L19: purine nucleotide biosynthesis, salvage, & degradation

Question 1

Pathway for purine biosynthesis

Answer 1

• liver and immune cells primarily biosynthesize purines
• starts with PRPP, building a purine ring on the ribose ring
• requires great input of energy
• one-carbon transfers are provided by N10-formyl-tetrahydrofolate
• Inosine 5’P is first fully formed purine nucleotide

Question 2

what is IMP?

Answer 2

• Inosine 5’P

- it is a key metabolic intermediate because it can readily tautomerize

Question 3

why doesn’t IMP have a role in RNA or DNA?

Answer 3

because it has mutagenic H-bonding (acts as both an H-bond donor and an H-bond acceptor

Question 4

pathway for purine biosynthesis pic

Answer 4

slides 106-109

Question 5

conversion of IMP to AMP

Answer 5

IMP + aspartate + GTP -> adenylosuccinate -> AMP

Question 6

key points of adenylosuccinate mechanism

Answer 6

• IMP tautomerizes to form -OH at C6
• GTP phosphorylates IMP to form the 6-phospho-IMP intermediate
• Aspartate amino group displaces phosphate

Question 7

conversion of IMP to GMP

Answer 7

IMP + NAD+ + H2O -> xanthosine-5’-P

xanthosine-5’-P + ATP + Gln -> guanine-5’-monophosphate

Question 8

key points of IMP dehydrogenase reaction

Answer 8

• enz-SH reversibly forms an adduct with inosine 5’-phosphate
• hydride is transferred to NAD+ as ring is oxidized
• H2O displaces Enz-SH to form XMP

Question 9

Mechanism of GMP synthase

Answer 9

1. adenylation of the purine -OH group
2. Glutamine hydrolysis and NH3 transfer through the
   tunnel
3. nucleophilic attack by NH3 to form GMP

Question 10

control of purine nucleotide synthesis

Answer 10

1. AMP inhibits AMP-sucsynthetase directing IMP to GMP
2. high GMP directs IMP to AMP
3. high IMP feedback inhibits 5’-Prib-NH2 formation
4. high AMP and GMP feedback inhibit PRPP synthetase
5. high AMP activates AMP-protein kinase, turning off purine pathway enzymes

Question 11

what does hypoxanthine guanine-phosphoribosyl transferase (HGPRT) do?

Answer 11

it salvages purines from RNA and DNA degradation

Question 12

what nucleic acid is not salvaged by HGPRT?

Answer 12

Adenine

Question 13

why is adenine not salvaged by HGPRT?

Answer 13

it is the most abundant purine and high levels of it would block IMP and GMP re-synthesis

Question 14

what is gout?

Answer 14

it is pathologic precipitation of excess uric acid

Question 15

affects of gout

Answer 15

• crystals accumulate in joints
• crystals activate inflammasome response
• cause severe inflammation and pain
• white blood cells attack

Question 16

how is gout treated?

Answer 16

it is treated with allopurinol (a xanthine oxidase inhibitor)

Question 17

what is the product of ribonucleotide reductase?

Answer 17

2’-deoxy-ribonucleottides

Question 18

what gives evidence of a transition from the RNA world to the DNA world

Answer 18

substrate specificity

Question 19

in eukaryotes substrates are __?

Answer 19

di - 2

Question 20

in prokaryotes substrates are ___?

Answer 20

tri- 3

Question 21

is RNR strictly controlled?

Answer 21

yes, it must be to yield the right balance of dNTPs needed for DNA synthesis

Question 22

RNR catalysis is controlled by ____?

Answer 22

allosteric specificity sites

Question 23

Nucleotide-binding at specificity sites allows RNR ___?

Answer 23

• to sense relative abundances of NDPs

- to make the right amount of each deoxynucleotide

Question 24

RNR achieves balanced production by_____?

Answer 24

• changing catalytic efficiency with each substrate

- preventing the overproduction of any single dNTP