Transcript Metabolismus purinů a pyrimidinů

Metabolism of purines and
pyrimidines
Vladimíra Kvasnicová
Structure of purine and pyrimidine
nucleotides
• nucleotide = ester of phosphoric acid and a nucleoside
• nucleoside = N-containing base + monosaccharide
• -N-glycosidic bond between base and saccharide
• nucleotide bases: aromatic heterocycles
 purines:
pyrimidine + imidazol ring
 pyrimidines:
pyrimidine ring
PURINE BASES
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
ribonucleoside
deoxyribonucleoside
N-glycosidic bond
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
ribonucleotide
deoxyribonucleotide
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
PYRIMIDINE
BASES
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
ribonucleosides
deoxyribonucleoside
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Ribonucleotides
* N-glycosidic bond
* ester bond
* anhydride bond
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook
of Biochemistry with Clinical Correlations, 4th ed. Wiley-Liss,
Inc., New York, 1997. ISBN 0-471-15451-2
Classification of nucleotides
• purine nucleotides: contain adenine, guanine,
hypoxanhine or xanthine
• pyrimidine nucleosides: contain cytosine, uracil or
thymine
• ribonucleotides (saccharide = ribose)
• deoxyribonukleotidy (saccharide = deoxyribose)
 formed by reduction of ribonucleoside
diphosphates (NADPH)
Purine nucleotides
a) include an aromatic cycle in the structure
b) can contain either adenine or thymine
c) include N-glycosidic bond
d) are composed of a nucleoside bound to
phosphoric acid by an anhydride bond
Purine nucleotides
a) include an aromatic cycle in the structure
b) can contain either adenine or thymine
c) include N-glycosidic bond
d) are composed of a nucleoside bound to
phosphoric acid by an anhydride bond
Pyrimidine nucleotides
a) include an imidazol ring in the structure
b) include thymidine- and cytidine monophosphate
c) contain an ester bond
d) can include 3 phosphate groups in their
structure
Pyrimidine nucleotides
a) include an imidazol ring in the structure
b) include thymidine- and cytidine monophosphate
c) contain an ester bond
d) can include 3 phosphate groups in their
structure
Occurrence of nucleotides
• essential for all cells
• mainly 5´-nucleosidedi and triphosphates
• ribonucleotides: concentration of a sum of
them is constant (mM), only their ratio varies
(main ribonucleotide of cells: ATP)
• deoxyribonucleotides: their concentration
depends on a cell cycle (µM)
Properties of nucleotides
• strong absorption of UV radiation (260 nm)
• purines are less stable under acidic conditions
than pyrimidines
• polar terminal phosphate groups
 alternative names: adenylate or adenylic acid, ...
Nucleotides in a metabolism
1) energetic metabolism
ATP = principal form of chemical energy
available to cells – „as money of the cell“
(30 kJ/mol / spliting off phosphate)
 phosphotransferase reactions (kinases)
 muscle contraction, active transport
2) monomeric units of RNA and DNA
 substrates: nucleoside triphosphates
3) physiological mediators
 cAMP, cGMP
(„second messengers“)
4) components of coenzymes
 NAD, NADP, FAD, CoA
5) activated intermediates
 UDP-Glc, GDP-Man, CMP-NANA
 CDP-choline, ethanolamine, diacylglycerol
 SAM  methylation
 PAPS  sulfatation
6) allosteric efectors
- regulation of key enzymes of metabolic
pathways
3´-phosphoadenosine-5´-phosphosulfate (PAPS)
used as the sulfate donor in metabolic reactions
(sulfatation)
Obrázek je převzat z http://web.indstate.edu/thcme/mwking/amino-acid-metabolism.html (leden 2007)
Purine and pyrimidine nucleotides can be
used
a) as nucleoside triphosphates for nucleic acid
synthesis
b) in energetic metabolism of cells
c) for activation of metabolic intermediates of
saccharides and lipids
d) in enzymatic reactions: some coenzymes are
nucleotides
Purine and pyrimidine nucleotides can be
used
a) as nucleoside triphosphates for nucleic acid
synthesis
b) in energetic metabolism of cells
c) for activation of metabolic intermediates of
saccharides and lipids
d) in enzymatic reactions: some coenzymes are
nucleotides
PRPP = 5-phosphoribosyl-1-pyrophosphate
• common substrate of both purine and pyrimidine
synthesis
• its synthesis is a key reaction of synthesis of
the nucleotides
• PRPP-synthetase is regulated by feed back
inhibition by nucleoside di and triphosphates
• precursors:
* ribose-5-phosphate (from HMPP)
* ribose-1-phosphate
(phosphorolysis of nucleosides)
• function:
 regulation of
nucleotide synthesis
 substrate of
nucleotide synthesis
PRPP = PRDP
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Synthesis of purine nucleotides
• de novo = new building of a nucleotide rings
• salvage reactions=synthesis from bases or nucleosides
 less energy need than for de novo synthesis
 they inhibit de novo synthesis
 substrates: a) base (adenine, guanine, hypoxanthine)
PRPP
b) ribonucleosides
ATP
Synthesis of purine nucleotides de novo
• high consumption of energy (ATP)
• cytoplasm of many cells, mainly in the liver
• substrates:
* 5-phosphoribosyl-1-diphosphate
(= PRDP = PRPP)
* amino acids
(Gln, Gly, Asp)
* tetrahydrofolate derivatives, CO2
• coenzymes:
* tetrahydrofolate (= THF)
* NAD+
• important intermediates:
 5´-phosphoribosylamine
 inosine monophosphate (IMP)
• products: nucleoside monophosphates (AMP, GMP)
• interconversion of purine nucleotides:
 via IMP = common precursor of AMP and GMP
(inosine monophosphate: base = hypoxanthine)
Synthesis of purine nucleotides
C
Y
T
O
P
L
A
S
M
Obrázek převzat z http://web.indstate.edu/thcme/mwking/nucleotide-metabolism.html (leden 2007)
IMP
GMP
AMP
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Synthesis of pyrimidine nucleotides
• de novo = new building of a nucleotide rings
• salvage reactions=synthesis from bases or nucleosides
 substrates:
a) * base (not cytosine)
* PRPP
b) * ribonucleosides
* ATP
Synthesis of pyrimidine nucleotides de novo
• cytoplasm of cells (exception: one enzyme is found at
mitochondria /dihydroorotate-DH)
• substrates:
*
*
*
*
carbamoyl phosphate (Gln,CO2,2ATP)
aspartate
PRPP
methylene-THF (only for thimidine)
Karbamoyl phosphate is formed in urea synthesis as well
(only in mitochondria of hepatocytes)
• important intermediates:
* orotic acid
* orotidine monophosphate (OMP)
* uridine monophosphate (UMP)
• products:
* cytidine triphosphate (from UTP)
* deoxythimidine monophosphate
(from dUMP)
Synthesis of pyrimidine nucleotides
C
Y
T
O
P
L
A
S
M
mitochondrion
Obrázek převzat z http://web.indstate.edu/thcme/mwking/nucleotide-metabolism.html (leden 2007)
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Synthesis of 2-deoxyribonucleotides
enzyme: ribonucleotide reductase
+ small protein „thioredoxin“
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Synthesis of thymidine monophosphate
Obrázek je převzat z učebnice: Devlin, T. M. (editor): Textbook of Biochemistry with Clinical Correlations, 4th
ed. Wiley-Liss, Inc., New York, 1997. ISBN 0-471-15451-2
Regulation of nucleotide synthesis
• PRPP-synthetase is inhibited by both purine
and pyrimidine nucleoside di- and
triphosphates
• nucleotide synthesis: feed back inhibition
• nucleoside diphosphate reductase:
activated by nucleoside triphosphates,
inhibited by deoxyadenosine triphosphate
(dATP)
Regulation of nucleotide synthesis
regulatory enzyme
activation
glutamine-PRPP
amidotransferase
(purines)
 PRPP
carbamoylphosphate
synthetase II
 PRPP
 ATP
(pyrimidines)
= cytosolic
inhibition
 IMP, GMP,
AMP
(allosteric
inhibition)
 UTP
Degradation of purines and pyrimidines
• exogenous: mostly not used for resynthesis
• endogenous:
 enzymes
*
*
*
*
*
nucleases (split off nucleic acids)
nucleotidases (...nucleotides)
nucleoside phosphorylases (nucleosides)
deaminase (adenosine)
xanthinoxidase
(hypoxanthine, xanthine)
inhibited by allopurinol (pharmacology)
Degradation of purines
„uric acid“
Degradation of pyrimidines
• products:
 purines → NH3, uric acid –
it has antioxidative properties
(partially excreted with urine; failure: hyperuricemia, gout)
physiological range:
serum 220 – 420 µmol/l (men)
140 – 340 µmol/l (women)
urine
0,48 – 5,95 mmol/l
 pyrimidines: C, U → -alanine, CO2, NH3
T
→ -aminoisobutyrate, CO2, NH3
Principal differences between metabolism
of purines and pyrimidines
purines
formation of
N-glycosidic
bond
in 1st step of their
biosynthesis
location of
biosynthesis
cytoplasm
products of
degradation
(PRDP is the 1st substrate)
uric acid
(poor solubility in H2O),
NH3
pyrimidines
a heterocyclic ring is
formed first, then it
reacts with PRDP
cytoplasm + 1 enzyme
is in a mitochondrion
CO2, NH3, -AMK
(soluble in H2O)
Synthesis of nucleotides
a) uses products of pentose cycle
b) includes phosphoribosyl diphosphate (PRDP =
PRPP) as a substrate
c) needs derivatives of folic acid
d) proceeds in a cytoplasm only
Synthesis of nucleotides
a) uses products of pentose cycle
b) includes phosphoribosyl diphosphate (PRDP =
PRPP) as a substrate
c) needs derivatives of folic acid
d) proceeds in a cytoplasm only
Synthesis of purine nucleotides
a) uses ammonia as a nitrogen donor
b) proceeds in a cytoplasm
c) can start from nucleosides produced by
degradation of nucleic acids
d) includes uric acid as an intermediate
Synthesis of purine nucleotides
a) uses ammonia as a nitrogen donor
b) proceeds in a cytoplasm
c) can start from nucleosides produced by
degradation of nucleic acids
d) includes uric acid as an intermediate
Synthesis of pyrimidine nucleotides
a) starts by the reaction: PRDP + glutamine
b) proceeds only in a cytoplasm of cells
c) includes orotic acid as an intermediate
d) includes inosine monophosphate as an
intermediate
Synthesis of pyrimidine nucleotides
a) starts by the reaction: PRDP + glutamine
b) proceeds only in a cytoplasm of cells
c) includes orotic acid as an intermediate
d) includes inosine monophosphate as an
intermediate
In a degradation of purine nucleotides
a) ammonia is released
b) CO2 is produced
c) the enzyme xanthine oxidase participates
d) uric acid is produced as the end product
In a degradation of purine nucleotides
a) ammonia is released
b) CO2 is produced
c) the enzyme xanthine oxidase participates
d) uric acid is produced as the end product
In a degradation of pyrimidine nucleotides
a) -amino acids are produced
b) the enzyme xanthine oxidase participates
c) orotic acid is formed
d) ammonia is produced
In a degradation of pyrimidine nucleotides
a) -amino acids are produced
b) the enzyme xanthine oxidase participates
c) orotic acid is formed
d) ammonia is produced