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Zhihong Li (李志红)Department of Biochemistry
Biochemistry Ⅱ
Main TopicsMetabolism of Nucleotides (4h)
DNA replication; RNA transcription; Protein synthesis (12h)
Gene expression and regulation (4h); Recombinant DNA technology (4h)
Signal transduction; Oncogene (6h)
Diabetes mellitus (2h); Lipoproteins Metabolism (4h)
Cholesterol Metabolism (2h); Bile acids Metabolism (2h)
Examination
Heme Synthesis (2h); Bile Pigments Metabolism (2h)
Liver function tests (2h); Metabolism of xenobiotics (2h)
Plasma Proteins and Immuno Proteins (2h)
Free Radicals and Antioxidants (2h)
Mineral Metabolism:(micro-elements) (2h)
Water and Electrolyte Balance; Acid Base Balance (4h)
Hormones (6h)
Biochemical changes during Pregnancy (2h)
N
N
N
NH
N
N
Lecture 1
Metabolism of Nucleotides
Contents
• Review: Structure of nucleic acid• Degradation of nucleic acid• Synthesis of Purine Nucleotides• Degradation of Purine Nucleotides• Synthesis of Pyrimidine Nucleotides• Degradation of Pyrimidine Nucleotides
Nucleoside and Nucleotide
Nitrogenous base ribose
Nitrogenous base ribose phosphate
Nucleoside =
Nucleotide =
pyrimidine purineOR
Riboseor
2-deoxyribose
N--glycosylbond
Structure of nucleotides
Purines vs Pyrimidines
Section 1
Degradation of nucleic acid
Nucleoprotein
Nucleic acid Protein
Nucleotide
NucleosidePhosphate
Base Ribose
Nucleotidase
Nucleosidase
Degradation of nucleic acid
In stomach Gastric acid and pepsin
In small intestine Endonucleases: RNase and DNase
Significances of nucleotides
1. Precursors for DNA and RNA synthesis
2. Essential carriers of chemical energy, especially ATP
3. Components of the cofactors NAD+, FAD, and coenzyme A
4. Formation of activated intermediates such as UDP-glucose and CDP-diacylglycerol.
5. cAMP and cGMP, are also cellular second messengers.
Section 2
Synthesis of Purine Nucleotides
There are two pathways leading to nucleotides
• De novo synthesis: The synthesis of nucleotides begins with their metabolic precursors: amino acids, ribose-5-phosphate, CO2, and one-carbon units.
• Salvage pathways: The synthesis of nucleotide by recycle the free bases or nucleosides released from nucleic acid breakdown.
§ 2.1 De novo synthesis• Site:
– in cytosol of liver, small intestine and thymus
• Characteristics:
a. Purines are synthesized using 5-phosphoribose(R-5-P) as the starting material step by step.
b. PRPP(5-phosphoribosyl-1-pyrophosphate) is active donor of R-5-P.
c. AMP and GMP are synthesized further at the base of IMP(Inosine-5'-Monophosphate).
N10-Formyltetrahydrofolate
N10-Formyltetrahydrofolate
1. Element sources of purine bases
First, synthesis Inosine-5'-Monophosphate, IMP
2. Synthesis of Inosine Monophosphate (IMP)
• Basic pathway for biosynthesis of purine ribonucleotides
• Starts from ribose-5-phosphate(R-5-P)• Requires 11 steps overall• occurs primarily in the liver
OH
1 ATP
AMP
2
Gln:PRPP amidotransferase
ribose phosphate pyrophosphokinase
Step 1:Activation of ribose-5-phosphate
Step 2: acquisition of purine atom N9
5- 磷酸核糖胺 ,PRA
•Steps 1 and 2 are tightly regulated by feedback inhibition
Committed step
甘氨酰胺核苷酸
3
Step 3: acquisition of purine atoms C4, C5, and N7
glycinamide synthetase
甲酰甘氨酰胺核苷酸
4
•Step 4: acquisition of purine atom C8
GAR transformylase
FH4 (or THF)
N10—CHO—FH4
甲酰甘氨咪核苷酸
5
Step 5: acquisition of purine atom N3
6
5- 氨基咪唑核苷酸
•Step 6: closing of the imidazole ring
5- 氨基 -4- 羧基咪唑核苷酸
Carboxyaminoimidazole
ribonucleotide (CAIR)
7
Step 7: acquisition of C6
AIR carboxylase
5- 氨基 -4-(N- 琥珀酸 )
- 甲酰胺咪唑核苷酸
Carboxyaminoimidazole ribonucleotide (CAIR)
Step 8: acquisition of N1
SAICAR synthetase
5- 氨基 -4- 甲酰胺咪唑核苷酸
Step 9: elimination of fumarate
adenylosuccinate lyase
5- 甲酰胺基 -4- 甲酰胺咪唑核苷酸
Step 10: acquisition of C2
AICAR transformylase
Step 11: ring closure to form IMP
• Once formed, IMP is rapidly converted to AMP and GMP (it does not accumulate in cells).
N10-CHOFH4
N10-CHOFH4
3. Conversion of IMP to AMP and GMP
Note: GTP is used for AMP synthesis.
Note: ATP is used for GMP synthesis.
IMP is the precursor for both AMP and GMP.
kinase
ADPkinase
ADP
ATP
ATP ADP
AMP
ATP
kinase
GDPkinase
ADP
GTP
ATP ADP
GMP
ATP
4. ADP, ATP, GDP and GTP biosynthesis
5. Regulation of de novo synthesis
The significance of regulation:
(1) Fulfill the need of the body, without wasting.
(2) [GTP]=[ATP]
• Purine nucleotide biosynthesis is regulated by feedback inhibition
§ 2.2 Salvage pathway
• Purine bases created by degradation of RNA or DNA and intermediate of purine synthesis were costly for the cell to make, so there are pathways to recover these bases in the form of nucleotides.
• The significance of salvage pathway :– a. Save the fuel.– b. Some tissues and organs such as brain and bone marr
ow are only capable of synthesizing nucleotides by salvage pathway.
• Two phosphoribosyl transferases are involved:– APRT (adenine phosphoribosyl transferase) for adenine.– HGPRT (hypoxanthine guanine phosphoribosyl transferas
e) for guanine or hypoxanthine.
Purine Salvage Pathway
N
NN
N
NH2
O
Guanine
N
N N
O
N
Hypoxanthine
O
OHHO
2-O3POH2C
N
N N
O
N
IMP
O
OHHO
2-O3POH2C
N
NN
N
NH2
O
GMP
.
.
Adenine AMP
PRPP PPi
adenine phosphoribosyl transferase
PRPP PPi
hypoxanthine-guaninephosphoribosyl transferase
(HGPRT)
Absence of activity of HGPRT leads to Lesch-Nyhan syndrome.
Lesch-Nyhan syndrome
• first described in 1964 by Michael Lesch and William L. Nyhan.
• there is a defect or lack in the HGPRT enzyme• Sex-linked metabolic disorder: only males• the rate of purine synthesis is increased about 200-fold
– Loss of HGPRT leads to elevated PRPP levels and stimulation of de novo purine synthesis.
• uric acid level rises and there is gout• in addition there are mental aberrations• patients will self-mutilate by biting lips and fingers off
Lesch-Nyhan syndrome
§ 2. 3 Formation of deoxyribonucleotide
• Formation of deoxyribonucleotide involves the reduction of the sugar moiety of ribonucleoside diphosphates (ADP, GDP, CDP or UDP).
• Deoxyribonucleotide synthesis at the nucleoside diphosphate(NDP) level.
SS
H2OMg2+
NADPH + H+NADP+
SHSH
thioredoxin
ribonucleotide reductase
NDP£¨N=A, G, C, U£©
dNDP
dNTP
ATP
ADP
kinase
O BaseCH2
HOH
OP PO BaseCH2
OHOH
OP P
thioredoxin
thioredoxin reductase
FAD
Deoxyribonucleotide synthesis at the NDP level
§ 2. 4 Antimetabolites of purine nucleotides
• Antimetabolites of purine nucleotides are structural analogs of purine, amino acids and folic acid.
• They can interfere, inhibit or block synthesis pathway of purine nucleotides and further block synthesis of RNA, DNA, and proteins. Widely used to control cancer.
1. Purine analogs
• 6-Mercaptopurine (6-MP) is a analog of hypoxanthine.
N
N NH
N
OH
N
N NH
N
SH
6-MPhypoxanthine
6-MP 6-MP nucleotide
de novo synthesis
salvage pathway
HGPRT
amidotransferase
IMP
AMP and GMP
--
-
-
-
• 6-MP nucleotide is a analog of IMP
2. Amino acid analogs
• Azaserine (AS) is a analog of Gln.
H2N C CH2
O
CH2 CH
NH2
COOH Gln
C
O
CH2 CH
NH2
COOH ASNN CH2 O
3. Folic acid analogs
• Aminopterin (AP) and Methotrexate (MTX)
R=H:AP
folic acid
N
NN
N
NH2
H2N
CH2 N C
R O
NH CH
COOH
R=CH3:TXT
CH2 CH2 COOH
N
NN
N
OH
H2N
CH2 N C
H O
NH CH
COOH
CH2 CH2 COOH
MTX
folate FH2 FH4
NADPH + H+
NADP+NADPH + H+
NADP+
FH2 reductase FH2 reductase
AP or MTX
- -
•The structural analogs of folic acid(e.g. MTX) are widely used to control cancer (e.g. leukaemia).
•Notice: These inhibitors also affect the proliferation of normally growing cells. This causes many side-effects including anemia, baldness, scaly skin etc.
Section 3
Degradation of Purine Nucleotides
nucleotide
nucleotidaseH2O
Pi
nucleosidenucleoside
phosphorylase
Pi R-1-P
purine
R-5-P
PRPP
pentose phosphate pathway
salvage pathway
oxidationuric acid
N
HCN
C
CC
N
CH
N
NH2
Ribose-P
AMP
HN
HCN
C
CC
N
CH
N
O
Ribose-P
IMPHN
HCN
C
CC
NH
CH
N
O
HN
CNH
C
CC
NH
CH
N
O
O
HN
CNH
C
CC
NH
C
N
O
O
O
GMP
Hypoxanthine
Uric Acid Xanthine
Xanthine Oxidase
N
HCN
C
CC
N
CH
N
NH2
Ribose-P
AMP
HN
HCN
C
CC
N
CH
N
O
Ribose-P
IMPHN
HCN
C
CC
NH
CH
N
O
HN
CNH
C
CC
NH
CH
N
O
O
HN
CNH
C
CC
NH
C
N
O
O
O
GMP
Hypoxanthine
Uric Acid Xanthine
Xanthine Oxidase
(2,6,8-trioxypurine)
Adenosine Deaminase
• Uric acid is the excreted end product of
purine catabolism in primates, birds, and some other animals.
• The rate of uric acid excretion by the normal adult human is about 0.6 g/24 h, arising in part from ingested purines and in part from the turnover of the purine nucleotides of nucleic acids.
Uric acid
• The disease gout, is a disease of the joints, usually in males, caused by an elevated concentration of uric acid in the blood and tissues.
• The joints become inflamed, painful, and arthritic, owing to the abnormal deposition of crystals of sodium urate.
• The kidneys are also affected, because excess uric acid is deposited in the kidney tubules.
GOUT
The uric acid and the gout
Uric acid Over 8mg/dl, in the plasma
Gout, Urate crystallization in joints, soft tissue, cartilage and kidney
Hypoxanthine
Xanthine Out of body
In urine
Diabetese nephrosis
……
Advanced GoutClinically Apparent Tophi
1
1. Photos courtesy of Brian Mandell, MD, PhD, Cleveland Clinic.2. Photo courtesy of N. Lawrence Edwards, MD, University of Florida.3. ACR Clinical Slide Collection on the Rheumatic Diseases, 1998.
21
3
HN
HCN
C
CC
NH
CH
N
O
Hypoxanthine
HN
HCN
C
CC
NH
N
HC
O
Allopurinol
Allopurinol – a suicide inhibitor used to treat Gout
Xanthine oxidase
Xanthine oxidase
Adenosine Deaminase(ADA) and Severe Combined Immunodeficiency Disease (SCID)
• SCID is caused by an Adenosine Deaminase Deficiency (ADA)– Gene is located on chromosome #22– ADA is involved in purine degradation – Accumulation of nucleotide metabolites = TOXIC t
o developing T lymphocytes – Deficiency results in failure to develop functional T
and B lymphocytes– Patients cannot withstand infection die if untrea
ted
Boy in the Bubble
Gene therapy
• September 14, 1990 @ NIH, French Anderson and R. Michael Blaese perform the first GT Trial– Ashanti (4 year old girl)
• Her lymphocytes were gene-altered (~109) used as a vehicle for gene introduction using a retrovirus vector to carry ADA gene
– Cynthia (9 year old girl) treated in same year
Section 4
Synthesis of Pyrimidine Nucleotides
• shorter pathway than for purines• Pyrimidine ring is made first, then attached to ribo
se-P (unlike purine biosynthesis)• only 2 precursors (aspartate and glutamine, plus H
CO3-) contribute to the 6-membered ring
• requires 6 steps (instead of 11 for purine)• the product is UMP (uridine monophosphate)
§ 4.1 De novo synthesis
1. Element source of pyrimidine base
N
CN
C
CC
12
34
5
6Asp
CO2
Gln
•Carbamoyl phosphate synthetase(CPS) exists in 2 types: •CPS-I, a mitochondrial enzyme, is dedicated to the urea cycle and arginine biosynthesis. •CPS-II, a cytosolic enzyme, used here. It is the committed step in animals.
Step 1: synthesis of carbamoyl phosphate
Step 2: synthesis of carbamoyl aspartate
ATCase: aspartate transcarbamoylase
•Carbamoyl phosphate is an “activated” compound, so no energy input is needed at this step.
Step 3: ring closure to form dihydroo
rotate
Step 4: oxidation of dihydroorotate to orotate
Step 5: acquisition of ribose phosphate moiety
•ribose phosphate originates from PRPP
Step 6: decarboxylation of OMP
The big picture
3. UTP and CTP biosynthesis
UDP
ADP
UTP
ATP ADP
UMP
ATP
kinase kinase
4. Formation of dTMP
The immediate precursor of thymidylate (dTMP) is dUMP.
The formation of dUMP either by deamination of dCMP or by hydrolyzation of dUDP. The former is the main route.
dTMP dTDP dTTP
dUMP
dUDP dCMP dCDP
N5,N10-methylene-tetrahydrofolic Acid
ATP ATP
ADP ADP
dTMP synthetase
UDP
dTMP synthesis at the nucleoside monophosphate level.
dUMP
dUDP
dCMPdTMP
H2O
Pi
H2O
NH3
NADPH
NADP+
thymidylate synthase HN
N
O
O
R 5' Pd
CH3
reductase
HN
N
O
O
R 5' Pd
+ H+
FH2
FH4
N5, N10-CH2-FH4 FH2
5. Regulation of de novo synthesis
carbamoyl phosphate
carbamoyl aspartate
UMP
ATP + CO2 + Gln
PRPP
UTP CTP
ATP + R-5-P
purine nucleotide
pyrimidine nucleotide
§ 4. 2 Salvage pathway
+ ATP
+ ATP
+ ATP
UMPCMP
dTMP + ADP
dCMP + ADP
uridinecytidine
deoxythymidine
deoxycytidine
thymidine kinase
deoxycytidine kinase
uridine-cytidine kinase+ ADP
+ PRPP + PPiuracilthymineorotic acid
pyrimidine phosphate ribosyltransferase UMP
dTMPOMP
§ 4. 3 Antimetabolites of pyrimidine nucleotides
• Antimetabolites of pyrimidine nucleotides are similar with them of purine nucleotides.
1. Pyrimidine analogs
• 5-fluorouracil (5-FU) is a analog of thymine.
HN
NH
O
O
FHN
NH
O
O
CH3
thymine5-FU
2. Amino acid analogs
• Azaserine (AS) inhibits the synthesis of CTP.
3. Folic acid analogs
• Methotrexate (MTX) inhibits the synthesis of dTMP.
4. Nucleoside analogs
• Arabinosyl cytosine (ara-c) inhibits the synthesis of dCDP.
N
N
NH2
O
ara-c
O
H
OH H
H
CH2OH
H OH
N
N
NH2
O
cytosine
O
H
OH OH
H
CH2OH
H H
Section 5
Degradation of Pyrimidine Nucleotides
H2OH2O
H2N CH2 CH2 COOH H2N CH2 CH COOH
CH3
N
NH
O
NH2H2O NH3
HN
NH
O
O
CH2
CH2NH2
NH
O
HOOC
HN
NH
O
O
CH3
CH2
CHNH2
NH
O
HOOC
CH3
cytosine uracil thymine
¦Â-ureidopropionate
¦Â-ureido-isobutyrate
CO2 + NH3
¦Â-alanine ¦Â-aminoisobutyrate
Highly soluble products
Summary of purine biosynthesis
dATP
dGTP
AMP
GMP
ADP
GDP
dADP
dGDP
IMP
ATP
GTP
CTP
Summary of pyrimidine biosynthesis
UDP UTP
CDP
dUDP
dCDP
dUMP
dCMP
dTMP
UMP
dTDP dTTP
dCTP
Summary of Nucleotide Synthesis
• Purines built up on ribose– PRPP synthetase: key step– First, synthesis IMP
• Pyrimidine rings built, then ribose added– CPS-II: key step– First, synthesis UMP
• Salvage is important
Points• Synthesis of Purine Nucleotides
– De novo synthesis: Site, Characteristics, Element sources of purine bases
– Salvage pathway: definition, significance, enzyme, Lesch-Nyhan syndrome
– Formation of deoxyribonucleotide: NDP level– Antimetabolites of purine nucleotides:
• Purine, Amino acid, and Folic acid analogs
• Degradation of Purine Nucleotides– Uric acid, gout
• Synthesis of Pyrimidine Nucleotides– De novo synthesis: Characteristics, Element sources of pyri
midine bases– Salvage pathway– Antimetabolites of pyrimidine nucleotides
• Catabolism of Pyrimidine Nucleotides