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Chapter 8
Nucleotide Metabolism
Nucleotides are bilding blocks of nucleic acids.
They are non-essential nutrients , because they can be
synthesized in the body.
Nucleic acids occur in the nucleoprotein.
Dietary nucleoprotein is digested in the stomach to yield
protein and nucleic acids.
Nucleic acids are further digested in the small intestine to
generate nucleotides.
Nucleotides are absorbed into intestinal mucosa cells , where
they are degraded to three components : base , pentose and
phosphate.
Pentose is absorbed but base is degraded and excreted.
Fnuctions of Nucleotides
1. They serve as building blocks of nucleic acids.
2. ATP plays an important pole in energy transformation.
3. ATP , ADP, and AMP may function as allosteric regulators and participate in regulation of many metabolic pathways. ATP involves in covalent modification of enzymes.
4. CAMP and cGMP are second messengers.
5. They are components of some coenzymes , such as
ADP in FAD , ADP in HSCoA etc.
6 . They are carriers of active metabolic intermediates
such as UDPG , SAM , CDP-DG etc.
Purine Nucleotide Metabolism
Anabolism
There are two pathways of synthesis of purine nucleotides :
the de novo synthesis pathway and the salvage pathway.
The former is the main synthesis pathway of nucleotides ,
the latter is important I brain and bone marrow.
The de novo synthesis of purine nucleotide means using
phosphoribose , amino acids , one carbon units and CO2
as raw materials to synthesize purine nucleotide from the
beginning.
fig 8-2
The pathway can be divided into two stages.
Stage one : formation of inosine monophosphate ( IMP )
Stage two : conversion of IMP to either AMP or GMP
Stage One
PRPP synthetase
R5P + ATP---------------------------PRPP + AMP
amidotransferase
PRPP + Gln---------------------------PRA + Glu
Once PRA is formed , the building of the purine ring
structure begins.
In nine successive reactions the first purine nucleotide
IMP is formed.
fig. 8-3
Stage Two
The conversion of IMP either to AMP or GMP requires
two reactions.
GTP,Mg++,adenylosuccinate synthase
IMP + Asp-------------------------------adenylosuccinate
adenylosuccinate lyase
Adenylosuccinate-----------------------AMP + fumarate
IMP dehydrogenase
IMP + H2O + NAD+---------------XMP + NADH + H+
ATP, Mg++, GMP synthase
XMP + Gln------------------------------------GMP + Glu
Nucleoside triphosphates are the most common nucleotide
used in metabolism.
ATP is synthesized from ADP and Pi via oxidative phosphorylation or substrate level phosphorylation.
ADP is synthesized from AMP in a reaction catalyzed by
adenylate kinase.
AMP + ATP------------------------- 2ADP
Other NTPs are also synthesized in ATP-requiring reactions
catalyzed by corresponding NMP kinases.
NMP + ATP-------------------------NDP + ADP
NDP kinase catalyzes the formation of NTP.
NDP + ATP-------------------------NTP + ADP
Regulation of de novo Pathway
PRPP activates amidotransferase.
IMP , AMP and GMP inhibit PRPP synthetase.
AMP inhibits conversion of IMP to GMP and GMP inhibits
conversion of IMP to GMP.
ATP stimulates conversion of IMP to GMP and GTP stimulates
conversion of IMP to AMP.
That ensures a balanced synthesis of both families of purine nucleotides.
Salvage Pathway of Purine Nucleotides
Many cells have mechanisms to retrieve purine bases and
purine nucleosides. They are used to synthesize purine nucleotides.
This is the salvage pathway.
From Base to Nucleotides
APRT
A + PRPP--------------------------------AMP + ppi
HGPRT
H + PRPP-------------------------------- IMP + ppi
HGPRT
G + PRPP--------------------------------GMP + ppi
From Nucleoside to Nucleotide
AR kinase
AR + ATP--------------------------------AMP + ADP
In comparison to de novo pathway, salvage pathway
is energy-saving.
In brain and bone marrow tissues salvage pathway is the
only pathway of nucleotide synthesis.
Deficiency of HGPRT causes Lesch Nyhan syndrome.
Formation of Deoxynucleotides
Deoxynucleotides are formed by reducing ribonucleotide diphosphates.
ribonucleotide reductase
NDP + NADPH + H+-----------------dNDP + H2O + NADP+
In the reaction hydrogen atoms are not directly donated by NADPH.
Thioredoxin, a protein with two sulfhydryl groups mediates the trans-
fer of hydrogen atoms from NADPH to ribonucleotide reductase.
Then the enzyme catalyzes the reduction of NDP, to form dNDP.
NDP reductase
NDP + thioredoxin ( SH )2 --------dNDP + thioredoxin (-S-S-)
The regeneration of reduced thioredoxin is catalyzed by thioredoxin
reductase.
thioredoxin (-S-S-) +NADPH +H+ thioredoxin ( SH )2+NADPH
NDP reductase is an allosteric enzyme, Its activity is controled by
various NTPs and dNTPs.
Antimetabolites of Purine Nucleotides
Antimetabolites of purine nucleotides are analogues of purine,
amino acids or folic acid.
They either act as competitive inhibitors of enzymes in purine
nucleotides synthesis or can be incorporated into purine nucleotides.
Thus they block purine nucleotides synthesis or interfere in nucleic acids synthesis.
6-MP and 6-MG are purine analogues
6-MP can be converted to 6-MP nucleotide in the body.
6-MP nucleotide is structurally similar to IMP and inhibits
conversion of IMP to AMP and GMP.
It also blocks synthesis of PRA from PRPP , synthesis of AMP
fromA , synthesis of GMP and IMP from G and H respectively.
Azaserine and diazonorleucine are amino acid analogues.
They are analogues of Gln and interfere with Gln in purine
nucleotide de novo synthesis.
Aminopterine and MTX are folic acid analogues.
They inhibit DHF reductase and block transfer of one carbon
unit. Thus purine nucleotide systhesis is blocked.
Catabolism of Purine Nucleotide
AMP undergoes hydrolysis and deamination, the A residue is
converted to H. H is oxidized , yielding X and X is oxidizd ,
yielding uric acid.
GMP is hydrolyzed and G is released. G is converted to X and
X is oxidized yielding uric acid.
In the human body the purine ring can not be degraded.
Uric acid contains the purine ring and is less soluble in
water.
Certain genetic defects in purine metabolism can cause
high blood levels of uric acid and results in a disease
known as gout.
In the disease sodium urate crystals are deposited in and
around joints and in the kidney.
Allopurinol is used to treat gout.
It is an analogue of H and inhibits xanthine oxidase which
catalyzes the oxidation of H and X.
Thus it inhibits uric acid formation.
Pyrimidine Nucleotide Metabolism
There are also two synthesis pathways of pyrimidine nucleotides:
denovo and salvage pathway.
De Novo Synthesis Pathway
In de novo pathway the pyrimidine ring is assembled first and then
linked to ribose phosphate.
The carbon and nitrogen atoms in the pyrimidine ring are derived
from bicarbonate, aspartate, and glutamine.
fig 8-9
It begins with the formation of carbamoyl phosphate.
Carbamoyl phosphate synthetase II in cytoplasm catalyzes the
reaction.
Gln + HCO3¯+ 2ATP----------------CP + Glu + 2ADP + Pi
Carbamoyl phosphate reacts with aspartate to form carbamoyl
aspartate . The closure of the pyrimidine ring is then calalyzed
dihydroorotase.
The product undergoes oxidation, addition of ribose phosphate
and decarboxylation, and is converted to UMP.
fig 8-10
UMP is a precursor for the other pyrimidine nucleotides.
Two sequential phosphorylation reactions form UTP which
then accepts an amide nitrogen from Gln to form CTP.
UMP-------UDP--------UTP UTP + Gln---CTP + Glu
d
dUMP is produced by dephosphorylation of dUDP.
The methylation of dUMP yields dTMP ( TMP ).
Regulation of de novo Pathway
In mammalian UMP inhibits CPS II.
Puring and pyrimidine nucleotide synthesis are coodinately
regulated.
Salvage Pathway
Pyrimidine phosphoribosyl transferase catalyzes the
following reaction.
Py + PRPP-------------- PyMP + ppi
Uridine kinase catalyzes the formation of UMP from
uridine and ATP.
UR + ATP------------- UMP + ADP
Antimetabolites of Pyrimidine Nucleotides
5FU is structurally similar to T.
It is converted to FdUMP or FUTP in the body.
FdUMP blocks the synthesis of dTMP.
FUTP can be incorporated in an RNA molecule in the
form of FUMP and thus inactivates it.
The machanism of amino acid and folic acid analogues
interfering with pyrimidine nucleotide synthesis is similar
to that with purine nucleotide synthesis.
Some nucleoside analogues such as arabinocytidine and
cyclocytidine are also important antimetabolites.
Arabinocytidine inhibits reauction of CDP to dCDP.
Catabolism of Pyrimidine Nucleotide
Pyrimidine nucleotides are hydrolyzed, yielding the building
blocks of pyrimidine, ribose and phosphate.
C and U are degraded to CO2, H2O and β alanine.
T is degraded to CO2, H2O and β aminoisobutyric acid.
fig 8-11