Nucleotide metabolism

De novo synthesis of purine

Salvage pathway for purine

Purine degradation

balance of purine

purine nucleotide cycle
in muscle

De novo synthesis of pyrimidine

Nucleotide biosynthesis

de novo biosynthesis pathway

salvage purines & pyrimidines
from diet & degradative pathways

好的抗癌、抗菌目標

ribose可代謝產能,
nitorgenous base不行

嘌呤環上N的來源

11 steps
ribose-5-P → IMP

image

N-1:aspartate (aspartic acid)

C-2、C-8:N10-formyl-THF

N-3、N-9:glutamine amide N

C-4、C-5、N-7:glycine

C-6:CO2

STEP 1

STEP 2

STEP 11

酵素

PRPP synthetase / ribose-5-phosphate pyrophosphokinase

反應

ribose-5-phosphate → PRPP

PRPP:5-phosphoribosyl-α-pyrophosphate

加 glutamine

反應

PRPP → phosphoribosyl-β-amine

酵素

Gln:PRPP amino-transferase

azaserine

很像glutamine→
抑制glutamine dependent酵素

反應

FAICAR →inosine monophosphate (IMP)

FAICAR:N-formylaminoimidazole-4-carboxamide ribonucleotide

酵素

IMP synthase

Rate-limiting step

First-committed step

purine & pyrimidine ribonucleotides

inhibit

click to edit

需要 6 ATP

multifunctional enzymes

A-1/A-2/A-3

STEP 3.4.6

B-1/B-2

STEP 7.8

C-1/C-2

STEP 10.11

tetrahydrofolate

folate

缺乏folic acid會導致核苷酸不足
→貧血anemia

acceptors & donors of
one-carbon units

active form of folate

for all oxidation level carbon
(除了CO2)

oxidation level

formate (2)

-CH=O

N10-formyl-THF

formaldehyde (0)

methanol (-2)

precursor

6-methyl pterin

PABA

glutamine

sulfonamides
磺胺類藥物

compete

click to edit

block folic acid formation

analogs

methotrexate

aminopterin

anti-cancer

in purine synthesis
STEP 4.10

IMP → AMP or GTP

AMP synthesis

GTP synthesis

STEP 1

反應

酵素

adenylsuccinate synthetase

STEP 2

IMP + aspartate → adenylsuccinate

反應

adenylsuccinate → AMP + fumerate

需要GTP

酵素

adenylsuccinase / adenylsuccinate lyase

STEP 1

反應

IMP → XMP

酵素

IMP dehydrogenase

STEP 2

反應

XMP → GMP

酵素

GMP synthetase

需要ATP

regulation

IMP synthesis

R-5-P pyrophosphokinase

AMP synthesis

ADP & GDP (-)

Gln:PRPP aminotransferase

AMP, ADP, ATP & GMP, GDP,GTP & IMP (-)

PRPP (+)

GMP synthesis

adenylsuccinate synthetase

AMP (-)

IMP dehydrogenase

GMP (-)

NMP → NDP & NTP

specific nucleoside monophosphate

AMP + ATP → 2ADP

酵素:adenylate kinase

GMP + ATP → GDP + ADP

酵素: guanylate kinase

nucleoside diphosphate kinase

all NDP + ATP → NTP + ADP

回收核酸分解出的purine:adenine, guanine, hypoxanthine

發生在extrahepatic tissues

發生在liver

佔嘌呤生物合成90%

不消耗ATP

adenine + PRPP ↔ AMP + PPi

酵素:adenine phosphoribosyl transferase (APRT)

hypoxanthine + PRPP ↔ IMP + PPi
guanine + PRPP ↔ GMP + PPi

酵素: hypoxanthine-guanine phosphoribosyl transferase (HGPRT)

缺乏HGPRT

Lesch-Nyhan syndrome

purine synthesis 增加200-fold

血液中 uric acid 增加
(purine代謝產生尿酸)

severe arthritis

最終產物:uric acid

AMP

adenosine

adenosine deaminase (ADA)

inosine

AMP deaminase

IMP

purine nucleoside
phosphorylase (PNP)

nucleotidase

nucleotidase

hypoxanthine

XMP

nucleotidase

xanthosine

PNP

xanthine

xanthine oxidase

GMP

nucleotidase

guanosine

PNP

guanine

guanine deaminase

xanthine oxidase

uric acid

四肢累積過多尿酸

gout 痛風

allopurinol 可抑制 xanthine oxidase

de novo synthesis
(10%)

purine necleotides in use

uric acid
(10%)

recycling
(90%)

wastes

運動時,AMP deaminase 將AMP → IMP,釋放NH3

因為AMP量下降,促進adenylate kinase製造更多ATP

IMP 可再回收為AMP

嘧啶環的來源

3C、N:aspartate

N:glutamine

C:HCO3-

主要的嘧啶

cytosine

uracil

thymine

6 steps
glutamine → UMP

STEP 1

glutamine + HCO3- + 2ATP + H2O → carbamoyl phosphate + glutamate

酵素

carbamoyl phosphate synthase II (CPS II)

STEP 2

carbamoyl-P + aspartate → carbamoyl-Asp

activator:PRPP
inhibitor:UTP

酵素

aspartate transcarbamoylase (ATCase)

STEP 3

carbamoyl-Asp → dihydroorotate (DHO) + OH-

酵素

dihydroorotase

STEP 4

DHO → orotate (一種嘧啶)

酵素

DHO dehydrogenase

STEP 5

orotate + PRPP → orotidine 5'-monophosphate (OMP)

酵素

orotate phosphoribosyltransferase

PRPP 提供 ribose-P

STEP 6

OMP → UMP + CO2

酵素

OMP decarboxylase

multifinctional polypeptides

CPS II, ATCase, dihydroorotase,

orotate PRT, OMP decarboxylase

metabolic channeling more efficient

3 steps
UMP → CTP

STEP 1

STEP 2

STEP 3

UMP + ATP → UDP + ADP

酵素

nucleoside monophosphate kinase

UDP + ATP → UTP + ADP

酵素

nucleoside diphosphate kinase

UTP + glutamine + ATP → CTP + glutamate

酵素

CTP synthetase

regulation

animals

bacteria

CPS II

PRPP, IMP, ATP (+)

UDP, UTP (-)

ATCase

ATP (+)

CTP (-)

pyrimidine degradation

人類:free pyrimidines bases不會salvaged
(只有nucleotides中的會被回收)

cytosine, uracil

分解代謝產生

β-alanine, ammonium, CO2

β-alanine 可被回收合成coenzyme A

thymine

分解代謝產生

β-aminoisobutyric acid (BAIBA), ammonium, CO2

form deoxyribonucleotides

細胞中90%的核酸是RNA

NDP的2'C去氧→dNDP

酵素

ribonucleotide reductase

3 nucleotide binding sites

C:substrate (active site)
on R2

NDPs

α2β2

A:activity-determining (overall activity site)
on R1

ATP(+) & dATP(-)

S:specificity-determining (substrate specific site)
on R1

ATP

thioredoxin reductase & thioredoxin提供還原力
(reversible sulfide:sulfhydryl transition)
(S-S/SH HS 氧化還原循環)

Tyr122 free radical
1>> removal of Ha hydrogen(Cys439)
2>> creation of C-3' redical

disulfide formation between Cys225 & Cys462
1>> dehydration
2>> form dNTP

dTTP

dGTP

dATP

偏向反應

CDP, UDP

偏向反應

GDP, ADP

偏向反應

ADP

SCID:severe combined immunodeficiency

缺乏ADA >> deoxyadenosine 不能轉為deoxyinosine

nucleoside kinase 將其轉為dAMP → dATP

dATP 抑制 ribonucleotide reductase

無法合成dNTP, 無法複製DNA

Thymine nucleotides合成

dUDP, dCDP → dTMP

dUDP → dUTP →

dUTPase

→ dUMP → dTMP

dCDP → dCMP

dCMP deaminase

→ dUMP → dTMP

dUMP → dTMP

酵素:thymidylate synthase

methylate甲基化dUMP的5號位

甲基提供者: N5,N10-methylene-THF

reductive methylation