biosynthesis and degradation of nucleotide

von Gierke’s disease or glycogen storage disease type I caused by

defiency in G6P, causing low blood glucose, enlarge fatty liver, high blood uric acid and gout

nucleotide consists of

sugar + base + phosphate

sugar

ribose or dexoyribose that lack 2OH group

base

nitroegnous base including purine and pyrimidine

phosphate

phosphate group

purine

adenine, guanine, hypoxanthine

pyrimidine

uracil, cytosine, thymine

purine nucleotide biosynthesis and metabolism include

PRPP synthesis, de novo of IMP, conversion AMP/GMP from IMP, salvage pathway, purine nucleotide metabolism

PRPP synthesis

R5P from PPP, PRPP synthetase transfers pyrophosphate to R5P forming PRPP

excess PRPP means excess

purines thus excess uric acid thus hyperuricemia

PRPP is an important

precursor for purine and pyrimidine nucleotide synthesis

PRPP can enter 2 pathways

de novo or salvage

de novo pathways is where

nitrogenous base synthesized

salvage pathway where

nitrogenous base recycled from other pathway and transferred to PRPP by transferase

De novo synthesis of IMP

from AA, needs N10-formyl-THF and folic acid derivative, uses 5 ATPs

purine and pyrimidine de novo biosynthesis are

energetically expensive processes

one molecule of hypoxanthine syntehsized on

ribose ring of PRPP to form IMP

IMP → AMP

hypoxanthine converts to adenine, by product is fumarate that enter TCA, uses GTP energy

IMP → GMP

hypoxanthine converted to xanthine then guanine, uses ATP energy

cells do not accumulate

IMP cause they are either converted into AMP/GMP since they serve as intermediate for their biosynthesis

AMP or GMP can be further phosphorylated by adenylate or guanylate kinase to form

ADP or GDP

salvage pathway uses nitrogenous bases from

other pathways such as from diet or dying cells

salvage pathway is the

preferred and energy efficient pathway

the bases are transferred to the PRPP in salvage pathway are by 2 type of transferases

APRT and HGPRT

APRT

adenine phosphoribosyl transferase

HGPRT

hypoxanthine and guanine phosphoribosyl transferase

nucleoside phosphorylated back to ________ and base is rececyled by ________ BUT in excessive purine diet or cell death the excess purine can be _______

nucleotide, salvage pathway, degraded

xanthine is ________, xanthine oxidase is key enzyme that metabolize _________________ into uric acid

common intermediate, hypoxanthine and xanthine

uric acid can be metabolized by ___________ into ammonia

intestinal bacteria

what is an XO inhibitor

allopurinol

purine nucleotides are first degraded into

free purines by 3 steps

step 1 of purine degradation

purine nucleotide dephosphorylation

5′-nucleotidase removes phosphate → nucleoside. ADA converts adenosine → inosine

step 2 of purine degradation

release of nitrogenous base

PNP removes the sugar → free purine base

step 3 of purine degradation

conversion into uric acid

XO converts hypoxanthine → xanthine → uric acid

final product of purine degradation is ________

uric acid

since humans lack ________ uric acid must be eliminated by ______

uricase, kidneys

in the blood uric acid exists as

monosodium urate (MSU)

MSU has

poor solubtility causing crystal to form into joints causing gouty arthritis

lesh-nyhan syndrome is

first neuropsychiatric abnormality attributed to single enzyme

2 ways uric acid can be formed due to deficiency in HGPRT

1.Deficiency in HGPRT → hypoxanthine and guanine accumulation → uric acid

2.Deficiency in HGPRT → PRPP accumulation → increased de novo synthesis of purine nucleotides → uric acid

what is more likely to be metabolize by XO but not recycled by salvage pathway

IMP and GMP, causing gout like symptoms

if more PRPP enter de novo synthesis it

increases pool of purine nucleotides, causing gout like symptoms

in G6P deficiency glucose 6 P isnt converted to glycose but directed to

glycolysis or PPP

increase glycolysis pathway from G6P deficiency leads to

triglyceride production in liver

increase PPP from G6P deficiency leads to

purine biosynthesis and increased uric acid

de novo synthesis of UMP

base is de novo synthesized from DHODH, transferred to PRPP to make UMP by UMP synthase, uses 2ATP

leflunomide is a DHODH inhibitor

used to treat rheumatoid arthritis because it inhibits lymphocyte activation and inflammation

UTP and CTP made from

UMP

CTP formed from UTP by

CTP synthetase

UTP → CTP driven by

ATP hydrolysis

CTP is rate limiting enzyme for

synthesis of cytosine nucleotide from de novo and uridine salvage pathway

DON is inhibitor of

CTP synthetase

salvage pathway of UMP nucleotide

base is uracil from diet/nucleic acid breakdown, UPRT is used, then forms UMP, overall minor pathway for pyrimidine nucleotide biosynthesis

in contrast to free purines salvage of free pyrimidines is

very minor pathway in most animals

CMP and UMP degraded to base like purines by _________ then glycosidic bond cleaves to release _______ and that is reduced in liver forming _______ which is soluble and recycled in other pathways

dephosphorylation, uracil, b-alanine

RNR converts ADP, GDP, CDP, UDP to

dADP, dGDP, dCDP, dUDP

dADP, dGDP, dCDP, dUDP are then phosphorylated into

dNTPs

TMP is not derived from ____ since theres no corresponding nucleotide

RNR

TMP is formed through

thymine formation pathway

Deoxyribonucleotides are synthesized from _________ by _________

corresponding ribonucleotides, ribonucleotide reductase (RNR)

RNR activity tightly controlled by

dNTP level

since thymine replaces uracil in DNA and thymine is used for deoxynucleotide there is no

ribose form of TMP

RNR plays role in regulating

pool size of ribonucleotides vs deoxyribonucleotides

TMP (Thymidylate) Biosynthesis

synthesized from dUMP, dUMP converted to TMP by adding methyl group, methyl donor turns into DHF after donating, folate cycle regenerate THF and methyl donor using DHFR and SHMT

what converts dUMP to TMP by adding methyl group

thymidylate synthase

what is the methyl donot in TMP biosyntehsis

N⁵,N¹⁰-methylene-THF

what does folate cycle use to regenerate THF and N⁵,N¹⁰-methylene-THF

DHFR and SHMT

methotrexate

inhibit DHFR causing block in folate recycling causing decrease in TMP and purine synthesis

fluorouracil

converts into FdUMP and inhibit thymidylate synthase thus decreasing TMP synthesis

cancer cells that rapidly use TMP for DNA replication make

thymidylate synthase and folate recycling important chemotherapy targets

UCK

phosphorylates uridine or cytidine to CMP and UMP

salvage of free pryrimidine base is ______ for most animals, thus instead cells recycle pyrimidine nucleosides by _________ them into ________

minor pathway, phosphorylating, nucleotides

TDK

phosphorylates thymidine into TMP, TDK important for DNA synthesis in cells

ADK and IGK also used for

purine nucleoside phosphorylation

TMP degraded into ________ then converted into _________ which can be used in __________ and ___________

β-aminoisobutyrate, methylmalonyl-CoA, fatty acid synthesis, energy metabolism

overall pyrimidines recycle

nucleosides using kinases

overall purines rely on

base salvage pathways more

TDK is important for supplying

TMP for DNA synthesis

why is pyrimidine base salvage less important

since pyrimidine metabolites are soluble they can be recycled by other pathways

why is purine base salvage more important

since purines can make poorly soluble uric acid which has to be excreted, recycling them is more important

folate deficiency inhibits _______ bc it inhibits ______, it also inhibits _________ thereby _________

purine biosynthesis, IMP synthesis, folate recycle, TMP synthesis

folate deficiency does NOT affect

UMP or CMP synthesis

xanthine oxidase (XO) is the key enzyme towards __________ to ________ then _______ and _______ is an inhibitor of XO

metabolize purine, xanthine, uric acid, allopurinol