Download Nucleotides

Section 8.
Amino Acid
Metabolism
One-carbon metabolism, purine
metabolism
11/29/05
Nucleotide Roles
• Activated precursors for DNA and RNA synthesis:
dNTPs and NTPs.
• Activation of other moieties for synthesis: UDPglucuronate, CDP-diacylglycerol, etc.
• Energy transduction: ATP for muscle contraction
and ion transport.
• Control reactions: GDP GTP of G-proteins.
• Control of metabolism: cAMP, ADP/ATP ratio,
enzyme phosphorylation.
• Constituents of other small molecules: NADH,
FAD, CoA, etc.
1
O
Purines
N
HN
N
• There are two pathways
for nucleotide
biosynthesis: de novo
and salvage.
• Hypoxanthine is an
intermediate on the de
novo pathway to AMP
and GMP and guanine,
and on the salvage
pathway to AMP.
2
NH
Hypoxanthine
NH2
N
N
N
NH
Adenine
O
N
HN
H2N
N
NH
Guanine
Purine Biosynthesis: the committed step
O
H
ATP AMP
H
O
O
O
CH
2
O P O
O P O CH2
OH
O
O
O
OH
OH
HO
HO
ribose 5-phosphate
• Addition of an amino
group from glutamine to
PRPP to form 5phosphoribosyl-1-amine
is the committed step.
3
O
O
P
O P O
O
O
5-phosphoribosyl-1-pyrophosphate
(PRPP)
Glutamine
Amidophophoribosyl
transferase
O
CH2
O PO
O
Glutamate
O
NH2
H
HO
+ PPi
OH
5-phosphoribosyl-1-amine
ADP
+
Pi
glycine
NH2
+ ATP
N10- formyl THF
Glutamine
THF
+ ATP
Glutamate
+ ADP + Pi
H2O
ribose-P
5-phosphoribosyl1-amine
asp
CO2
+
ATP
Purine Biosynthesis
fumarate
ADP
+ Pi
+
O
N10-formylTHF
THF
H2O
HN
N
H
N
N
see Figs. 25.7
& 25.8
ribose-P
Inosinate
(IMP)
4
H
AMP and GMP
Asp
+ GTP
NH
H
fumarate
GDP
+ Pi
O
NH2
Adenylosuccinate
N
H
N
N
H
N
ribose-P
N
N
N
AMP
H
ribose-P
IMP
NAD+
NADH
Gln
+ ATP
Glu +
AMP + PPi
HN
N
Xanthylate
• GMP and AMP are
phosphorylated to di5 and triphosphates.
O
H2N
N
N
ribose-P
see Fig. 25.9
GMP
H
Control Mechanisms
O
O P O CH2
O
O
H
O
O
P
O
O P O
O
OH
O
HO
• All purine nucleotides
inhibit
5-phosphoribosyl-1-pyrophosphate
(PRPP)
amidophosphoribosyl
transferase activity.
Glutamine
Amidophophoribosyl
• For balance:
transferase
Glutamate
– AMP inhibits the
O
synthesis of
NH2
O
CH
2
O P O
adenylosuccinate.
+ PPi
H
O
– GMP inhibits the
OH
HO
synthesis of xanthylate.
5-phosphoribosyl-1-amine
6
AMP inhibits upper pathway
Asp
+ GTP
fumarate
GDP
+ Pi
AMP
Adenylosuccinate
IMP
NAD+
Gln
+ ATP
NADH
Xanthylate
GMP inhibits lower pathway
7
Glu +
AMP + PPi
GMP
Single Carbon Carriers
• Biotin
-CO2• S-adenosyl methionine (SAM)
-CH3
• Tetrahydrofolate (THF)
-CH3
Single carbons for:
-CH2– carnitine
– choline
=CH– creatine
-HC=O
– epinephrine
-C=NH
8
– purines
– thymine
– others
HCO3+ ATP
O
Biotin
HN
ADP
+ Pi
NH
O
C
N
NH
O
O
S
O
O
S
Biotin
O
O
• Biotin carries a carboxyl group, activated by ATP hydrolysis.
• The coenzyme is covalently attached to a flexible lysine
sidechain in the enzyme active site.
O
HN
NH
lysine sidechain
HN
N
S
O
9
enzyme
O
O O
+
H3 N
ATP
S-Adenosyl
methionine O
(SAM) H N
H3 C S
+
O
+
NH2
N
N
N
CH2O
N
OH
HO
S-adenosylmethionine
3
+ Pi + PPi
~CH3
For synthetic
methylations
O O
+
H3 N
NH2
(SAM)
N
S
H3 C S
methionine
O O
+
H3 N
-CH3
ADP
SH
homocysteine
• Homocysteine is
methylated by N5-methyl-THF.
10 • B12 is required.
N
N
H2O
CH2
N
O
OH
HO
S-adenosylhomocysteine
• Dietary folic
acid is
reduced to the
coenzyme
form THF.
• The 5- and
10- nitrogens
can carry
single
carbons in all
oxidation
states except
carboxylate.
11
Tetrahydrofolate
N
H2 N
N
N
HN
OH
O
O
N
O
O
NH
folate
O
ascorbate
2 NADPH
2 NADP+
N
H2 N
NH
N
O
NH
OH
HN
O
NH
O
O
O
5
10
tetrahydrofolate (THF or H4F)
THF Carbon
States
• The carbon
comes from
serine or
formate.
• Oxidation,
reduction and
transamination
occur while
bound to THF.
Fig. 24.13
12
Cobalamine
• Derived from
vitamin B12.
• Required for
methylation of
homocysteine.
13
Styer 4th
GLYCINE
NH
SERINE
NH
NH
N
N
H2C
THF
NH
N5,N10-methylene-THF
NADPH
NADP+
Pi + PPi
methionine
S-adenosyl
methionine
homocysteine
S-adenosyl
homocysteine
B12
~CH3
NH
NADP+
N
CH3
NADPH
NH
adenosine
N5-methyl-THF
H2O
NH
NH
NH
HC
ATP
N
N5,N10-methenyl-THF
NH
O
NH4+
N
C
H
GLYCINE
NH
N
14
CH
NH
HN
N5-formimino-THF
N10-formyl-THF
Single Carbon Scheme
N
N
ribose + Pi
O
NH2
N
HN
NH NH
adenylate
deaminase
N
(IMP)
H2O2
HN
O
15
O
O2 + H2O
H2O2
O
N
N NH
H
urate
O
HN
O
OH
O
N
NH NH
xanthine
xanthine
oxidase
O
NH
HN
N
N NH
H
uric acid
O
HN
+
H
xanthine
oxidase
O2 + H2O
Purine Degradation
NH
hypoxanthine
ribose-5'-phosphate
• Xanthine oxidase contains
molybdenum (Mo) and ironflavoprotein.
• Urate is an antioxidant.
N
HN
NH NH
ribose-5'-phosphate
O
O
O
N
H
NH
uric acid
Gout
N
N
HO
O
N
HN
NH
urate
O
O
OH
N
N
xanthine
oxidase
• Urate can precipitate as
a sodium salt, causing
gout.
OH
• Allopurinol is oxidized
N
N
by xanthine oxidase to
N NH
alloxanthine, a potent HO
inhibitor of xanthine
alloxanthine
oxidase.
• Less urate is produced.
• The use of the salvage
pathway increases.
16
O2
N
NH
allopurinol
N
HN
NH
hypoxanthine
xanthine
oxidase
O2
OH
N
N
HO
N
NH
xanthine
Salvage Pathway
hypoxanthine + PRPP
guanine + PRPP
hypoxanthine-guanine
phosphoribosyl
transferase
hypoxanthine-guanine
phosphoribosyl
transferase
IMP + PPi
GMP + PPi
• Nucleotides can also be made from intact bases, either
from dietary sources or from degraded nucleotides that
were made by the de novo pathway.
17 • The salvage pathway is prevalent in the brain.
Lesch-Nyhan Syndrome
• Caused by very low levels of hypoxanthineguanine phophoribosyltransferase.
• This disrupts the salvage pathway for purine
nucleotide biosynthesis.
• High [PRPP] & [urate] and increased de
novo purine biosynthesis.
• Symptoms are gout, mental deficiency,
spasticity, compulsive self-mutilation,
aggression.
18
Next topic: Pyrimidines,
catecholamines, serotonin &
histamine.