경희대학교 의과대학·의학전문대학원

2019 년 1 학기

Text : Harper’s Biochemistry Refs: Devlin’s, Lippincott’s

Ch. 32. Nucleotides

2019 년 1 학기

Text : Harper’s Biochemistry Refs: Devlin’s, Lippincott’s

Figure 32-1 Purine and pyrimidine.

The atoms are numbered according to the international system.

Biomedical importance:

 Monomeric unit of nucleic acids (precursors of nucleic acids)  Metabolic functions

(energy metabolism, protein synthesis, regulation of enzyme activity, signal transduction)  Coenzymes (vitamins and vitamin derivatives)

 Donors & Acceptors of phosphate

 Key biosynthetic intermediates of sugars & lipids  Second messengers (cAMP & cGMP)

 Chemotherapy of cancer, AIDS, & immune suppressors

•Nucleic acids:

P

Figure 32-3 Ribonucleosides

Chemistry of purines, pyrimidines, nucleosides, & nucleotides

1) Base(s); ~ carbon/ nitrogen-containing heterocylic compounds

~ opposite directions of numbering six-atom rings (Fig. 32-1) 2) Nucleosides are N-Glycosides; (Fig. 32-3)

~ (d)D-Ribose (sugar) – Base(s)(Purine/Pyrimidine) ~ Sugar:

Ribonucleosides (D-ribose)

deoxyribonucleosides (2-deoxy-D-ribose)

~ Ribose(sugar) – Base(s)(Purine/Pyrimidine): -N-glycosidic bond,

Sugar - (N-1)pyrimidine, Sugar – (N-9)purine

•Nucleic acids: Phosphate(s) –

(d)D-Ribose (sugar) - Base(s)(Purine/Pyrimidine)

Deoxyadeno-sine

Figure 32-2 Tautomerism of the oxo and amino

functional groups of purines and pyrimidines

Physiological conditions Favor amino & oxo

Chemistry of purines, pyrimidines, nucleosides, & nucleotides

1) Base(s);

2) Nucleosides are N-Glycosides;

~ (d)D-Ribose (sugar) – Base(s)(Purine/Pyrimidine) 3) Nucleotides are phosphorylated nucleosides (Fig. 32-4)

~ In general, nucleosides with a phosphoryl group on the 5’-hydroxyl group of sugar

~ Additional phosphoryl group is ligated by acid anhydride bonds (high energy group transfer potential)

to the phosphoryl group of a mononucleotide, forming nucleoside diphosphate and triphos-phate (Fig. 32-4). --- AMP,, ADP,, ATP

•Nucleic acids:

Phosphate(s) – (d)D-Ribose (sugar) - Base(s)(Purine/Pyrimidine)

Chemistry of purines, pyrimidines, nucleosides, & nucleotides

1) Base(s);

2) Nucleosides are N-Glycosides;

~ (d)D-Ribose (sugar) – Base(s)(Purine/Pyrimidine) 3) Nucleotides are phosphorylated nucleosides (Fig. 32-4)

4) Heterocyclic N-Glycosides exist as Syn and Anti conformers (Fig. 32-5) * Anti conformers predominate.

Figure 32-5 The syn and

anti

conformers of adenosine dif -fer

with respect to orientation about the N-glycosidic bond

Steric hindrance by the heterocycle:

No freedom of rotation about the -N-glycosidic bond of nucleosides or nu-cleotides

Chemistry of purines, pyrimidines, nucleosides, & nucleotides

•Nucleic acids:

Phosphate(s) – (d)D-Ribose (sugar) - Base(s)(Purine/Pyrimidine)

Table 32-1 Purine/Pyrimidine Bases, (d)Ribonucleosides, and

(d)Ribonucleotides Ade-nine(A) Gua-nine(G) Cyto-sine(C) X = H X(9 ) X(9 ) X(1 ) X(1 ) dX

X = (d)Ribose X = (d)Ribose Phosphate

Uracil (U) Thymine (T) Adenosine Cytidine Uridine Thymidine Guanosine

Figure 32-1 Purine and

pyrim-idine Adenosine monophosphate (AMP) Thymidine monophosphate (TMP) Guanosine monophosphate (GMP) Cytidine monophosphate (CMP) Uridine monophosphate (UMP) UMP TMP AMP dAMP

Figure 32-6 Structure of (d)AMP, UMP,

Figure 32-7 Four uncommon

but naturally occurring pyrimidines and purines Modification of polynucleotides can generate additional structures

 Four uncommon (additional) bases in DNA and RNA (Fig. 32-7)

Figure 32-1 Purine and

pyrimi-dine DNA

Figure 32-8 Structures of hypoxanthine, xanthine, and

uric acid

Modification of polynucleotides can generate additional structures  Intermediates in the catabolism of adenine and guanine;

~ Hypoxanthine, Xanthine, Uric acid (Fig. 32-8) (Chapter 33)

Figure 32-1 Purine and

Fig. 32-9 Caffeine, a trimethylxanthine (coffee).

The dimethylxanthines theobromine (cocoa) and theo-phylline (tea) are similar but lack the methyl group at N-1 and at N-7, respectively.

Modification of polynucleotides can generate additional structures  Methylated heterocycles (bases) of plants;

~ Caffeine (Xanthine derivatives; Fig. 32-8) (Fig. 32-9) _{Figure 32-1 Purine and} pyrimi-dine

Caffeine

 Nucleotides absorb ultraviolet light

~ Nucleotides absorb light at a wavelength at 260 nm

Fig. 32-10 cAMP, 3’,5’-cyclic AMP,

and cGMP, 3’,5’-cyclic GMP

Nucleotides serve diverse physiological functions

 ATP (1 mmol/L): principal biological transducer of free energy  Second messenger: cAMP (1 nmol/L) & (GTP) cGMP (Fig. 32-10)

 Sulfate donor:

Adenosine-3'-phosphate-5-phosphosulfate (Fig. 32-11),

Figure 32-1 Purine and

pyrim-idine

Figure 32-11 Adenosine

3’-phos-phate-5’-phosphosulfate.

 Methyl group donor:

S-Adenosylmethionine (Fig. 32-12)

Table. 32-2 Many coenzymes and related

com-pounds are

Derivatives of adenosine monophosphate

Nucleotides serve diverse physiological functions  UDP-sugar derivatives:

~ biosynthesis of glycogen, glucosyl disaccharides etc…(Chap 47&48)  CTP:

~ lipid synthesis (Chap 24)  Coenzymes:

~ NAD, FAD, CoASH (Table 32-2)

Figure 32-1 Purine and

Figure 32-13 Selected synthetic pyrimidine and purine

analogs

Synthetic nucleotide analogs are used in chemotherapy

 Synthetic purine & pyrimidine analogs ~ Anti-cancer agents: 5-fluorouracil, 6-thiogua-nine and 6-mercaptopurine, 6-azauridine, 8-azagua6-thiogua-nine  incorporated into DNA prior to cell division (Fig. 32-13)

 Allopurinol for hyperuricemia and gout

 Acylovir (Acycloguanosine) for Herpes virus infection

Allopuri-nol

Figure 32-13 Selected synthetic pyrimidine and purine

analogs 5-Fluo-rouracil Acyclovir 6-Azauri-dine 6-mercaptop-urine 6-Thiogua-nine 8-Azagua-nine 6 6 8 6 5

Figure 32-14 Arabinosylcytosine (cytarabine) and

aza-thioprine

Synthetic nucleotide analogs are used in chemotherapy  Synthetic purine & pyrimidine analogs;

~ Cytarabine: chemotherapy of cancer (Fig. 32-14)

Clinical use of drugs

developed by Elion and Hitch-ings

"for their discoveries of important principles for drug treatment"

1988 Nobel Prize in Physiology or Medicine

Purine, Pyrimi-dine base,

nucle-oside Analogs Indications 6-Mercaptopurine, thioguanine Leukemia Azathioprine Organ transplanta-tion, Autoimmune disease Allopurinol Gout

Acyclovir Herpes virus infec-_tion

14 Simple precur-sors Base s Nucleo-sides Nucleo-tides DNA, RNA

(Synthetic) Nonhydrolyzable nucleotide triphosphate analogs serve as research tools  (Figure 32-15) Synthetic derivatives of nucleoside triphosphates incapable of

undergo-ing hydrolytic release of the terminal phosphoryl group. (Pu/Py, a purine or pyrimidine base; R, ribose or deoxyribose.) Shown are the parent (hydrolysable) nucleoside

triphosphate (top) and the unhydrolyzable β–methylene (center) and γ–imino derivates (bottom).

Synthesis of polynucleotides

 A nucleoside 5’-triphosphate is added to the 3’-hydroxyl group of a growing peptide chain ~ 5’-phosphoryl group of a mononucleotide (1st_{) can esterify with 2}nd _{hydroxyl group,}

forming a phosphodiester.

~ Phosphodiester bonds link the 3’- and 5’-carbons of adjacent monomers.

~ 2nd _{hydroxyl group is the 3’-OH of the pentose of the 2}nd _nucleotide.

 forming a dinucleotide (pentose moieties are linked by a 3’,5’-phosphodiester bond to form the “backbone” of RNA and DNA.

 The formation of a dinucleotide removes water(H₂O) between two mononucleotides

Polynucleotides are directional macromolecules

 The base sequence (or primary structure) of a polynucleotide  P: phosphodiester bond, a single letter (Bases)

a vertical line; Pentose

 5’-end being the one with a free or phosphorylated 5’-hydroxyl.

P A OH P P P T C A pApTpCpA 3’  ATCA 5’ 5’ 3’ 5’ 3’ 5’ 3’ 5’ 3’ 5’ 3’

DNA & RNA are polynucleotides  RNAs are far less stable than DNA

 2’-hydroxyl group of RNA (absent from DNA: 그림 )

functions as a nucleophile during hydrolysis of the 3’,5’-phosphodiester bond AMP dAMP

그림

• Under physiologic conditions, the amino and oxo tautomers of purines,

pyrim-idines, and their derivatives predominate.

• Nucleic acids contain, in addition to A, G, C, T, and U, traces of 5-methylcytosine,

5-hydroxymethylcytosine, pseudouridine (tRNA), or N-methylated heterocycles.

• Most nucleosides contain D-ribose or 2-deoxy-D-ribose linked to

N-1

of a

pyrimi-dine or to

N-9

of a purine by a -

glycosidic bond

whose anti- conformers

predomi-nate.

• A primed numeral locates the position of the phosphate on the sugars of

mononu-cleotides (eg, 3'-GMP, 5'-dCMP). Additional phosphoryl groups linked to the first by

acid anhydride bonds

form nucleoside diphosphates and triphosphates.

SUMMARY I

• Nucleoside triphosphates have high group transfer potential and participate in

covalent bond syntheses. The cyclic phosphodiesters cAMP and cGMP function as

intracellular second messengers.

• Mononucleotides linked by 3',5'-phosphodiester bonds form polynucleotides,

direc-tional macromolecules with distinct 3'- and 5'-ends. For pTpGpTp or TGCATCA,

the 5-end is at the left, and all phosphodiester bonds are 3

'-> 5'.

• Synthetic analogs of purine and pyrimidine bases and their derivatives serve as

anticancer drugs

either by inhibiting an enzyme of nucleotide biosynthesis or by

being incorporated into DNA or RNA

SUMMARY II

Nucleotides

Text : Harper’s Biochemistry Refs: Devlin’s, Lippincott’s

Ch. 32. Nucleotides

Biomedical importance:

 Biosynthesis is stringently regulated and coordinated by feedback mechanisms

 Genetic diseases of purine metabolism: Gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency

•Nucleic acids:

P

Purines & pyrimidines are dietarily nonessential:  Synthesis from intermediates

 Ingested nucleic acids  digested to (mononucleotides)bases and Oxidized & excreted in the urine.  Injected compounds are incorporated: Injected [3H] thymidine is incorporated into

newly synthesized DNA, thus used to measure the rate of DNA synthesis.

Biosynthesis of purine nucleotides:  Growth and Tissue regeneration.

 Sources of each atom (Figure 33-1)  Isotopic precursors to pigeons.

Three processes for purine nucleotides biosynthesis:  Synthesis from amphibolic intermediates; Fig. 33-2, 33-3

 Phosphoribosylation; Fig. 33-4

 Phosphorylation of purine nucleotides N10

-Formyl-tetrahydrofolate

Figure 33-1 Sources of the nitrogen and carbon

atoms

of the purine ring

N5_{, N}10 -Formyl-tetrahydrofolate N10_{-Formyl-tetrahydro-folate} 10 5 Figure 44-15

Amide nitrogen of glutamine Glycine Aspartate

Inosine MonoPhosphate (IMP) is synthesized from amphibolic intermediates

 α-D-ribose 5-phosphate (PRPP; PhosphoRibosylPyroPhosphate) IMP (Figure 33-2)  PRPP; an intermediate in the purine salvage pathway and in the biosynthesis of pyrimidine

nucleotides, NAD+_{, and NADP}+_.

 Glutamine, Glycine, N5_,N10_{-Formyl tetrahydrofolate}

PRPP

Figure 33-2 Purine biosynthesis from ribose-5-phosphate and

ATP Glutamine Glutamine Glycine N5_{, N}10 -Formyl-tetrahydrofolate

Inosine MonoPhosphate (IMP) is synthesized from amphibolic intermediates  α-D-ribose 5-phosphate  (PRPP)

 Glutamine, glycine, N5_,N10_{-Formyl tetrahydrofolate}

 Aspartate, N10_{-Formyl tetrahydrofolate  IMP (Figure 33-2)}

Aspartate

N10 -Formyl-tetrahydrofolate

IMP

Figure 33-2 Purine biosynthesis from ribose-5-phosphate and

Inosine MonoPhosphate (IMP)  AMP & GMP (Figure 33-3)  IMP to AMP (ADP  ATP)

 IMP to XMP  GMP (GDP  GTP)

Figure 33-3 Conversion of IMP to AMP and GMP

IMP

AMP

GMP

XMP

Multifunctional catalysts participate in purine nucleotide biosynthesis (Figure 33-2)  In eukaryotes, multifunctional enzymes

 reactions (3,4&6),,,,, (7&8),,,, (10&11)

Antifolate drugs block purine nucleotide biosynthesis (Figure 33-2)  Rnxs 4, 10 can be blocked by antifolate drugs

~ Compounds that block purine synthesis by inhibiting formation of folic acid  cancer chemotherapy

Figure 33-2 Purine biosynthesis from ribose-5-phosphate and

ATP N5_{, N}10 -Formyl-tetrahydrofolate N10 -Formyl-tetrahydrofolate

IMP

1

0

4

Reaction 2 Reaction 5 Glutamine analogs block purine nucleotide biosynthesis (Figure 33-2)

 diazanorleucine (reaction 2) (reaction 13 &14)  Azaserine (reaction 5)

Synthetic Purine analogs block purine nucleotide biosynthesis (Figure 33-2/ Fig. 32-13)  6-mercaptopurine (reaction 13 &14)

 mycophenolic acid (reaction 14)

IMP

GMP

XMP

AMP

“Salvage reactions” convert purines & their nucleotides to mononucleotides (Figure 33-2/33-3/ 33-4)  “Salvage reaction”:

 far less energy than de novo synthesis. 1) Free purines to purine 5’-mononucleotide Pu + PR-PP  Pu-RP + Ppi

** Two phosphoribosyl transferases (Figure 33-4)  Adenine + PRPP ; AMP (APRTase)

 hypoxanthine - guanine + PRPP ; IMP or GMP (HPRTase)

2) Phosphoryl transfer from ATP to nucleoside Pu-R + ATP -> PuR-P + ADP

 Liver mainly provides purines & purine nucleosides for salvage and for utilization

 A low level of PRPP glutamyl amidotransferase (Fig. 33-2, reaction 2) in human requires exogenous

purines

 Brain, erythrocytes, and PMN leukocytes :  unable to synthesize 5’-phosphoribosylamine (Fig. 33-2, structure III) and thus, utilize

exogenous purines to form nucleotides.

_{Figure 33-4 Phosphoribosylation of adenine,}

hypoxanthine, and guanine to form AMP, IMP, GMP

PRPP

PRPP

PRPP

APRTase

H(G)PRTase

“Salvage reactions” convert purines & their nucleotides to mononucleotides (Figure 33-2/33-3/ 33-4)  Lesch-Nyhan Syndrome; an overproduction of hyperuricemia

 a defect in hypoxanthine-guanine phosphoryribosyl transferase, an enzyme of a purine salvage (Figure 33-4)

 Rise in PRPP results in purine overproduction (Figure 33-2/33-3 )

Figure 33-4 Phosphoribosylation of adenine,

hypoxanthine, and guanine to form AMP, IMP, GMP

PRPP

Figure 33-2 Purine biosynthesis

Hepatic Purine Biosynthesis is stringently regulated (Figure 33-2/33-3/ 33-4)  AMP & GMP feedback regulate PRPP glutamyl aminotransferase

 Major determinant of purine synthesis: PRPP concentration

 Feedback inhibition of rxn 1,2 by AMP, GMP, ADP, GDP (Figure 33-5)

Reactions ① and ② are catalyzed by PRPP synthase and by PRPP glutamyl

amido-transferase, respectively. : Feedback inhibition by intermediates of the pathway

Figure 33-5 Control of the rate of de novo purine nucleotide

biosynthe-sis.

PRPP

Hepatic Purine Biosynthesis is stringently regulated (Figure 33-2/33-3/ 33-4)  AMP & GMP feedback regulate PRPP glutamyl aminotransferase

 AMP & GMP feedback regulate their formation from IMP (Figure 33-6)  AMP & GMP feedback inhibition (Rxn 12, 14) (Figure 33-3)

 Cross requirement of GTP (Rnx 12) or ATP (Rnx 15) (Figure 33-3) decreases synthesis of one purine nucleotide

when there is a deficiency of the other nucleotide.

Figure 33-6 Regulation of the interconversion of IMP to adenosine

nu-cleotides

and guanosine nucleotides 12 14 Adenylsuccinate synthase

1

2

1

4

GTP

ATP

Figure 33-3

Hepatic Purine Biosynthesis is stringently regulated (Figure 33-2/33-3/ 33-4)  AMP & GMP feedback regulate PRPP glutamyl aminotransferase

 AMP & GMP feedback regulate their formation from IMP (Figure 33-6)  AMP & GMP feedback inhibition (Rxn 12, 14) (Figure 33-3)

 AMP & GMP inhibit salvage reaction (Figure 33-4)  AMP(GMP) inhibit

Hypoxanthine-guanine phosphoribosyltransferase (H(G)PRTase)

Figure 33-4 Phosphoribosylation of adenine,

hypoxanthine, and guanine to form AMP, IMP, GMP

PRPP

PRPP

PRPP

Reduction of ribonucleotide diphosphates forms deoxyribonucleoside diphosphates (dNDPs )  Reduction of 2'-hydroxyl by ribonucleotide reductase complexes (Fig. 33-7): NDPs → dNDPs  Enzyme complex is functional only when cells are actively synthesizing DNA

 Reduction requires thioredoxin, thioredoxin reductase, NADPH+H+

Figure 33-7 Reduction of ribonucleoside

Figure 33-8 Regulation of the purine and pyrimidine

ri-bonucleotides to their respective 2’-deoxyriri-bonucleotides

Reduction of ribonucleotide diphosphates forms deoxyri-bonucleoside diphosphates (d-NDPs )  (Figure 33-8)  Reduction of ribonucleoside diphosphates (NDPs) to deoxyri-bonucleoside diphosphates (d-NDPs):::

~ Complex regulatory controls based on balanced production of deoxyribonucleotides for DNA synthesis (Figure 33-9 연결 - 참조 ) Purine nucleotides Biosynthesis (Fig. 33-2) Pyrimidine nucleotides Biosynthesis (Fig. 33-9)

Biosynthesis of Pyrimidine Nucleotides

 Biosynthetic pathway for pyrimidine nucleotides (Figure 33-9)

 1st reaction: cytosolic carbamoyl phosphate synthase II (CPS II) (mitochondrial CPS I of urea cycle;; Figure 28-12)

CO₂ + glutamine +ATP -> ->-> UMP -> UDP

CO

+ glutamine +ATP

1

Carbamoylphosphate

Synthase II(

CPSII

)

Biosynthesis of Pyrimidine Nucleotides

 Biosynthetic pathway for pyrimidine nucleotides (Figure 33-9)  1st reaction: cytosolic carbamoyl phosphate synthase II (mitochondrial CPS I of urea cycle;; Figure 28-12)

CO₂ + glutamine +ATP -> ->-> UMP -> UDP

CO

+ glutamine +ATP

1

Figure 33-9 The biosynthetic pathway for pyrimidine

nu-cleotides

Carbamoylphosphate

Synthase II

(CPS II)

 Biosynthetic pathway for pyrimidine nucleotides (Figure 33-9)

 1st reaction: cytosolic carbamoyl phosphate synthase II (cf, mitochondria CPS I of urea cycle)

CO₂ + glutamine +ATP -> ->-> UMP -> UDP

 UDP-> UTP-> CTP, UDP-> dUDP-> dUMP-> TMP

Figure 33-9 The biosynthetic pathway for pyrimidine

nu-cleotides

dTMP

UDP

UMP

dUMP

CTP

UTP

(

Fig. 33-7)

d

UDP

 Conversion of dUMP to dTMP by thymidylate synthase and

dihydrofolate reductase (Rnx 12)  FdUMP

::: inhibition of thymidylate synthase

Figure 32-13

1

2

dUMP

dTMP

Folic acid

Methotrexate

 Conversion of dUMP to dTMP by thymidylate synthase and dihydrofo-late reductase (Rnx 12)

 Methotrexate (anticancer drug;) ::: inhibitor of dihydrofolate reductase

1

2

dUMP

dTMP

- Allopuri-nol Certain pyrimidine analogs are substrates for enzymes of pyrimidine nucleotide biosynthesis  Orotate phosphoribosyltransferase (rxn 5, Fig. 33-9) converts the drug allopurinol (Fig, 32-13) to nucleotide in which ribosyl phosphate is attached to N-1 of the pyrimidine ring.

 Orotate phosphoribosyltransferase (rxn 5, Fig. 33-9) phosphoribosylates the anticancer drug

5-fluorouracil  nucleotide

5

Figure 33-9 The biosynthetic pathway for pyrimidine

nu-cleotides

Orotate Phosphoribosyl

transferase _rouracil

5-fluo-5 Carbamoylphosphate Synthase II (CPS II) Fig, 32-13 Fig, 32-13

The deoxyribonucleosides of Uracil & Cytosine are salvaged

 Salvage reactions converts

~ pyrimidine ribonucleosides uridine & cytidine  nucleotides (UMP, UDP, UTP….CMP, CDP, CTP) ~ pyrimidine deoxyribonucleosides thymidine & deoxycytidine  nucleotides (dTMP….dCMP…)  ATP-dependent phosphoribosyltransferases (kinases) catalyze

~ 2’-deoxy-(cytidine/ guanosine/ adenosine)  their corresponding nucleoside triphosphates (dCTP,, dGTP,, dATP)

 Orotate phosphoribosyltransferase (rxn 5, Fig. 33-9), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP)

5

Figure 33-9 The biosynthetic pathway for pyrimidine

nu-cleotides

Orotate Phosphoribosyl

transferase _{Orotic acid}

(OA) OMP

Regulation of Pyrimidine Nucleotide Biosynthesis

 Gene expression & enzyme activity both are regulated

(

 Rxn 1 (CPSII) is inhibited by UTP but activated by PRPP

 Rxn 2 (ATCase) is inhibited by CTP but activated by ATP (Fig. 33-10)

CO₂ + glutamine +ATP

Figure 33-9 The biosynthetic pathway for pyrimidine

nu-cleotides Carbamoylphosphate Synthase II 1 2 (-)UTP (+)PRPP Aspartate transcarbamoylase (-)CTP (+)ATP

 Purine & pyrimidine nucleotides biosynthesis are coordinately regulated

 Cross regulation: PRPP synthase (rxn , Fig. 33-2) is inhibited by both pu and py nucleotides

Figure 33-8 Regulation of the purine and pyrimidine

ri-bonucleotides to their respective 2’-deoxyriri-bonucleotides

P ur in e nu cl eo ti de s B io sy nt he si s ( F ig . 3 3-2 ) P yr im id in e nu cl eo ti de s B io sy nt he si s ( F ig . 3 3-9 ) 1 1

Regulatory aspects of purine & pyrimidine nu-cleotides reduction to their respective 2’-de-oxyribonucleotide (Fig. 33-10)

 Humans Catabolize Purines to Uric acid ~ Adenosine and Guanosine

 uric acid in human (Fig. 33-11)

 Uricase converts uric acid to allantoin (wa-ter-soluble). ~~ Since humans lack uricase, end product of purine catabolism is uric acid.

Uric acid Guanosine Adenosine deaminase Inosine Adenosine Guanine Hypoxanthine Xanthine

Figure 33-11 Formation of uric acid from purine nucleo -sides

 Gout is a Metabolic Disorder of Purine Catabolism  Defect in PRPP synthase : V_max (rxn 1, Fig 33-2) ~ Overproduction and overexcretion of purine catabolites  When serum urate levels exceed the solubility limit, ~ Sodium urate crystallizes,

~ causing inflammatory reaction, gouty arthritis (통풍관절 염 )

 Most cases of gout: abnomalities in renal handling of uric acid. ③ Treatment: Allopurinol (Fig. 33-11 참조 )

PRPP

Uric acid

Other Disorders of Purine Catabolism

: Genetic disorders of purine catabolisms

 Lesch-Nyhan syndrome

~ Hyperuricemia

~ Defect in HGPRTase (Fig. 33-4)

 rise in PRPP  purine overproduction (Fig. 33-2)

 Von Gierke's disease

~ Glucose-6-phosphatase deficiency

 increase in ribose 5-phosphate (Fig. 33-2)  purine overproduction & hyperuricemia

 Hypouricemia

(increased excretion of hypoxanthine and xanthine; Fig. 33-11) ~ Xanthine oxidase deficiency

due to a genetic defect or to severe liver damages  Xanthinuria

PRPP

PRPP

Fig. 33-2

Adenosine deaminase Inosine Adenosine Guanosine Uric acid Guanine Hypoxanthine Xanthine

Other Disorders of Purine Catabolism

 Adenosine deaminase deficiency (Fig. 33-11) ~ an immunodeficiency disease

 T&B cells are sparse and dysfunctional

 Purine nucleoside phosphorylase deficiency(Fig. 33-11) (InosineHypoxnathine; GuanosineGuanine)

~ severe deficiency of T cells with normal B cell function Immune dysfucntions:

 result from accumulation of dATP & dGTP,  Inhibit ribonucleotide reductase, (Fig. 33-9)  Thereby deplete cells of DNA precursors

Purine nucleoside phosphorylase Ribonucleotide reductase UDP dUDP dUMP dTMP

Catabolism of Pyrimidines Produces

Water-soluble Metabolites

 β-ureidopropionase (from pyrimidine precursors)

(Fig. 33-12)

~ water-soluble end product of pyrimidine catabolism

CO

~ β-aminoisobutyrate  forms succinyl-CoA

(Fig. 20-2)

~ Excretion of β-aminoisobutyrate increases

in leukemia & severe x-ray radiation exposure due to increased destruction of DNA

~ β-hydroxybutyric aciduria;

 deficiency in dihydropyrimidine dehydrogenase ~ uraciluria-thyminuria

 pyrimidine catabolism disorder

Cytosine Dihydropyrimidine dehydrogenase -Ureidopropionase -Alanine _{-Amidoisobutyrate} CO₂ + NH₃

Fig. 33-12

Overproduction of pyrimidine catabolites is only

rarely associated with clinically significant

ab-normalities

( vs abnormal purine catabolisms)

 Since end-products of pyrimidine catabolism are highly water-soluble, pyrimidine overproduction results in few clinical signs or symptoms

 Orotic acidurias ~ Reye syndrom;

 severely damaged mitochondria to utilize carbamoyl phosphate

(Fig. 28-12; def.mt CPS1; urea cycle),

 cytosolic overproduction of orotic acid (Fig. 33-9).

*** Type I (Rnx 5/6), II (Rnx 6)-orotic aciduria ~ Deficiency of a urea cycle enzyme:

 mt ornithine transcarbamoylase deficiency (Fig. 28-12)  CAP(cytosol) increase

 results in excretion of pyrimidine precursors  orotic aciduria

~ Drugs may precipitate in orotic aciduria:

: allopurinol (rxn 5) and 6-azauridine (rnx 6)

5

₁

6

Fig. 33-9

• Ingested nucleic acids are degraded to purines and pyrimidines. New

purines and pyrimidines are formed from amphibolic intermediates

and thus are dietarily nonessential.

• Several reactions of IMP biosynthesis require folate derivatives and

glutamine. Consequently, antifolate drugs and glutamine analogs

in-hibit purine biosynthesis.

• Oxidation and amination of IMP forms AMP and GMP, and

subse-quent phosphoryl transfer from ATP forms ADP and GDP. Further

phosphoryl transfer from ATP to GDP forms GTP. ADP is converted to

ATP by oxidative phosphorylation. Reduction of NDPs forms dNDPs.

• Hepatic purine nucleotide biosynthesis is stringently regulated by the

pool size of PRPP and by feedback inhibition of PRPP-glutamyl

amidotransferase by AMP and GMP.

SUMMARY I

• Coordinated regulation of purine and pyrimidine nucleotide

biosyn-thesis ensures their presence in proportions appropriate for nucleic

acid biosynthesis and other metabolic needs.

• Humans catabolize purines to uric acid (pKa 5.8), present as the

rela-tively insoluble acid at acidic pH or as its more soluble sodium urate

salt at a pH near neutrality. Urate crystals are diagnostic of gout.

Other disorders of purine catabolism include Lesch–Nyhan

syn-drome, von Gierke disease, and hypouricemias, etc…..

• Since pyrimidine catabolites are water-soluble, their overproduction

does not result in clinical abnormalities. Excretion of pyrimidine

pre-cursors can, however, result from a deficiency of ornithine

transcar-bamoylase because excess carbamoyl phosphate is available for

pyrimidine biosynthesis.

SUMMARY II