Transcript Nükleotid Metabolizması - mustafaaltinisik.org.uk

Nucleotide Metabolism
Bases/Nucleosides/Nucleotides
NH2
NH2
C
N
C
N
CH
C C N
H N
H
N
HC
HOH2 C
NH2
N
N
N
O-
O
H
H
HO
H
H
H
Base=
Base
Adenine
N
HC
CH
Base + Sugar=
Nucleoside
Deoxyadenosine
N
CH
N
O
O
O
N
P O P O P OCH2 O
H
H
OOOH
H
HO
H
Base + Sugar + Phosphate=
Nucleotide
Deoxyadenosine
5’-triphosphate
(dATP)
Cellular Roles of Nucleotides
• Energy metabolism (ATP)*
• Monomeric units of nucleic acids*
• Regulation of physiological processes
– Adenosine controls coronary blood flow
– cAMP and cGMP serve as signaling molecules
•
•
•
•
Precursor function-GTP to tetrahydrobiopternin
Coenzyme components- 5’-AMP in FAD/NAD+
Activated intermediates- UDP Glucose
Allosteric effectors- regulate themselves and
others
How I hope to make this at least
bearable if not mildly interesting
• Purines and Pyrimidines
– Synthesis (de novo and salvage pathways)
– Degradation
– Relevant disease states
– Relevant clinical applications (Friday)
You are not responsible for any structures
Purines and Pyrimidines
NH2
C
N
C
N
CH
C C N
H N
H
Adenine
HN
O C
O
C
C
C
CH3
O
C
Two
Purines
N
H N
C
CH
C C N
N
H2 N
H
Guanine
NH2
C
H
N
C
C
C
N
H
H
Two
O
N
H
Pyrimidines
H
Thymine/Uracil
Cytosine
Synthesis Pathways
• For both purines and pyrimidines there are two means
of synthesis (often regulate one another)
– de novo (from bits and parts)
– salvage (recycle from pre-existing nucleotides)
de novo Pathway
Salvage Pathway
Many Steps Require an
Activated Ribose Sugar (PRPP)
5’
de novo Synthesis
• Committed step: This is the point of no return
– Occurs early in the biosynthetic pathway
– Often regulated by final product (feedback
inhibition)
X
Purine Biosynthesis (de novo)
• Atoms derived from:
–
–
–
–
–
Aspartic acid
Glycine
Glutamine
CO2
Tetrahydrofolate
Inhibited by
AMP, GMP, IMP
X
• Also requires
– 4 ATP’s
Committed Step
Purines are synthesized on the Ribose ring
Purine Biosynthesis (de novo)
(A bunch of steps you don’t
need to know)
O
C
H N
C
O
C
N
N
C
CH
C N
N
H N
C
CH
C C N
N
H2 N
(Inosine Monophosphate)
ATP
GTP
Feedback
Inhibition
NH2
C
N
C
N
CH
C C N
H N
Purine Degradation
• Sequential removal of bits
and pieces
• End product is uric acid
• Uric acid is primate-specific
Other species
further metabolize
uric acid
Excreted in
Urine
Xanthine
Oxidase
Excess Uric Acid Causes Gout
• Primary gout (hyperuricemia)
– Inborn errors of metabolism that lead to overproduction
of Uric Acid
• Overactive de novo synthesis pathway
– Leads to deposits of Uric Acid in the joints
– Causes acute arthritic joint inflammation
Xanthine
Oxidase
X
Allopurinol
Avoid:
Offal foods such as liver, kidneys, tripe, sweetbreads and tongue
Immunodeficiency Diseases
Associated with Purine Degradation
• Defect in adenosine deaminase
– Removes amine from adenosine
• SCID- severe combined
immunodeficiency
• “Bubble Boy” Disease
• Defect in both B-cells and Tcells (Disease of Lymphocytes)
• Patients extremely susceptible to
infection - hence the Bubble
Lymphocyte
Therapies for SCID
• Can be diagnosed in infants through a
simple blood test (white cell count)
• Bone marrow transplant for infants
– Familial donor
• Continued administration of adenosine
deaminase (ADA-PEG)
• Gene therapy- repair defective gene in Tcells or blood stem cells
Salvage Pathway for Purines
Hypoxanthine
+ PRPP = IMP or GMP + PPi
or
Hypoxanthineguanosylphosphoribosyl transferase
Guanine
(HGPRTase)
Adenine
+ PRPP =
AMP + PPi
Adeninephosphoribosyl transferase
(APRTase)
Lesch-Nyhan Syndrome
• Absence of HGPRTase
• X-linked (Gene on X)
– Occurs primarily in males
• Characterized by:
–
–
–
–
–
Increased uric acid
Spasticity
Neurological defects
Aggressive behavior
Self-mutilation
Total Aside on X-linked Diseases
• Why are X-linked
diseases generally
found only in males?
• Females have two X
chromosomes - would
need to mutate both
copies to see a
recessive phenotype
• Males have a single X
chromosome
XY
XX
Think about
Fragile X Syndrome
Biosynthesis of Pyrimidines
• Synthesized from:
–
–
–
–
Glutamine
CO2
Aspartic acid
Requires ATP
HN
O C
O
C
N
H
C
C
Uracil
CH3
H
O
NH2
C
H
N
C
C
C
N
H
H
Cytosine
• Pyrimidine rings are synthesized independent of
the ribose and transferred to the PRPP (ribose)
• Generated as UMP (uridine 5’-monophosphate)
Regulation of Pyrimidine
Biosynthesis
• Regulation occurs at first step in the pathway
(committed step)
X
• 2ATP + CO2 + Glutamine = carbamoyl phosphate
Inhibited by UTP
If you have lots of UTP around this means you won’t
make more that you don’t need
Hereditary Orotic Aciduria
• Defect in de novo synthesis of pyrimidines
• Loss of functional UMP synthetase
– Gene located on chromosome III
• Characterized by excretion of orotic acid
• Results in severe anemia and growth
retardation
• Extremely rare (15 cases worldwide)
• Treated by feeding UMP
Why does UMP Cure
Orotic Aciduria?
Carbamoyl
Phosphate
• Disease (-UMP)
– No UMP/excess orotate
Orotate
X
UMP
Synthetase
Feedback
Inhibition
• Disease (+UMP)
– Restore depleted UMP
– Downregulate pathway via feedback inhibition (Less orotate)
Biosynthesis: Purine vs Pyrimidine
• Synthesized on PRPP
• Regulated by GTP/ATP
• Generates IMP
• Requires Energy
• Synthesized then added to
PRPP
• Regulated by UTP
• Generates UMP/CMP
• Requires Energy
Both are very complicated multi-step process which
your kindly professor does not expect you to know in detail
Pyrimidine Degradation/Salvage
• Pyrimindine rings can be fully degraded to
soluble structures (Compare to purines that
make uric acid)
• Can also be salvaged by reactions with PRPP
– Catalyzed by Pyrimidine
phosphoribosyltransferase
Degradation pathways are quite distinct for purines and
pyrimidines, but salvage pathways are quite similar
Wait a minute:
So far we’ve only made
GMP, AMP, and UMP
We need the dNTPs according to
the Know-it-All Professor who
taught us that a couple of months ago
Two Problems
• These are monophosphates (i.e. GMP)- we
need triphosphates (i.e. GTP) for both DNA
and RNA synthesis
• These are ribonucleotides- that’s fine for
RNA but we also need to make DNA
Synthesis of ribonucleotides first
supports the RNA world theory
Specific Kinases Convert NMP
to NDP
Nucleoside
Monophosphates
Nucleoside
Diphosphates
Monophosphate
Kinases
• Monophosphate kinases are specific for the bases
Adenylate Kinase
AMP + ATP
2ADP
Guanylate Kinase
GMP + ATP
GDP + ADP
Conversion of Ribonucleotides
to Deoxyribonucleotides
HOCH
OH
HOCH
OH
BASE
BASE
2
2
O
O
5´
5´
H 1´
H H 1´
4´
4´ H
H 3´ 2´ H
H 3´ 2´ H
RibonucleotideHO
OH
HO H Reductase
Deoxyribonucleoside
Ribonucleoside
Somehow we need to get rid of this oxygen
Ribonucleotide Reductase
•
•
•
•
•
Catalyzes conversion of NDP to dNDP
Highly regulated enzyme
Regulates the level of cellular dNTPs
Activated prior to DNA synthesis
Controlled by feedback inhibition
dNDP to dNTP (the final step)
• Once dNDPs are generated by
ribonucleotide reductase a general kinase
can phosphorylate to make the dNTP’s
• So far we’ve made GTP, ATP, and UTP
(which can be aminated to form CTP)
• What about TTP?
You’ll have to tune in tomorrow
Plan for Tomorrow
• Brief Explanation of how dUMP is
converted to dTMP
• Some clinically relevant treatments based
on these pathways that are used to combat:
– Cancer
– Bacterial Infections
– Viral Infections
Take Home Concepts from
Today’s Lecture
• Nucleotides can be made through two pathways
– (de novo and salvage)
•
•
•
•
Pathways are regulated by feedback inhibition
Specific degradation pathways exist
Molecular basis of metabolic diseases mentioned
What steps are necessary to generate a dNTP from
the initial NMP made