Nucleotide Metabolism

1
Chapter 8

• Nucleotide Metabolism

2
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.
3
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.
4

• Pentose is absorbed but base is degraded and
  excreted.
• Fnuctions of
  Nucleotides
• They serve as building blocks of nucleic acids.
• ATP plays an important pole in energy
  transformation.
• ATP , ADP, and AMP may function as allosteric
  regula-
• tors and participate in regulation of many
  metabolic path-
• ways. ATP involves in covalent
  modification of enzymes.

5
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.
6
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.
7
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
8
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---------------------------?PR
PP AMP
amidotransferase PRPP Gln-----------------------
----?PRA Glu
9
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
10
Stage
Two The conversion of IMP either to AMP or GMP
requires two reactions.
GTP,Mg,adenylosuccinate synthase IMP
Asp-------------------------------?adenylosuccinat
e adenylosuccinate
lyase Adenylosuccinate-----------------------?AMP
fumarate
11
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 phos-
phorylation or substrate level phosphorylation.
12
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
13
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 nuc- leotides.
14
Salvage Pathway of Purine
Nucleotides Many cells have mechanisms to
retrieve purine bases and purine nucleosides.
They are used to synthesize purine nu-
cleotides. This is the salvage pathway.
15
From Base to Nucleotides
APRT A
PRPP--------------------------------?AMP ppi
HGPRT H
PRPP--------------------------------? IMP ppi
HGPRT G
PRPP--------------------------------?GMP ppi
16
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.
17
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.
18
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
)2NADPH NDP reductase is an allosteric enzyme,
Its activity is controled by various NTPs and
dNTPs.
19
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 nucleo- tides. Thus they block purine
nucleotides synthesis or interfere in nuc- leic
acids synthesis.
20
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.
21
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.
22
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.
23
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.
24
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.
25
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
26
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.
27
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
28
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.
29
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
30
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.
31
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.
32
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 ß alanine. T is
degraded to CO2, H2O and ß aminoisobutyric
acid. fig 8-11