Chapter 33/34: Purines and Pyrimidines metabolism II/III

Question 1

In the salvage pathway for purine synthesis, _______ + _________ yields IMP and PPi via the enzyme ____________.

Answer 1

1. hypoxanthine
2. PRPP
3. hypoxanthine-guanine phosphorybosyltransferase (HGPRT)

Question 2

In the salvage pathway for purine synthesis, _______ + _________ yields GMP and PPi via the enzyme ____________.

Answer 2

1. Guanine
2. PRPP
3. Hypoxanthine-guanine phosphorybosyltransferase (HGPRT)

Question 3

In the salvage pathway for purine synthesis, _______ + _________ yields AMP and PPi via the enzyme ____________. AMP can also be synthesized indirectly (MORE COMMON) from _____ (hypoxanthine + PRPP).

Answer 3

1. Adenine
2. PRPP
3. Adenine phosphorybosyltransferase (APRT)
4. IMP (instead of free adenine)

Question 4

Hypoxanthine-guanine phosphorybosyltransferase (HGPRT) is inhibited by what?

Answer 4

its products IMP and GMP (feedback inhibition)

Question 5

In purine degradation, ____% is recycled by salvage enzymes and only ____% is converted to _________ and excreted.

Answer 5

1. 90%
2. 10%
3. Uric acid

Question 6

Degradation first converts purines and purine nucleotides to __________. This is then converted via __________ to xanthine and ultimately to _________, which is excreted by the kidneys or GI tract

Answer 6

1. hypoxanthine
2. xanthine oxidase (converts BOTH hypoxanthine –> xanthine –> uric acid)
3. uric acid

Question 7

IMP/AMP degradation:

1st step: The **hydrolytic removal of phosphate (nucleotides –> nucleosides) from AMP or IMP via ______________. AMP forms _______ and IMP forms _______.

2nd step: Adenosine is converted to inosine by ___________, which removes an amino group.

3rd step: __________ is removed from inosine via Purine nucleoside phosphorylase (PNP). This reaction yields _________.

4th: hypoxanthine can then be converted to xanthine and then uric acid via _______.

Answer 7

1. ***nucleotidases, adenosine, inosine
2. adenosine deaminase (ADA)
3. ribose (*as ribose-1-P), hypoxantine
4. xanthine oxidase

Question 8

GMP degradation:

1st step: The **hydrolytic removal of phosphate (nucleotides –> nucleosides) from GMP via ______________ to form _______.

2nd step: __________ is removed from guanosine via Purine nucleoside phosphorylase (PNP). This reaction yields _________.

***3rd step: _________ removes an amino group from guanine to form xanthine. Xanthine can then be converted into uric acid for excretion.

Answer 8

1. nucleotidases, guanosine
2. ribose (as R-1-P), guanine
3. *Guanine deaminase (or guanase)

Question 9

Important enzymes in AMP, GMP and IMP degradation

Answer 9

Nucleotidases: nucleotide (AMP, GMP, IMP) –> nucleoside (adenosine, guanosine, inosine)
Purine nucleoside phosphorylase (PNP): removes ribose as R-1-P to form hypoxanthine (AMP/IMP) or guanine (GMP)
Xanthine oxidase: converts hypoxanthine –> xanthine (AMP/IMP) and xanthine –> uric acid (ALL)

Question 10

The ________ pathway in purine metabolism leads to more ATP and GTP production whereas the _______ pathway generates more purine breakdown products. High de novo activity increases purine turnover, resulting in higher _________ concentrations. Higher salvage pathway activity leads to decreased __________ and reduces plasma uric acid. An increase in the salvage pathway depletes _______ levels.

Answer 10

1. salvage
2. de novo
3. plasma uric acid
4. de novo synthesis
5. PRPP

Question 11

What disease is caused by increased plasma uric acid levels (hyperuricemia)? What forms and what areas are most affected?

Answer 11

dx: GOUT
product: Uric acid crystals can form (can form kidney stones)
affected areas: **joints and kidney tubules (slow blood flow, synovial fluid favors the precipitate), ***big toe (higher gravitational pull)

Question 12

Rapid inflammation of the joints (usually the big toe, can affect other joints too) due to hyperuricemia is called ________? Repeated instances can lead to ________? The inflammation is caused by ____________.

Answer 12

1. A gouty attack, VERY painful (HOT, red, swollen/tender joints)
2. arthritis
3. leukocytes

Question 13

What dx results from any of the following:

1. Increased uric acid due to increased NUCLEIC ACID turnover. Causes?***
2. Increased uric acid due to increased ATP degradation. Causes?
3. Accelerated ATP catabolism and decreased renal excretion of uric acid. Causes?
4. Increased tubular reabsorption of uric acid. Causes?

Answer 13

SECONDARY GOUT

1. causes: leukemias, lymphomas, and after chemotherapy.**
2. causes: glycogen storage diseases and tissue hypoxia.
3. causes: Ethanol intake or primary renal dx
4. causes: dehydration and diuretics

Question 14

Lead exposure can lead to ___________. This causes lead nephropathy.

Answer 14

Saturnine gout

History:
1. prevalent in 18th-century England among the upper classes due to lead plumbing
2. prevalent with the romans because they drank from vessels containing lead

Question 15

Causes of hyperuricemia?

1. Effect of any of the following? Idiopathic, renal failure, Medication, Dehydration, Alcohol consumption, Obesity/diabetes, High fructose corn syrup, Chemotherapy/radiation
2. Effect of any of the following? increased purine degradation, Consumption of purine-rich foods (animal origin)
3. Effect of the following? Von Gierke disease

Answer 15

1. Underexcretion of uric acid by the kidneys
2. Overproduction of purines
3. Glycogen storage dx: can’t break down glycogen = lactic acidosis. Lactic acid competes for excretion with uric acid, causes a build- up of uric acid.

Question 16

When treating gout, your doctor recommends a low-purine diet. Which of the following should you avoid?

A. Sugary foods/beverages with high fructose corn syrup.
B. Fruits and vegetables, especially apples
C. Seafood
D. Meat, especially liver and game meats.
E. Black coffee and tea
F. Alcoholic beverages, especially beer

Answer 16

A, C, D, F

Question 17

What medicine can be used for the treatment of gout? What is its effect?

Answer 17

Allopurinol: it inhibits xanthine oxidase, stopping the rxn of
hypoxanthine to xanthine and ultimately to uric acid
**stops uric acid formation

Higher levels of hypoxanthine does NOT have a negative effect –>
it gets recycled back to the purine salvage pathway by HGPRT!

Question 18

**What X-linked dx is caused by polymorphisms (defects) in HGPRT, resulting in increased levels of PRPP and decreased nucleotide production? What is the result?

Answer 18

Dx: Lesch-Nyhan syndrome
Increased PRPP: Self- mutilation, Mental retardation, Hyperuricemia
Decreased nucleotide production: decreased dopamine production, lesions in striatal dopaminergic pathways

Question 19

**What autosomal recessive metabolic disorder is caused by a deficiency in adenosine deaminase? What is the effect?

Answer 19

disorder: Severe combined immunodeficiency disease (SCID)
effect: Primary immunodeficiency, lack of both B-cell and T-cell function = increased infections (bacterial, viral and fungal)

Question 20

What is required for pyrimidine biosynthesis? Where does it take place?

Answer 20

Required: Glutamine, HCO3- (bicarbonate), H2O, ATP (x2)

Ammonia (NH3) is derived from Gln hydrolysis

Location: Liver (because it can handle the ammonia waste)

Question 21

First step in de novo pyrimidine synthesis requires what enzyme?
2 ATP + HCO3- + NH3 –> carbamoyl phosphate + 2 ADP + Pi

Answer 21

enzyme: carbamoyl phosphate synthetase II (CPS II)

Question 22

activators and inhibitors of carbamoyl phosphate synthetase (CPS II)?

Answer 22

activated by: PRPP, ATP, glutamine
inhibited by: UMP, UDP, UTP (end products)

Question 23

Once carbamoyl phosphate is formed, _____ is added via _____________ to make carbamoyl aspartate. After a couple of reactions, the enzyme dihydroorotase makes ________, which has the basic pyrimidine ring shape.

Answer 23

1. aspartate
2. aspartate transcarbamoylase (ATCase)
3. Orotate

Question 24

Activators (turns ON pyrimidine production) and inhibitors (turns OFF pyrimidine production) of aspartate transcarbamoylase (ATCase)?

Answer 24

activated by: Aspartate (substrate) = ON
inhibited by: Cytidine triphosphate (CTP) = OFF

Question 25

Once orotate is formed, it combines with PRPP to form ______. What enzyme is used and what molecule is released?

Answer 25

Forms: orotidylate (OMP) Enzyme: orotate phosphoribosyl transferase Released: pyrophosphate

Question 26

Orotic aciduria is an autosomal recessive condition characterized by excessive excretion of what in the urine? It is caused by a deficiency of what enzyme in the presence of NORMAL levels of ammonia?

Answer 26

excessive excretion: orotic acid ***enzyme deficiency: UMP synthase

Question 27

Once orotidylate (OMP) is formed, it is decarboxylated via orotidylate decarboxylase to form _______. What inhibits orotidylate decarboxylase?

Answer 27

product: UMP (the uracil base) orotidylate decarboxylase (aka UMP synthase) is inhibited by: UMP, CMP

Question 28

What is the main product of pyrimidine synthesis?

Answer 28

UMP!!!

Question 29

UMP is converted to UTP by adding 2 ATPS, what 2 enzymes are used?

Answer 29

1st P: UMP kinase (UMP --> UDP) 2nd P: Nucleoside Diphosphate Kinase (NDPK) **the enzyme that adds –P to all NDPs (UDP --> UTP)

Question 30

***Once formed, UTP then gets converted to ______ by cytidylate synthetase, which requires what 2 things?

Answer 30

1. CTP (only nucleotide synthesized directly as a triphosphate, does NOT require NDPK) 2. *** cytidylate synthetase requires glutamine and ATP.

Question 31

CTP (RNA) --> dCTP (DNA) via what enzyme?

Answer 31

Apyrase (dephosphorylates CTP)

Question 32

Order of synthesis for pyrididines?

Answer 32

U --> C --> T "U CuTie"

Question 33

Conversion of UMP to dUMP step 1: UMP --> UDP via ___________ step 2: UDP --> dUDP via __________ step 3: dUDP --> dUTP via ___________ *****step 4: dUTP --> dUMP via ____________

Answer 33

1. UMP kinase: mono --> di 2. ***Ribonucleotide Reductase (RNR): catalyzes all conversions of NDP (ribose) --> dNDP (deoxyribose) 3. Nucleoside Diphosphate Kinase (NDPK)**: di --> tri ******IMPT 4. dUTPase: dUTP is recognized by DNA polymerase, converted to dUMP QUICKLY to avoid it being put into DNA

Question 34

dUMP --> dTTP steps: 1. dUMP --> dTMP via ________ 2. dTMP --> dTDP via ________ 3. dTDP --> dTTP via _________

Answer 34

1. Thymidylate synthase 2. dTMP kinase 3. NDPK **NDPK is not regulated. If it finds a NDP or a dNDP, it WILL turn it into NTP or dNTP.

Question 35

dUTP can also be directly made from deamination of _________ -

Answer 35

dCTP

Question 36

****IMPT: The KEY enzyme for DNA replication that converts dUMP --> dTMP is _________. What does it require? What inhibits it?

Answer 36

thymidylate synthase: If it's not functional, you can’t replicate DNA! requires: N5-N10-methylene-THF which is oxidized to FH2 (DHF) inhibited by 5-FU: gets converted to FdUMP which covalently binds/PERMANENTLY inhibits thymidylate synthase. It blocks dTMP synthesis and indirectly inhibits **DNA synthesis

Question 37

**Foalte recycling: FH2 (DHF) is reduced to FH4 (THF) by what enzyme? What inhibits this enzyme?

Answer 37

enzyme: dihydrofolate reductase inhibited by methotrexate: competitively inhibits DHF by binding with DHF reductase --> stops dTMP synthesis. Indirectly blocks DNA synthesis

Question 38

Pyrimidine rings (cytosine, uracil, thymine) can be degraded to _____ and ________, which can then be excreted via the lungs and kidneys.

Answer 38

1. CO2 and ammonia

Question 39

Pyrimidine degradation MAIN steps: 1. UTP and CTP --> UMP and CMP via ______ 2. UMP and CMP --> uridine and cytidine via _______ 3. Uridine and Cytidine --> sugar-P + pyrimidine (cytosine, uracil) via ______. This rxn requires _____. 4. Cytosine --> uracil via _______ ***5. Uracil --> 5,6-dihydrouracil via ___________. This rxn uses _______. ****6. In the last few steps 5,6-dihydrouracil --> _________, releasing ____ and _____. These are the 3 final products of cytosine and uracil degradation.

Answer 39

1. apyrase (hydrolase) 2. pyrimidine-specific 5’-nucleotidases (NMP --> nucleoside + P) 3. Pyrimidine phosphorylase, requires Pi (phosphate) 4. cytosine deaminase ***5. dihydropyridine dehydrogenase, uses NADPH. ***6. acetyl CoA, NH4+ and CO2

Question 40

Common enzymes between Cytosine/Uracil vs Thymine degradation 1. Uracil --> 5,6-dihydrouracil AND Thymine --> dihydrothymine (RLS) both use what enzyme? What does this rxn also require? 2. This enzyme yields beta-alanine (cytosine/uracil) and beta-aminoisobutyrate (thymidine)

Answer 40

1. dihydropyridine dehydrogenase, also requires NADPH 2. beta-ureidopropionase

Question 41

**What does the breakdown of Thymine yield?

Answer 41

**Succinyl CoA (can go into TCA cycle), NH4+, CO2

Question 42

Nucleoside monophosphate kinases (NMPK) vs Nucleoside Diphosphokinase (NDPK)

Answer 42

Question 43

ATCase enzyme regulation is important for purine/pyrimidine balance. A high concentration of purine nucleotides causes ______ to activate ATCase and synthesize pyrimidines. A high concentration of pyrimidines causes ______ to inhibit ATCase, decreasing pyrimidine synthesis.

Answer 43

1. ATP/Aspartate 2. CTP

Question 44

What is the anti-HIV drug that lacks 3'-OH (mechanism of chain termination*****) and blocks DNA polymerization? How does it target the viral RNA?

Answer 44

1. Azido-deoxythymidine (AZT): The azido-group blocks DNA polymerization and there is no ***3’ –OH (mechanism of chain termination) 2. The viral RNA-dependent DNA polymerase (reverse transcriptase) is 100X more sensitive to AZT than the host cell DNA polymerase is. Therefore, it does not inhibit DNA synthesis in healthy cells.

Question 45

What is the active/catalytic subunit of Ribonucleotide reductase (RNR)? What 2 allosteric sites does it have and what type of molecule binds to each?

Answer 45

1. R1 2. Specificity site, Activity site 3. Specificity site BINDS TRIPHOSPHATES (ATP, dATP). The active site BINDS DIPHOSPHATES

Question 46

Ribonucleotide reductase (RNR) has 2 subunits. The R2 subunit has a ____ radical which is a key for enzymatic activity

Answer 46

Tyr (tyrosine) radical

Question 47

In the R1 subunit, the ________ site tells the enzyme which NDPs can bind and transform into dNDPs. On the other hand, the _______ site turns the enzyme on and off.

Answer 47

1. Specificity 2. Activity

Question 48

What is the effect of ATP on the R1 subunit vs the effect of dATP on the subunit and HIGH and MODERATELY HIGH concentrations?

Answer 48

ATP: binds to the specificity site and ACTIVATES RNR (NDP --> dNDP) dATP (HIGH):