Lecture 50

Question 1

What are 6 key roles of nucleotides in cellular processes?

Answer 1

1) Activated precursors of RNA & DNA
2) Adenine nucleotides are components of the major co-enzymes: NAD, NADP, FMN, FAD, & CoA
3) Nucleotide derivatives are activated intermediates in biosynthetic processes (UDP-glucose, SAM)
4) Serve as metabolic regulators (e.g. cAMP and the activation of cell signaling)
5) Serve as major currency of energy in all cells (ATP & GTP)
6) Several metabolic diseases have their etiology in nucleotide metabolism

Question 2

What are the purine DNA nucleotide bases?

Answer 2

1) Adenine

2) Guanine

Question 3

What are the pyrimidine DNA nucleotide bases?

Answer 3

1) Thymine

2) Cytosine

Question 4

What are the purine RNA nucleotide bases?

Answer 4

1) Adenine

2) Guanine

Question 5

What are the pyrimidine RNA nucleotide bases?

Answer 5

1) Uracil

2) Cytosine

Question 6

What is the difference between adenine, adenosine, & adenosine monophosphate (AMP)?

Answer 6

1) Adenine is a base
2) Adenosine contains the adenine base in addition to a ribose sugar (or deoxyribose)
3) Adenosine monophosphate (AMP) contains the adenine base, a ribose (or deoxyribose sugar), & a phosphate group

Question 7

How can you form a diphosphate?

Answer 7

Using nucleoside monophosphate kinase & a tri-phosphate, you can phosphorylate a monophosphate to form a diphosphate

Question 8

How can you form a triphosphate?

Answer 8

Using nucleoside diphosphate kinase & a tri-phosphate, you can phosphorylate a diphosphate to form a triphosphate

Question 9

What are two common purine bases aside from adenine and guanine?

Answer 9

1) Hypoxanthine (6 oxy purine)
2) Xanthine (2,6 dioxy purine)
3) Adenine (6 amino purine)
4) Guanine (2 amino, 6-oxy purine)

Question 10

What are two biosynthetic pathways for obtaining purine and pyrimidine bases?

Answer 10

1) De novo pathway (building the bases from non-purine molecules)
2) Salvage pathway (the reutilization of bases from dietary or catabolic sources)

Question 11

How can you form uracil from cytosine?

Answer 11

Oxidative deamination

Question 12

Which of the following types of cells has the highest activity of ribonucleotide reductase and why? (Epithelial, nerve, or muscle)

Answer 12

When there is a need for DNA synthesis in a cell, such as in a rapidly dividing cell, RNA will be converted to DNA by Ribonucleotide reductase. This occurs most often in an epithelial cell, which needs to rapidly divide

Question 13

When a pharmaceutical company produces a purine or pyrimidine based drug, it is best administered as a base, nucleoside, or nucleotide?

Answer 13

Nucleoside because it is unphosphorylated and can easily cross the cell membrane

Question 14

What is a byproduct of the pentose phosphate pathway that is used in the biosynthesis of purines?

Answer 14

Ribose 5-phosphate

Question 15

What is the first step of purine biosynthesis in the de novo pathway?

Answer 15

1) Ribose 5-phosphate + ATP → PRPP + AMP (uses ribose phosphate pyrophospho-kinase (PRPP) synthetase)
2) Can be inhibited by products IMP, AMP & GMP through feedback inhibition

Question 16

Once PRPP is formed, what is the first unique step to purine biosynthesis in the de novo pathway?

Answer 16

1) Phosphate pyrophospho-kinase (PRPP) + Glutamine → 5-phosphoribosylamine + Glutamate + PPi
2) Can be inhibited by products IMP, AMP, & GMP through feedback inhibition

Question 17

Describe the de novo pathway of purine biosynthesis

Answer 17

1) It is a nonregulatory pathway (once it begins, it goes to completion)
2) The end product is Inosine Mono-Phosphate (IMP)
3) The Nitrogens that make up IMP are taken from aspartate, glycine, & glutamine
4) The carbons that make up IMP are taken from 10-formyl THF, 5,10-methenyl THF, CO2, & Glycine

Question 18

What is Inosine Monophosphate (IMP) used for?

Answer 18

It is a precursor for both AMP & GMP

Question 19

How is AMP formed from IMP?

Answer 19

1) IMP + Aspartic Acid + GTP → Adenylosuccinate + GDP (uses Adenylosuccinate synthetase)
2) Adenylosuccinate → Fumarate + AMP (uses Adenylosuccinase)
3) The presence of GTP positively regulates this pathway

Question 20

How is GMP formed from IMP?

Answer 20

1) IMP + H2O + NAD+ → Xanthosine monophosphate + NADH + H+ (uses IMP dehydrogenase)
2) Xanthosine monophosphate + Glutamine + ATP → GMP + Glutamate + AMP + PPi (uses GMP synthetase)
3) The presence of ATP positively regulates this pathway

Question 21

How is Adenine formed from Hypoxanthine-IMP?

Answer 21

Hypoxanthine-IMP + Aspartate → Adenine (Aspartate donates N)

Question 22

How is Guanine formed from Xanthine-XMP?

Answer 22

Xanthine-XMP + Glutamine → Guanine (Glutamine donates N)

Question 23

What is unique about the salvage pathways of purine synthesis?

Answer 23

Instead of synthesizing new purines (like the de novo pathway), it re-utilizes existing purines

Question 24

What are two main enzymes with different specificities associated wit the salvage pathways?

Answer 24

1) Adenine phosphoribosyl transferase (APRT)

2) Hypoxanthine-guanine phosphoribosyl transferase (HGPRT)

Question 25

Describe the reaction catalyzed by APRT

Answer 25

PRPP + Adenine → AMP

Question 26

Describe the reaction catalyzed by HGPRT

Answer 26

PRPP + Hypoxanthine/Guanine → IMP/GMP

Question 27

What enzyme catalyzes the degradation of a nucleotide into a nucleoside?

Answer 27

Nucleotidase

Question 28

What enzyme catalyzes the degradation of a nucleoside into a base?

Answer 28

Phosphorylase

Question 29

What are two diseases correlated with a deficiency in HGPRT?

Answer 29

Gaut & Lesch–Nyhan syndrome

Question 30

How can adenosine be degraded to uric acid?

Answer 30

1) Adenosine + H2O → Inosine + NH4+ (uses Adenosine deaminase (ADA))
2) Isosine + Pi → Hypoxanthine + Ribose-1-phosphate (uses purine nucleoside phosphorylase)
3) Hypoxanthine → Xanthine (uses xanthine oxidase)
4) Xanthine → Uric Acid (uses xanthine oxidase)

Question 31

How can guanosine be degraded to uric acid?

Answer 31

1) Guanosine + Pi → Guanine + ribose-1-phosphate (uses purine nucleoside phosphorylase)
2) Guanine + H2O → Xanthine + NH4+ (uses guanine deaminase)
3) Xanthine → Uric Acid (uses xanthine oxidase)

Question 32

What causes Gout?

Answer 32

Gout results from hyperuricemia. This can be caused by:

1) Decreased Uric Acid excretion: 80% of gout – idiopathic, renal disease, diabetes insipidus, hypertension, Downs syndrome, & many others
2) Increased Uric Acid production: 20% of gout
a) PRPP synthetase overactivity, hemolytic diseases, lymphoproliferative disease, & many others
b) HGPRT deficiency (Lesch-Nyhan syndrome)

Question 33

What exacerbates gout?

Answer 33

1) Alcohol
2) Purine rich diet
3) Obesity

Question 34

How exactly does PRPP synthetase overactivity lead to gout?

Answer 34

Lost regulation of PRPP synthetase allows it to continuously convert Ribose 5-phosphate to phosphoribosyl amine, which is then converted to purines. When there is an excess of purines, they must be degraded into uric acid

Question 35

How exactly does HGPRT deficiency lead to gout?

Answer 35

Without HGPRT, PRPP + Guanine cannot be converted to GMP. There is therefore a buildup of guanine, resulting in elevated uric acid levels

Question 36

How can gout be treated?

Answer 36

Administration of allopurinol inhibits xanthine oxidase, causing:

1) Decrease urate
2) Increase xanthine & hypoxanthine
3) Decrease PRPP

Question 37

What is Lesch-Nyhan syndrome?

Answer 37

1) X-linked recessive
2) Severe HGPRT deficiency, which causes decreased IMP & GMP, & Increased PRPP & de novo purine pathway
3) Hyperuremia: gouty arthritis, kidney stones, tophi
4) Neurologic disability: spasticity & hyperreflexia
5) Behavioral problems: cognitive dysfunction, aggression, & self-injury

Question 38

What is SCIDS (Severe Combined Immunodeficiency Syndrome)?

Answer 38

1) Autosomal recessive disorder
2) Mutations in Adenosine Deaminase (ADA) causes a deficiency of ADA
3) Infants subject to bacterial, candidiasis, viral, & protozoal infections
4) A buildup of of dATP is toxic and causes reduction of both T band C cells