Download File

METABOLISM OF PURINE
NUCLEOTIDES
&
PURINE DISORDERS
ROLES OF NUCLEOTIDES
Building blocks for DNA and RNA
“Second messengers” in signal transduction cascades
Energy “currency” of the cell
Components of major co-enzymes
Purine Biosynthesis
• Major site – (Adenine & Guanine) in the Liver.
• Synthesis starting from Ribose-5-phosphate
Denova pathway: Precursors contribute to form the Purine
ring: Glycine (C4, C5, N7), Glutamine ( N3, N9), THF (C2, C8
atoms) Aspartate (N1 atom), & CO2 (C6 atom).
• The construction of 6-membered ring forms Inosine- 5monophosphate (IMP) – 1st Purine.
• Energetics – expenditure of 6 high energy bonds.
Salvage pathway - Free & dietary purine can be converted
into corresponding nucleotides (no need ATP).
• RBC, Neutrophils & Brain cannot produce Purines by de
novo synthesis. But by only salvage pathway (HGPRTase).
Glucose-6-P
pentose phosphate
pathway
ATP
Ribose-5-phosphate
AMP
5-phosphoribosylpyrophosphate (PRPP)
PRPP synthetase
Gln
PRPP
Amino acids:
amidotransferase
Glu
Gly + Gln + Asp
Inosine mono
phosphate (IMP)
5-phospho-ribosylamine
Cofactors:
N10-formyl THF
Figure 1. Synthesis
of inosine monophosphate (IMP)
NAD+
aspartate + GTP
IMP
GDP + Pi
NADH
Gln + ATP
fumarate
Glu+AMP+PPi
guanosine monophosphate
(GMP)
Figure 2.
adenosine monophosphate
(AMP)
GDP
ADP
GTP
ATP
Formation of AMP and GMP from IMP
Ribose-5-phosphate
PRPP synthetase
PRPP
PRPP amidotransferase
5-phospho-ribosylamine
IMP
allosteric inhibition
GMP
AMP
GDP
ADP
Figure 3. Allosteric inhibition of purine biosynthesis;
also ATP stimulates formation of AMP. Regulatory enzyme is PRPP amido
transferase, controlled by feed back inhibition of nucleotides - IMP, AMP & GMP.
GMP, GDP or GTP
AMP, ADP or ATP
Adenosine
Guanosine
NH3
adenosine deaminase
Inosine
Ribose-1-P
purine nucleoside
phosphorylase
+PRPP Guanine
HGPRT
Ribose-1-P
Hypoxanthine
Xanthine
xanthine
oxidase
+PRPP
HGPRT
xanthine
oxidase
GMP
Figure 4. Degradation
IMP
Uric acid
of purines to uric acid and salvage of
purine bases via hypoxanthine-guanine phosphoribosyl transferase
METABOLISM OF PURINE
Glucose-6-P
pentose phosphate
pathway
ATP
Ribose-5-phosphate
AMP
X1a
5-phosphoribosylpyrophosphate (PRPP)
PRPP synthetase
Gln
PRPP
Amino acids:
amidotransferase
Glu
Gly + Gln + Asp
Inosine mono
phosphate (IMP)
5-phospho--ribosylamine
Cofactors:
N10-formyl THF
Figure 1. Hyperuricemia: Gout: X1a = PRPP synthetase defects
associated with a superactive enzyme characterized by an increased Vmax or an
enzyme with a reduced Km for ribose-5-P.
Disorder of Purine catabolism
• In humans, Purine rings are degraded to the metabolically
inert uric acid.
• Normal level of uric acid in blood is 2-7 mg/dl (Female)
& 3-8 mg/dl (Male).
• Uric acid excretion in urine is about 500 – 700 mg/day.
1. GOUT (Hyper uricemia)
• An inherited metabolic disease
• Incidence - 3 /1000 person
• Defect - partial deficiency of HGPRTase or increased
levels of PRPP caused by a hyperactive synthetase.
• Causes - ↓ Synthesis of GMP and IMP & ↑ in PRPP
levels --> ↑d Purine biosynthesis by de novo pathway.
Gout
1. Primary gout:
• Inborn error of metabolism due to over production of
uric acid
• Causes: Varient PRPP synthetase, PRPP gltamyl amido
transferase, HGPRT deficiency, Glu-6-Pase defiecincy.
2. Secondary gout:
• Various disease causing ↑or ↓uric acid excretion.
• E.g.: Cancer, Psoriasis, Trauma, Starvation,
Impairment in renal function leads to gout.
GOUT
Causes
• Uricosuria
• Accumulation of sodium urate crystals in the soft tissues
called Tophi -causes painful Gouty - Arthritis.
• Deposition of Na-urate crystals -Renal calculi
• Clinical finding- Red faced, Acidosis & Renal damage.
Treatment
• Reduced dietary Purine intake & restrict alcohol is advised.
• Uricosuric drugs. For example – Probenecid (Benemide),
Salicylates and Halofenate.
• Allopurinol (analog of hypoxanthine) competitively inhibits
Xanthine oxidase & thus uric acid synthesis (Suicide inhibition)
• Coichicine is an anti-inflammatory drug to relieve pain. Other useful
drugs include Indomethacin, Naproxen, Brufen, Corticosteroids etc.
2. LESCH-NYHAN SYNDROME
•
•
•
•
•
X-linked recessive disorder of Purine metabolism.
Incidence is 1 in 10,000 males
Defect - Total lack of HGPRTase, salvage pathway
Thus ↑ PRPP (de novo pathway), ↑ production of uric acid.
Diagnosis - Murexide test (reddish deposit) to detect urine uric acid
Clinical manifestations
• ↑ Production of uric acid causes severe gout, poor growth, and renal
failure due to Nephrolithiasis.
• Neurological abnormalities such as Mental retardation,
Aggressive behavior, learning disability & compulsive selfdestructive behavior.
• Irresistible urge to bite their finger & lips (Self mutilation)
• It shows that abnormal behavior can be caused by absence of a
single enzyme.
GMP, GDP or GTP
AMP, ADP or ATP
Adenosine
Guanosine
NH3
Inosine
Purine nucleoside
phosphorylase
Ribose-1-P
+PRPP Guanine
Xanthine
GMP
Ribose-1-P
Hypoxanthine
HGPRTase
X2
Adenosine deaminase
Inhibited by
allopurinol
xanthine
oxidase
xanthine
oxidase
+PRPP
HGPRTase
X2
IMP
Uric acid
Figure 4. X2 = moderate defect (>50% activity) leading to gout; severe defect
(very low activity) leads to Lesch-Nyhan
syndrome.
3. Immunodeficiency diseases
• Immunodeficiency diseases are associated with
Purine degradation disorders.
• Defect: Adenosine deaminase & Purine nucleoside
phosphorylase involved in uric acid synthesis.
• Causes: Severe combined immunodeficiency
(SCID) involving T-cell & usually B-cell
dysfunctions and thus impaired the immunity.
• Uric acid synthesis ↓& tissue level of N.S & N.T↑
• Treatment: Transferring ADA gene by Gene therapy.
4. Hypo uricemia
Hypoxanthine
• When serum uric acid level is less than
2 mg/dl represents hypouricemia.
• A rare congenital Xanthine oxidase deficiency
• Incidence is 1 in 45,000.
xanthine
oxidase
Xanthine
xanthine
oxidase
Uric acid
• It leads to the increased excretion of Xanthine &
Hypoxanthine.
• Causes: Excretion of xanthine in urine - Xanthinuria.
Frequently causes the formation of Xanthine stones in
the urinary tract.
SYNTHESIS OF PYRIMIDINE
NUCLEOTIDES
&
PYRIMIDINE DISORDERS
SYNTHESIS OF PYRIMIDINE
NUCLEOTIDES
• Synthesis of the Pyrimidine is less complex than
that of the purines, since the base is much simpler
(6 member ring).
• Precursors – Carbomyl phosphate & Aspartate
• De novo synthesis - in the Liver cytosol
• Energetic: Requires 2ATP
 ATP, PRPP
Carbamoyl phosphate
synthetase II (gln)
Glutamine +
2ATP + CO2
phosphate
-
UDP
UTP
Glu + Pi
+ ADP
CTP
Aspartate
Carbamoyl transcarbamoylase
Glu + Pi
+ 2 ADP
Gln +
ATP
Carbamoyl
-aspartate
Aspartate
Orotate
+2 ADP
UTP
+2 ATP
Orotate phosphoribosyl
transferase
UMP
+PRPP
OMP
Orotidylic acid
decarboxylase
Figure1 -
Biosynthesis of the Pyrimidine nucleotides UTP & CTP
Regulation of Pyrimidine Biosynthesis
CPS-II
Asp. Trans carbomylase
OMP Decarboxylase
CATABOLISM OF PYRIMIDINE
NUCLEOTIDES
• Pyrimidine degraded in the liver.
• The end products - are nitrogenous bases Cytosine,
Uracil and Thymine.
• The bases are then degraded to amino acids, namely
β-Alanine (from Cytosine & Uracil) &
β-Amino isobutyrate (from Thymine).
• These amino acids undergo transamination &
other reactions to finally produce Acetyl coA,
Succinate & CO2.
• Clinical significance: β-Aminoisobutyrate is excreted
in large quantity in Leukemia & when body subjected
to X-ray irradiation.
2. Reye’s syndrome
• Secondary Oratic aciduria
• Defect in Ornithine transcarbamoylase
(in urea cycle)
Causes
• Accumulation of Carbomyl phosphate
• This is then diverted for the increased synthesis and
excretion of oratic acid.
Clinical significances of
pyrimidine metabolism
Oraticaciduria
• Rare inherited disorders.
• Enzyme deficiency- Orotate phosphoribosyl
transferase and OMP decarboxylase.
• Defect - in UMP formation & thus ↑ Oratate synthesis in
blood & its excretion in the urine.
• Causes - Growth retardation, severe anemia caused by
hypo chromic erythrocytes and megaloblastic bone
marrow. Leukopenia is also common.
• Treatment – Diet rich in uridine & cytidine, which
↑UMP production via the action of nucleoside kinases.
UMP then inhibits CPS-II, thus ↓ orotic acid production.