Met Purine Met Learning Objectives 1 How Are Purines Synthesized 2 How Are Purines Catabolized 3 How Are Pyrimidines Synthesized and Catabolized 4 How Are ID: 935200
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Slide1
Nucleotide Metabolism
Pyrimidine
Met.
Purine
Met.
Slide2Learning Objectives
1. How
Are Purines Synthesized?
2.
How Are Purines Catabolized?
3.
How Are
Pyrimidines
Synthesized and
Catabolized
?
4.
How Are
Ribonucleotides
Converted to
Deoxyribonucleotides
?
5. How
Is
dUTP
Converted to
dTTP
?
6
.
Abnormal metabolism of uric acid.
7. Anticancer drugs targets
Slide3Two types of pathways lead to nucleotides: the
de novo pathways and the salvage pathways.
De novo
synthesis of nucleotides begins with their metabolic precursors:
amino acids
,
ribose 5-phosphate
,
CO
2
, and
NH
3
.
Salvage pathways
recycle the free bases and nucleosides released from nucleic acid breakdown.
Slide4Salvage and de Novo Pathways
Slide5The purine ring structure is built up one or a few atoms at a time, attached to
ribose
throughout the process.
The
pyrimidine
ring is synthesized as
orotate
, attached to
ribose phosphate
, and then converted to the common
pyrimidine
nucleotides required in nucleic acid synthesis.
Although the free bases are not intermediates in the de novo pathways, they are intermediates in some of the salvage pathways.
Slide6de Novo Pathway for Purine Nucleotide Synthesis
Slide7The synthesis of the purine ring is more
complex. The only major component is
glycine
,
which
donates C-4 and C-5, as well as N-7
. All of the other atoms in the ring are incorporated individually. C-6 comes from HCO3–. Amide groups from
glutamine provide
the atoms N-3 and N-9. The amino group donor for the inclusion of N-1 is
aspartate
,
which is converted into
fumarate
in the process, in the same way as in the urea cycle . Finally, the carbon atoms C-2 and C-8 are derived from
formyl
groups in N10-
formyl-tetrahydrofolate
Slide8The Purine Ring System Is Assembled on Ribose Phosphate
Glutamine
phosphoribosyl
amidotransferase
catalyzes this reaction.
Slide9De novo
purine
biosynthesis, like
pyrimidine
biosynthesis, requires PRPP, but for
purines
, PRPP provides the foundation on which the bases are constructed step by step. The initial committed step is the
displacement of pyrophosphate by ammonia
, rather than by a preassembled base, to produce
5-phosphoribosyl-1-amine, with the amine in the
β
configuration.
Slide10de Novo Purine Biosynthesis
Slide11Inosinate Formation
Slide12Generating AMP and GMP
Slide13Slide14Slide15Slide16Slide17Slide18Salvage Pathways Economize Intracellular Energy Expenditure
Two salvage enzymes with different specificities recover
purine
bases.
Adenine
phosphoribosyltransferase
catalyzes the formation of
adenylate
Slide19whereas
hypoxanthine-guanine
phosphoribosyltransferase
(HGPRT)
catalyzes the formation of
guanylate
as well as
inosinate
(
inosine
monophosphate
, IMP), a precursor of
guanylate
and
adenylate
Slide20Pyrimidine Nucleotides Are Made from Aspartate
, PRPP, and
Carbamoyl
Phosphate
The common
pyrimidine
ribonucleotides
are
cytidine
5-monophosphate (CMP;
cytidylate
) and
uridine
5-monophosphate (UMP;
uridylate
), which contain the
pyrimidines
cytosine and
uracil
. De novo
pyrimidine
nucleotide Biosynthesis proceeds in a somewhat different manner from
purine
nucleotide synthesis;
the six-
membered
pyrimidine
ring is made first and then attached to ribose 5-phosphate.
Slide21In the first step of the
carbamoyl
phosphate synthesis pathway, bicarbonate is
phosphorylated
by ATP to form
carboxyphosphate
and ADP. Ammonia then reacts with
carboxyphosphate
to form
carbamic
acid and inorganic phosphate.
Slide22Carbamoyl
phosphate reacts with
aspartate
to form
carbamoylaspartate
in a reaction catalyzed by
aspartate
Transcarbamoylase
.
Carbamoylaspartate
then
cyclizes
to form
dihydroorotate
which is then oxidized by
NAD+ to form
orotate
.
Slide23Orotate
reacts with PRPP to form
orotidylate
, a
pyrimidine
nucleotide. This reaction is driven by the hydrolysis of pyrophosphate. The enzyme that catalyzes this addition,
pyrimidine
phosphoribosyltransferase
,
Slide24Pyrimidine synthesis
Slide25Carbamoyl phosphate reacts with
aspartate
to yield
N-
carbamoylaspartate
in the first committed step of
pyrimidine
biosynthesis . This reaction is catalyzed by
aspartate
transcarbamoylase
Slide26By removal of water from
N-
carbamoylaspartate
, a reaction catalyzed by
dihydroorotase
, the
pyrimidine
ring is closed to form
L-
dihydroorotate
.
This compound is oxidized to the
pyrimidine
derivative
orotate
, a reaction in which NAD is the ultimate electron acceptor.
Slide27Once
orotate
is formed, the ribose 5-phosphate side chain, provided once again by PRPP, is attached to yield
orotidylate
.
Orotidylate
is then
decarboxylated
to
uridylate
, which is
phosphorylated
to UTP. CTP is formed from UTP by the action of
cytidylate
synthetase
.
Slide28de Novo Pathway for Pyrimidine Nucleotide Synthesis.
The C-2 and N-3 atoms in the
pyrimidine
ring
come from
carbamoyl
phosphate, where as the other atoms of the ring come from
aspartate
.
Slide29The
pyrimidine
ring is made up of three components:
the nitrogen atom N-1 and carbons C-4 to C-6 are derived from
aspartate
, carbon
C-2 comes from
HCO3-
, and the second nitrogen (N-3) is taken from the amide group of
glutamine
.
Slide30Slide31Degradation of Purines and Pyrimidines
Produces
Uric Acid and Urea, Respectively
Purine
nucleotides are degraded by a pathway in which they lose their phosphate through the action of
5-Nucleotidase .
Adenylate
yields adenosine,
which is
deaminated
to
inosine
by
adenosine
deaminase
,
and
inosine
is hydrolyzed to hypoxanthine (its
purine
base) and D-ribose. Hypoxanthine is oxidized successively to
xanthine
and then uric acid by
xanthine
oxidase
, a
flavoenzyme
with an atom of molybdenum
and four iron-sulfur centers in its prosthetic group. Molecular oxygen is the electron acceptor in this complex reaction.
Slide32Purine Catabolism
Purine
bases are converted first into
xanthine
and then into
urate
for excretion.
Xanthine
oxidase
catalyzes two steps in this process.
Slide33Urate Crystals.
Micrograph of sodium
urate
crystals. Joints and kidneys are damaged by these crystals in
gout.
Slide34Uric acid is the excreted end product of
purine
catabolism in
primates, birds, and some other animals
. A healthy adult human excretes uric acid at a rate of about 0.6 g/24 h; the excreted product arises in part from ingested
purines
and in part from turnover of the
purine
nucleotides of nucleic acids. In most mammals and many other vertebrates, uric acid is further degraded to
allantoin
by the action of
urate
oxidase
.
Slide35Slide36Slide37Lesch-Nyhan syndrome
A genetic lack of
hypoxanthine-guanine
phosphoribosyltransferase
activity
, seen almost exclusively in male children, results in a bizarre set of symptoms
.
Children
with this genetic disorder, which becomes manifest by the age of 2 years, are sometimes poorly coordinated and mentally retarded. In addition, they are extremely hostile and show compulsive self-destructive tendencies:
they mutilate themselves by biting off their fingers,
toes, and lips.
Slide38Excess Uric Acid Causes Gout
Long thought, erroneously, to be due to “high living,” gout is a disease of the joints caused by an elevated concentration of uric acid in the blood and tissues.
The joints become inflamed, painful, and arthritic, owing to the abnormal deposition of sodium
urate
crystals.
The kidneys are also affected, as excess uric acid is deposited in the kidney tubules.
Gout occurs predominantly in males. Its precise cause is not known, but it often involves an
underexcretion
of
urate
. A genetic deficiency of one or another enzyme of
purine
metabolism may also be a factor in some cases.
Slide39Gout is effectively treated by a combination of nutritional and drug therapies. Foods especially rich in nucleotides and nucleic acids, such as liver or glandular products, are withheld from the diet. Major alleviation of the symptoms is provided by the drug
allopurinol
,
which inhibits
xanthine
oxidase
, the enzyme that catalyzes the conversion of
purines
to uric acid.
Slide40multiple tophi on the hands (Panel A), feet, knees, Some of the tophi exuded a white, chalky material.
Laboratory studies were notable for a serum uric acid level of 8.5 mg per deciliter (506 µmol per liter), Xray hand:soft tissue swelling and pararticular erosions
Slide41The Gout-By James Gilray-1799
Johnson and Rideout NEJM, 350 (11): 1071, Figure 1 March 11, 2004
Slide42Slide43Control of Purine Biosynthesis.
Feedback inhibition controls both the overall rate of
purine
biosynthesis
and the balance between AMP and GMP production
Slide44Many Chemotherapeutic Agents Target Enzymes
in the Nucleotide Biosynthetic Pathways
The first set of agents includes compounds that inhibit
glutamine
amidotransferases
.
Recall
that
glutamine
is a nitrogen donor in at least half a dozen separate reactions in nucleotide biosynthesis. The binding sites for glutamine and the mechanism by which NH4 is extracted are quite similar in many of these enzymes. Most are strongly inhibited by glutamine analogs such as
azaserine
and
acivicin
Slide45Several Valuable Anticancer Drugs Block the Synthesis of
Thymidylate
One inhibitor that acts on
thymidylate
synthase
,
fluorouracil, is an important
chemotherapeutic agent. Fluorouracil itself is not the enzyme inhibitor. In the cell, salvage pathways convert it to the
deoxynucleoside
monophosphate
FdUMP
, which binds to and inactivates the enzyme. Inhibition by
FdUMP
is a classic example of mechanism-based enzyme inactivation. Another prominent chemotherapeutic agent,
methotrexate
, is an inhibitor
of
dihydrofolate
reductase
. This
folate
analog acts as a competitive inhibitor; the enzyme binds
methotrexate
with about 100 times higher affinity than
dihydrofolate
.
Slide46Methotrexate
is a valuable drug in the treatment of many rapidly growing tumors, such as those in acute leukemia and
choriocarcinoma
, a cancer derived from placental cells. However,
methotrexate
kills rapidly replicating cells whether they are malignant or not.
Slide47Anticancer Drug Targets
Slide48END
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