Purine and Pyrimidine Metabolism
Topics
Overview
Nomenclature
Hydrolysis of Polynucleotides
Purine Catabolism
Pyrimidine Catabolism
De novo Synthesis of Purine Nucleotides
De novo Synthesis of Pyrimidine Nucleotides
Interconversion of Nucleotides
Salvage of Bases
Formation of Deoxyribonucleotides
Synthesis of dTMP
Quiz Questions
Overview
One of the important specialized pathways of a number of amino acids is the
synthesis of purine and pyrimidine nucleotides. These nucleotides are
important for
a number of reasons. Most of them, not just ATP, are the sources of energy that
drive most of our reactions. ATP is the most commonly used source but GTP
is used
in protein synthesis as well as a few other reactions. UTP is the source of
energy for
activating glucose and galactose. CTP is an energy source in lipid
metabolism. AMP
is part of the structure of some of the coenzymes like NAD and Coenzyme A. And,
of course, the nucleotides are part of nucleic acids. Neither the bases nor the
nucleotides are required dietary components. (Another
perspective on this.) We can
both synthesize them de novo and salvage and reuse those we already have.
Nomenclature
Nitrogen Bases
There are two kinds of nitrogen-containing bases - purines and pyrimidines.
Purines
consist of a six-membered and a five-membered nitrogen-containing ring, fused
together. Pyridmidines have only a six-membered nitrogen-containing
ring. There
are 4 purines and 4 pyrimidines that are of concern to us.
Purines
- Adenine = 6-amino purine
- Guanine = 2-amino-6-oxy purine
- Hypoxanthine = 6-oxy purine
- Xanthine = 2,6-dioxy purine
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Adenine and guanine are found in both DNA and RNA. Hypoxanthine and
xanthine are not incorporated into the nucleic acids as they are being
synthesized
but are important intermediates in the synthesis and degradation of the purine
nucleotides.
Pyrimidines
- Uracil = 2,4-dioxy pyrimidine
- Thymine = 2,4-dioxy-5-methyl pyrimidine
- Cytosine = 2-oxy-4-amino pyrimidine
- Orotic acid = 2,4-dioxy-6-carboxy pyrimidine
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Cytosine is found in both DNA and RNA. Uracil is found only in RNA. Thymine is
normally found in DNA. Sometimes tRNA will contain some thymine as well as
uracil.
Nucleosides
If a sugar, either ribose or 2-deoxyribose, is added to a
nitrogen base, the resulting
compound is called a nucleoside. Carbon 1 of the sugar is attached
to nitrogen 9 of a
purine base or to nitrogen 1 of a pyrimidine base. The names of
purine nucleosides
end in -osine and the names of pyrimidine nucleosides end in
-idine. The
convention is to number the ring atoms of the base normally and to use l',
etc. to
distinguish the ring atoms of the sugar. Unless otherwise specificed, the
sugar is
assumed to be ribose. To indicate that the sugar is 2'-deoxyribose, a
d- is placed before
the name.
- Adenosine
- Guanosine
- Inosine - the base in inosine is hypoxanthine
- Uridine
- Thymidine
- Cytidine
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Nucleotides
Adding one or more phosphates to the sugar portion of a nucleoside results in a
nucleotide. Generally, the phosphate is in ester linkage to carbon
5' of the sugar. If
more than one phosphate is present, they are generally in acid anhydride
linkages to
each other. If such is the case, no position designation in the name is
required. If the
phosphate is in any other position, however, the position must be
designated. For
example, 3'-5' cAMP indicates that a phosphate is in ester linkage to both
the 3' and
5' hydroxyl groups of an adenosine molecule and forms a cyclic structure. 2'-GMP
would indicate that a phosphate is in ester linkage to the 2' hydroxyl
group of a
guanosine. Some representative names are:
- AMP = adenosine monophosphate = adenylic acid
- CDP = cytidine diphosphate
- dGTP = deoxy guanosine triphosphate
- dTTP = deoxy thymidine triphosphate (more commonly designated
TTP) - cAMP = 3'-5' cyclic adenosine monophosphate
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Polynucleotides
Nucleotides are joined together by 3'-5' phosphodiester bonds to form
polynucleotides. Polymerization of ribonucleotides will produce an RNA while
polymerization of deoxyribonucleotides leads to DNA. [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Hydrolysis of Polynucleotides
Most, but not all, nucleic acids in the cell are associated with protein.
Dietary
nucleoprotein is degraded by pancreatic enzymes and tissue nucleoprotein by
lysosomal enzymes. After dissociation of the protein and nucleic acid, the
protein is
metabolized like any other protein.
The nucleic acids are hydrolyzed randomly by nucleases to yield a
mixture of
polynucleotides. These are further cleaved by phosphodiesterases
(exonucleases) to a
mixture of the mononucleotides. The specificity of the pancreatic nucleotidases
gives the 3'-nucleotides and that of the lysosomal nucleotidases gives the
biologically important 5'-nucleotides.
The nucleotides are hydrolyzed by nucleotidases to give the
nucleosides and
Pi. This is probably the end product in the intestine with the
nucleosides being the primary form absorbed. In at least some tissues, the
nucleosides undergo phosphorolysis with nucleoside phosphorylases to
yield the
base and ribose 1-P (or deoxyribose 1-P). Since R 1-P and R 5-P are in
equilibrium, the
sugar phosphate can either be reincorporated into nucleotides or metabolized via
the Hexose Monophosphate Pathway. The purine and pyrimidine bases released are
either degraded or salvaged for reincorporation into nucleotides. There is
significant
turnover of all kinds of RNA as well as the nucleotide pool. DNA doesn't
turnover
but portions of the molecule are excised as part of a repair process.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Purine and pyrimidines from tissue turnover which are not salvaged are
catabolized
and excreted. Little dietary purine is used and that which is absorbed is
largely
catabolized as well. Catabolism of purines and pyrimidines occurs in a less
useful
fashion than did the catabolism of amino acids in that we do not derive any
significant amount of energy from the catabolism of purines and pyrimidines.
Pyrimidine catabolism, however, does produce beta-alanine, and the endproduct of
purine catabolism, which is uric acid in man, may serve as a scavenger of
reactive
oxygen species.
Purine Catabolism
The end product of purine catabolism in man is uric acid. Other
mammals have the
enzyme urate oxidase and excrete the more soluble allantoin as the end product.
Man does not have this enzyme so urate is the end product for us. Uric acid is
formed primarily in the liver and excreted by the kidney into the urine.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Nucleotides to Bases
Guanine nucleotides are hydrolyzed to the nucleoside guanosine which
undergoes
phosphorolysis to guanine and ribose 1-P. Man's intracellular nucleotidases are not
very active toward AMP, however. Rather, AMP is deaminated by the enzyme
adenylate (AMP) deaminase to IMP. In the catobilsm of purine
nucleotides, IMP is
further degraded by hydrolysis with nucleotidase to inosine and then
phosphorolysis to hypoxanthine.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Adenosine does occur but usually arises from S-Adenosylmethionine during the
course of transmethylation reactions. Adenosine is deaminated to inosine by an
adenosine deaminase. Deficiencies in either adenosine deaminase or
in the purine
nucleoside phosphorylase lead to two different immunodeficiency diseases by
mechanisms that are not clearly understood. With adenosine deaminase
deficiency,
both T and B-cell immunity is affected. The phosphorylase deficiency
affects the T
cells but B cells are normal. In September, 1990, a 4 year old girl was
treated for
adenosine deaminase deficiency by genetically engineering her cells to
incorporate
the gene. The treatment,so far, seems to be successful.
Whether or not methylated purines are catabolized depends upon the
location of
the methyl group. If the methyl is on an -NH2, it is removed along
with the -NH2 and the core is metabolized in the usual fashion. If
the methyl is on a ring nitrogen, the compound is excreted unchanged in the
urine.
Bases to Uric Acid
Both adenine and guanine nucleotides converge at the common intermediate
xanthine. Hypoxanthine, representing the original adenine, is
oxidized to xanthine
by the enzyme xanthine oxidase. Guanine is deaminated, with the
amino group
released as ammonia, to xanthine. If this process is occurring in tissues
other than
liver, most of the ammonia will be transported to the liver as glutamine for
ultimate excretion as urea.
Xanthine, like hypoxanthine, is oxidized by oxygen and xanthine oxidase with the
production of hydrogen peroxide. In man, the urate is excreted and the hydrogen
peroxide is degraded by catalase. Xanthine oxidase is present in significant
concentration only in liver and intestine. The pathway to the nucleosides,
possibly
to the free bases, is present in many tissues.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Gouts and Hyperuricemia
Both undissociated uric acid and the monosodium salt (primary form in blood) are
only sparingly soluble. The limited solubility is not ordinarily a problem
in urine
unless the urine is very acid or has high [Ca2+]. [Urate salts
coprecipitate with calcium salts and can form stones in kidney or bladder.]
A very
high concentration of urate in the blood leads to a fairly common group of
diseases
referred to as gout. The incidence of gout in this country is about 3/1000.
Gout is a group of pathological conditions associated with markedly
elevated levels
of urate in the blood (3-7 mg/dl normal). Hyperuricemia is not
always symptomatic,
but, in certain individuals, something triggers the deposition of sodium urate
crystals in joints and tissues. In addition to the extreme pain
accompanying acute
attacks, repeated attacks lead to destruction of tissues and severe
arthritic-like
malformations. The term gout should be restricted to hyperuricemia with the
presence of these tophaceous deposits.
Urate in the blood could accumulate either through an overproduction and/or an
underexcretion of uric acid. In gouts caused by an overproduction of
uric acid, the
defects are in the control mechanisms governing the production of - not
uric acid
itself - but of the nucleotide precursors. The only major control of
urate production
that we know so far is the availability of substrates (nucleotides,
nucleosides or free
bases).
One approach to the treatment of gout is the drug allopurinol, an
isomer of
hypoxanthine. [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Allopurinol is a substrate for xanthine oxidase, but the product binds so
tightly that
the enzyme is now unable to oxidized its normal substrate. Uric acid production is
diminished and xanthine and hypoxanthine levels in the blood rise. These
are more
soluble than urate and are less likely to deposit as crystals in the
joints. Another
approach is to stimulate the secretion of urate in the urine.
Summary
In summary, all, except ring-methylated, purines are deaminated (with the amino
group contributing to the general ammonia pool) and the rings oxidized to
uric acid
for excretion. Since the purine ring is excreted intact, no energy benefit
accrues to
man from these carbons.
Pyrimidine Catabolism
In contrast to purines, pyrimidines undergo ring cleavage and the usual end
products of catabolism are beta-amino acids plus ammonia and carbon dioxide.
Pyrimidines from nucleic acids or the energy pool are acted upon by
nucleotidases
and pyrimidine nucleoside phosphorylase to yield the free bases. The
4-amino group
of both cytosine and 5-methyl cytosine is released as ammonia.
Ring Cleavage
In order for the rings to be cleaved, they must first be reduced by
NADPH. Atoms 2
and 3 of both rings are released as ammonia and carbon dioxide. The rest of
the ring
is left as a beta-amino acid. Beta-amino isobutyrate from thymine or
5-methyl
cytosine is largely excreted. Beta-alanine from cytosine or uracil may either be
excreted or incorporated into the brain and muscle dipeptides, carnosine
(his-beta-ala) or anserine (methyl his-beta-ala).
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
General Comments
Purine and pyrimidine bases which are not degraded are recycled - i.e.
reincorporated into nucleotides. This recycling, however, is not sufficient
to meet
total body requirements and so some de novo synthesis is essential.
There are
definite tissue differences in the ability to carry out de novo
synthesis. De novo
synthesis of purines is most active in liver. Non-hepatic tissues generally have
limited or even no de novo synthesis. Pyrimidine synthesis occurs in
a variety of
tissues. For purines, especially, non-hepatic tissues rely heavily on
preformed bases -
those salvaged from their own intracellular turnover supplemented by bases
synthesized in the liver and delivered to tissues via the blood.
"Salvage" of purines is reasonable in most cells because xanthine oxidase,
the key
enzyme in taking the purines all of the way to uric acid, is significantly
active only
in liver and intestine. The bases generated by turnover in non-hepatic
tissues are
not readily degraded to uric acid in those tissues and, therefore, are
available for
salvage. The liver probably does less salvage but is very active in de
novo synthesis -
not so much for itself but to help supply the peripheral tissues.
De novo synthesis of both purine and pyrimidine nucleotides occurs
from readily
available components.
De Novo Synthesis of Purine Nucleotides
We use for purine nucleotides the entire glycine molecule (atoms 4, 5,7),
the amino
nitrogen of aspartate (atom 1), amide nitrogen of glutamine (atoms 3, 9),
components of the folate-one-carbon pool(atoms 2,
, carbon dioxide, ribose 5-Pfrom glucose and a great deal of energy in the form of ATP. In de novo
synthesis,
IMP is the first nucleotide formed. It is then converted to either AMP or
GMP.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
PRPP
Since the purines are synthesized as the ribonucleotides, (not as the free
bases) a
necessary prerequisite is the synthesis of the activated form of ribose
5-phosphate.
Ribose 5-phosphate reacts with ATP to form 5-Phosphoribosyl-1-pyrophosphate
(PRPP).
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
This reaction occurs in many tissues because PRPP has a number of roles - purine
and pyrimidine nucleotide synthesis, salvage pathways, NAD and NADP formation.
The enzyme is heavily controlled by a variety of compounds (di- and tri-phosphates,
2,3-DPG), presumably to try to match the synthesis of PRPP to a need for the
products in which it ultimately appears.
Commitment Step
De novo purine nucleotide synthesis occurs actively in the cytosol
of the liver
where all of the necessary enzymes are present as a macro-molecular
aggregate. The
first step is a replacement of the pyrophosphate of PRPP by the amide group of
glutamine. The product of this reaction is 5-Phosphoribosylamine.
The amine group
that has been placed on carbon 1 of the sugar becomes nitrogen 9 of the ultimate
purine ring. This is the commitment and rate-limiting step of the pathway.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
The enzyme is under tight allosteric control by feedback inhibition. Either
AMP,
GMP, or IMP alone will inhibit the amidotransferase while AMP
+ GMP or AMP +
IMP together act synergistically. This is a fine control and
probably the major factor
in minute by minute regulation of the enzyme. The nucleotides inhibit the enzyme
by causing the small active molecules to aggregate to larger inactive
molecules.
[PRPP] also can play a role in regulating the rate. Normal intracellular
concentrations of PRPP (which can and do fluctuate) are below the KM of the
enzyme for PRPP so there is great potential for increasing the rate of the
reaction by
increasing the substrate concentration. The kinetics are sigmoidal. The
enzyme is
not particularly sensitive to changes in [Gln] (Kinetics are hyperbolic and
[gln]
approximates KM). Very high [PRPP] also overcomes the normal
nucleotide
feedback inhibition by causing the large, inactive aggregates to
dissociate back to the
small active molecules.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Purine de novo synthesis is a complex, energy-expensive pathway. It
should be, and
is, carefully controlled.
Formation of IMP
Once the commitment step has produced the 5-phosphoribosyl amine, the rest of
the molecule is formed by a series of additions to make first the 5- and
then the
6-membered ring. (Note: the numbers given to the atoms are those of the
completed
purine ring and names, etc. of the intermediate compounds are not given.) The
whole glycine molecule, at the expense of ATP adds to the amino group to provide
what will eventually be atoms 4, 5, and 7 of the purine ring (The amino group of
5-phosphoribosyl amine becomes nitrogen N of the purine ring.) One more atom is
needed to complete the five-membered ring portion and that is supplied as 5,
10-Methenyl tetrahydrofolate.
Before ring closure occurs, however, the amide of glutamine adds to carbon
4 to start
the six-membered ring portion (becomes nitrogen 3). This addition requires ATP.
Another ATP is required to join carbon 8 and nitrogen 9 to form the
five-membered
ring.
The next step is the addition of carbon dioxide (as a carboxyl group) to
form carbon 6
of the ring. The amine group of aspartate adds to the carboxyl group with a
subsequent removal of fumarate. The amino group is now nitrogen 1 of the final
ring. This process, which is typical for the use of the amino group of
aspartate,
requires ATP. The final atom of the purine ring, carbon 2, is supplied by
10-Formyl
tetrahydrofolate. Ring closure produces the purine nucleotide, IMP.
Note that at least 4 ATPs are required in this part of the process. At no
time do we
have either a free base or a nucleotide.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Formation of AMP and GMP
IMP can then become either AMP or GMP. GMP formation requires
that IMP be first
oxidized to XMP using NAD. The oxygen at position 2 is substituted by the
amide N
of glutamine at the expense of ATP. Similarly, GTP provides the energy to
convert
IMP to AMP. The amino group is provided by aspartate in a mechanism
similar to
that used in forming nitrogen 1 of the ring. Removal of the carbons of
aspartate as
fumarate leaves the nitrigen behind as the 6-amino group of the adenine ring. The
monophosphates are readily converted to the di- and tri-phosphates.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Control of De Novo Synthesis
Control of purine nucleotide synthesis has two phases. Control of the
synthesis as a
whole occurs at the amidotransferase step by nucleotide inhibition
and/or [PRPP].
The second phase of control is involved with maintaining an appropriate
balance
(not equality) between ATP and GTP. Each one stimulates the synthesis
of the other
by providing the energy. Feedback inhibition also controls the branched
portion as
GMP inhibits the conversion of IMP to XMP and AMP inhibits the conversion of
IMP to adenylosuccinate.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Possible Scenario:
One could imagine the controls operating in such a way that if only one of
the two
nucleotides were required, there would be a partial inhibition of de novo
synthesis
because of high levels of the other and the IMP synthesized would be directed
toward the synthesis of the required nucleotide. If both nucleotides were
present in
adequate amounts, their synergistic effect on the amidotransferase would
result in
almost complete inhibition of de novo synthesis.
De Novo Synthesis of Pyrimidine Nucleotides
Since pyrimidine molecules are simpler than purines, so is their synthesis
simpler
but is still from readily available components. Glutamine's amide nitrogen and
carbon dioxide provide atoms 2 and 3 or the pyrimidine ring. They do so,
however,
after first being converted to carbamoyl phosphate. The other four atoms of
the ring
are supplied by aspartate. As is true with purine nucleotides, the sugar
phosphate
portion of the molecule is supplied by PRPP.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Carbamoyl Phosphate
Pyrimidine synthesis begins with carbamoyl phosphate synthesized in
the cytosol of
those tissues capable of making pyrimidines (highest in spleen, thymus,
GItract and
testes). This uses a different enzyme than the one involved in urea synthesis.
Carbamoyl phosphate synthetase II (CPS II) prefers glutamine to free
ammonia and
has no requirement for N-Acetylglutamate.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Formation of Orotic Acid
Carbamoyl phosphate condenses with aspartate in the presence of aspartate
transcarbamylase to yield N-carbamylaspartate which is then converted to
dihydroorotate.
In man, CPSII, asp-transcarbamylase, and dihydroorotase activities
are part of a
multifunctional protein.
Oxidation of the ring by a complex, poorly understood enzyme produces the free
pyrimidine, orotic acid. This enzyme is located on the outer face of the inner
mitochondrial membrane, in contrast to the other enzymes which are cytosolic.
Note the contrast with purine synthesis in which a nucleotide is formed
first while
pyrimidines are first synthesized as the free base.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Formation of the Nucleotides
Orotic acid is converted to its nucleotide with PRPP. OMP is then
converted
sequentially - not in a branched pathway - to the other pyrimidine
nucleotides.
Decarboxylation of OMP gives UMP. O-PRT and OMP decarboxylase
are also a
multifunctional protein. After conversion of UMP to the
triphosphate, the amide of
glutamine is added, at the expense of ATP, to yield CTP.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Control
The control of pyrimidine nucleotide synthesis in man is exerted primarily
at the
level of cytoplasmic CPS II. UTP inhibits the enzyme,
competitively with ATP. PRPP
activates it. Other secondary sites of control also exist (e.g. OMP
decarboxylase is
inhibited by UMP and CMP). These are probably not very important under normal
circumstances.
In bacteria, aspartate transcarbamylase is the control enzyme. There is only one
carbamoyl phosphate synthetase in bacteria since they do not have mitochondria.
Carbamoyl phosphate, thus, participates in a branched pathway in these organisms
that leads to either pyrimidine nucleotides or arginine.
Interconversion of Nucleotides
The monophosphates are the forms synthesized de novo although the
triphosphates are the most commonly used forms. But, of course, the three forms
are in equilibrium. There are several enzymes classified as nucleoside
monophosphate kinases which catalyze the general reaction:(= represents a
reversible reaction)
Base-monophosphate + ATP = Base-diphosphate + ADP
e.g. Adenylate kinase: AMP + ATP = 2 ADP
There is a different enzyme for GMP, one for pyrimidines and also enzymes that
recognize the deoxy forms.
Similarly, the diphosphates are converted to the triphosphates by nucleoside
diphosphate kinase:
BDP + ATP = BTP + ADP
There may be only one nucleoside diphosphate kinase with broad specificity. One
can legitimately speak of a pool of nucleotides in equilibrium with each
other.
Salvage of Bases
Salvaging of purine and pyrimidine bases is an exceedingly important process for
most tissues. There are two distinct pathways possible for salvaging the
bases.
Salvaging Purines
The more important of the pathways for salvaging purines uses
enzymes called
phosphoribosyltransferases (PRT):
PRTs catalyze the addition of ribose 5-phosphate to the base from PRPP to
yield a
nucleotide.:
Base + PRPP = Base-ribose-phosphate (BMP) + PPi
We gave already seen one example of this type of enzyme as a normal part of
de
novo synthesis of the pyrimidine nucleotides, - O-PRT.
As a salvage process though, we are dealing with purines. There are two enzymes,
A-PRT and HG-PRT. A-PRT is not very important because we generate
very little
adenine. (Remember that the catabolism of adenine nucleotides and nucleosides is
through inosine). HG-PRT, though, is exceptionally important and it
is inhibited by
both IMP and GMP. This enzyme salvages guanine directly and adenine indirectly.
Remember that AMP is generated primarily from IMP, not from free adenine.
Lesch-Nyhan Syndrome
HG-PRT is deficient in the disease called Lesch-Nyhan Syndrome, a severe
neurological disorder whose most blatant clinical manifestation is an
uncontrollable
self-mutilation. Lesch-Nyhan patients have very high blood uric acid
levels because
of an essentially uncontrolled de novo synthesis. (It can be
as much as 20 times the
normal rate). There is a significant increase in PRPP levels in various
cells and an
inability to maintain levels of IMP and GMP via salvage pathways. Both of these
factors could lead to an increase in the activity of the amidotransferase.
Salvaging Pyrimidines
A second type of salvage pathway involves two steps and is the major pathway for
the pyrimidines, uracil and thymine.
Base + Ribose 1-phosphate = Nucleoside + Pi (nucleoside phosphorylase)
Nucleoside + ATP - Nucleotide + ADP (nucleoside kinase - irreversible)
There is a uridine phosphorylase and kinase and a deoxythymidine phosphorylase
and a thymidine kinase which can salvage some thymine in the presence of dR
1-P.
Formation of Deoxyribonucleotides
De novo synthesis and most of the salvage pathways involve the
ribonucleotides.
(Exception is the small amount of salvage of thymine indicated above.)
Deoxyribonucleotides for DNA synthesis are formed from the ribonucleotide
diphosphates (in mammals and E. coli).
A base diphosphate (BDP) is reduced at the 2' position of the ribose
portion using
the protein, thioredoxin and the enzyme nucleoside diphosphate
reductase.
Thioredoxin has two sulfhydryl groups which are oxidized to a disulfide bond
during the process. In order to restore the thioredoxin to its reduced for
so that it can
be reused, thioredoxin reductase and NADPH are required.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
This system is very tightly controlled by a variety of allosteric
effectors. dATP is a
general inhibitor for all substrates and ATP an activator. Each substrate
then has a
specific positive effector (a BTP or dBTP). The result is a maintenance of an
appropriate balance of the deoxynucleotides for DNA synthesis.
Synthesis of dTMP
DNA synthesis also requires dTMP (dTTP). This is not synthesized in the
de novo
pathway and salvage is not adequate to maintain the necessary amount. dTMP is
generated from dUMP using the folate-dependent one-carbon pool.
Since the nucleoside diphosphate reductase is not very active toward UDP, CDP is
reduced to dCDP which is converted to dCMP. This is then deaminated to form
dUMP. In the presence of 5,10-Methylene tetrahydrofolate and the enzyme
thymidylate synthetase, the carbon group is both transferred to the
pyrimidine ring
and further reduced to a methyl group. The other product is
dihydrofolate which is
subsequently reduced to the tetrahydrofolate by dihydrofolate reductase.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Chemotherapeutic Agents
Thymidylate synthetase is particularly sensitive to availability of the folate
one-carbon pool. Some of the cancer chemotherapeutic agents interfere with this
process as well as with the steps in purine nucleotide synthesis involving
the pool.
Cancer chemotherapeutic agents like methotrexate (4-amino, 10-methyl
folic acid)
and aminopterin (4-amino, folic acid) are structural analogs of
folic acid and inhibit
dihydrofolate reductase. This interferes with maintenance of the folate pool and
thus of de novo synthesis of purine nucleotides and of dTMP
synthesis. Such agents
are highly toxic and administered under careful control.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
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