 Necleotide Metabolism (Pyrimidine)

8/12/2019 5-2 Necleotide Metabolism (Pyrimidine)

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Cytidine MonophosphateCytosine Cytidine

Base Nucleotide

Base (P04ester)

Nucleoside*

Base

Nucleotidase

Phosphorylase

The nomenclature of pyrimidine depends

on their linkage to a pentose


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shorter pathway than for purines

base is made first, then attached to ribose-P

(unlike purine biosynthesis)

only 2 precursors (aspartate and glutamine, plusHCO3

-) contribute to the 6-membered ring

requires 6 steps (instead of 11 for purine)

the product is UMP (uridine monophosphate)


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Condensation of glutamine, bicarbonate in thepresence of ATP

Carbamoyl phosphate synthetase exists in 2 types:

CPS-Iwhich is a mitochondrial enzyme andis dedicated to the urea cycle and arginine

biosynthesis)CPS-II, a cytosolic enzyme used here


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CPS-II is the major site of regulation in animals: UDP and

UTP inhibit the enzyme and ATP and PRPP activate it

It is the committed step in animals


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enzyme is aspartate transcarbamoylase(ATCase)

catalyzes the condensation of carbamoylphosphate with aspartate with the release of Pi

ATCase is the major site of regulation inbacteria; it is activated by ATP and inhibited by

CTPcarbamoyl phosphate is an activatedcompound, so no energy input is needed at thisstep


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enzyme: dihydroorotase

forms a pyrimidine from carbamoylaspartate

water is released in this process


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In E. coli, the first 3 enzymatic reactions arecatalyzed by 3 separate proteins/enzymes

In animals, all 3 steps are found in a multifunctionalenzyme (210 kD).

The acronym CAD is used as a name for the

multienzyme: carbamoyl phosphate synthetase,aspartate transcarbamoylase and dihydroorotase


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An irreversible reaction

Enzyme: dihydroorotate dehydrogenase

Oxidizing power is derived from quinones(thru coenzyme Q)


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Enzyme: orotate phosphoribosyl transferase

Ribose phosphate originates from PRPP

product is orotidine-5-monophosphate(OMP)

orotate phosphoribosyl transferase is also

used in salvage of uracil and cytosine to theircorresponding nucleotide


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enzyme: OMP decarboxylase

product: uridine monophosphate (UMP)

In animals, steps 5 and 6 are catalyzed bya single polypeptide with 2 active sites


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OCH2 H

H

OH

O

OH

H

P O

O

O-

P

O

O-

O-

OPHO

O

OH

H

HN

N

O

O COO-

Horotate phosphoribosyl transferase

PRPP

PPi

O

CH2

H

OH

H

OH

H

OPHO

O

OH

N

HN

O

O COO-

H

orotidine monophosphate (OMP)

O

CH2

H

OH

H

OH

H

OPHO

O

OH

N

HN

O

O

HCO2

OMP decarboxylase

(uses no cofactors)


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An inherited human diseasecaused by a deficiencyin the multifunctional enzyme that catalyzes the last2 steps in the pyrimidine synthesis

large amounts of orotic acid in urine

retarded growth and severe anemia

treat by administration (injection) of uridine and/or

cytidine


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UMP + ATP UDP + ADP

UDP + ATP UTP + ADP

nucleoside diphosphate kinase

CTP synthase (cytidylate synthetase)

N

N

O

H

O

Ribose 3phosphate

N

NO

Ribose 3phosphate

NH2glutamine +

ATPGlutamate +

ADP +Pi

CTPUTP

(in bacteria, ammonia donates the amino group)


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Glutamine +

HCO3-

+ATP

Carbamoyl

phosphate

carbamoyl phosph.

synthetase

Orotate

OMPUMPUTP + CTP

orotate

phosphoribosy

transferase

UTP and CTP are feeback inhibitors of CPS II


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O

HH

OHOH

CH2

H H

OP

O

OH

OHBase

O

HH

HOH

CH2

H H

OP

O

OH

OHBase

ribonucleotide reductase

dADP, dGDP, dUDP and dCDP are all synthesized by the same enzyme

Synthesized from nucleoside diphosphate (not mono or triphosphate) by

ribonucleotide reductase

All pathways shown previously led to synthesis of ribonucleotides


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Methylation of d-UMP via N5,N10-

methylene THF

Reaction inhibited by 5-fluorouracil (Efudex)

O


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HN

N

O

H

HO

OCH2

H

OH

H

H

H

OP

O-

O

-O

H

H

OCH2

H

OH

H

H

H

OP

O-

O

-O

HN

N

O

CH3

HO

HN

N

N

NH2N

O N

H

H

H

RH

H

HN

N

N

NH2N

O N

H

H

RH

d-UMP


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Catabolism of a pyrimidine


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5-2 Necleotide Metabolism (Pyrimidine)