Met 2: Purine/Pyrimidine

Question 1

Compare the structure of Purines and Pyrimidines

Classify the nitrogenous bases.

Answer 1

• Purines: two rings, Adenine, Guanine
• Pyrimidines: one ring, cytosine, thymine (only DNA), uracil (only RNA)

Question 2

______ bases are built onto the ribose sugar

_______ bases are built by making the base first and then attaching ribose later

Answer 2

Purine bases are built onto the ribose sugar

Pyrimidine bases are built by making the base first and then attaching ribose later

Question 3

AMP and GMP are produced from ____

Answer 3

AMP and GMP are produced from IMP

Question 4

Name the intermediates leading up to AMP and GMP

Answer 4

G6P -> Ribose 5 Phosphate -> 5 PRPP -> IMP ->

AMP and GMP

Question 5

Describe the first step in purine synthesis

Answer 5

Glucose 6 phosphate converted to ribose 5 phosphate in HMP shunt (by G6P dehydrogenase)

Question 6

Describe the second step of purine synthesis.

What is the enzyme and its inputs

Answer 6

Ribose 5 phosphate converted to 5 PRPP

by PRPP synthase

Requires ATP and Mg

Question 7

Describe the third step of purine synthesis.

What is the enzyme?

What are its inputs? (5)

Answer 7

5 PRPP converted to IMP

by G PRPP amidotransferase

Glycine, Glutamine, Aspartate donate NH3

THF, CO2 donate carbon

Question 8

Describe regulation of IMP conversion into AMP and GMP (2)

Answer 8

• Increased concentration of one nucleoside upregulates production of the other b/c
  
  • Conversion of IMP into GMP requires ATP
  • Conversion of IMP into AMP requires GTP
• Both pathways also have negative feedback (GMP and AMP inhibit their own production)

Question 9

What two main enzymes are regulated in purine synthesis?

Answer 9

PRPP Synthase

Glutamine PRPP Amidotransferase

Question 10

How are PRPP synthase and Glutamine PRPP amidotransferase regulated?

Answer 10

• PRPP synthase and glutamine PRPP amidotransferase are both regulated using negative feedback
• They are inhibited by IMP, AMP, GMP (downstream products)

Question 11

What enzyme in pyrimidine synthesis gets regulated?

What are its two inputs?

Name 2 allosteric regulators

Answer 11

Carbamoyl phosphate synthase 2

Inputs are CO2 and Glutamine

Activated by ATP, inhibited by UTP

Question 12

Name the 6 intermediates in pyrimidine synthesis

Answer 12

Glutamine + CO2 -> Carbamoyl phosphate -> orotic acid -> UMP -> UTP -> CTP

Question 13

What 3 substances provide the carbons for pyrimidine synthesis?

Answer 13

Glutamine, CO2, Aspartate

• Glutamine and CO2 are inputs for CPS2
• Aspartate comes in later

Question 14

What enzyme converts UTP into CTP?

What are its inputs?

Answer 14

CTP Synthase

Inputs: UTP and ATP

Question 15

CPS I and CPS II are found in which pathways?

In what location?

Answer 15

CPS I: Part of urea cycle, in mitochondria

CPS II: Part of pyrimidine synthesis, in cytosol

Question 16

Compare the allosteric regulators of CPS I and CPS II

Answer 16

CPS I: activated by N acetyl glu

CPS II: activated by ATP, inhibited by UTP

Question 17

What enzyme converts RNA nucleotides into DNA nucleotides?

Answer 17

Ribonucleotide reductase

Question 18

Describe the regulation of ribonucleotide reductase

Answer 18

• Activity site is inhibited by dATP (product) and activated by ATP (precursor)
• Substrate specificity site determines which deoxynucleotide gets made

Question 19

How is dTMP made?

Name one cofactor

Answer 19

dTMP (thymidine DNA nucleotide) is made from dUMP by thymidilate synthase

THF needed to donate methyl group

Question 20

What are the 4 intermediates in purine breakdown?

Answer 20

Adenosine -> Inosine -> Hypoxanthine -> Uric Acid

Question 21

Describe the enzymes involved in purine breakdown

Answer 21

1. Adenosine deaminase needed to convert adenosine into inosine
2. Xanthine oxidase needed to convert hypoxanthine into uric acid

Question 22

What is the product of pyrimidine breakdown?

Answer 22

Pyrimidine breakdown products are inputs for TCA cycle (acetyl coA, malonyl coA, succinyl coA)

Question 23

Describe the purpose of the purine salvage pathway

Answer 23

• Purines are normally built onto ribose rings
• Purine salvage pathway allows us to take purine bases that are already made and put them back onto rings

Question 24

What enzyme is needed for purine salvage?

What does it do and what are its inputs?

Answer 24

by Hypoxanthine Guanine Phosphoribosyl Transferase (HGPRT)

Converts adenine, guanine into AMP, GMP

Requires PRPP

Question 25

How does adenosine deaminase deficiency cause SCID?

Answer 25

• Without adenosine deaminase, can’t degrade adenosine into inosine
• dATP builds up
• This inhibits ribonucleotide reductase (produces DNA nucleotides from RNA nucleotides)
• Damages cell proliferation