Nucleotide Metabolism

Question 1

What is the dUTP problem?

Answer 1

Deoxyuridine triphosphate

- incorporation into DNA is mutagenic

Question 2

What can be done by virus to combat dUTP problem?

Answer 2

Poxvirus, Herpesvirus (+ retroviruses)

• encode dUTPase (dUTP –> dUMP)
• encode thymidine kinase (TMP –> TTP)

Question 3

What viral drugs target nucleotide metabolism?

Answer 3

Acyclovir + gancyclovir

AZT

Question 4

Acyclovir mechanism of action?

Answer 4

Phosphorylated by viral TK to acyclovir monophosphate in step towards incorporation into DNA

• not recognised by cellular TK so does not affect non-infected cells

Question 5

What viral enzymes affect nucleotide metabolism?

Answer 5

dUTPase
Thymidine kinase
Uracil-DNA glycosylase
Viral ribonucleotide reductase
HIV Vif
HIV Vpxi

Question 6

What is viral ribonucleotide reductase?

Answer 6

Ribonucleotides (NTP) –> deoxyribonucleotides (dNTP)

• encoded by herpesvirues and poxviruses

Question 7

How can cell fight back against virus?

Answer 7

APOBEC3G
ADAR
SAMHD1

Question 8

What is the mechanism of action of APOBEC3G?

Answer 8

Deaminates a single cytosine in APOB mRNA

–> premature stop codon

Question 9

What is the mode of action of APOBEC3G against HIV?

Answer 9

Will convert C –> U on -ve strand during reverse transcription

• -> hypermutation
• -> excision by UNG + degradation

Question 10

How can HIV block the action of APOBEC3G?

Answer 10

Vif (ubiquitin ligase)

- targets APOBEC3G for degradation by the proteosome

Question 11

What is the mechanism of action and effects of ADAR?

Answer 11

Deaminates adenosine –> inosine

Loss of termination codon (UGA –> UGG)
Mutation during replication
Less stable RNA structure
Cleavage of edited RNA by specific RNAase

Question 12

What is the mechanism of action of SAMHD1?

Answer 12

• becomes activated on interaction with dGTP
• triphosphate triphosphohydrase –> removes 3 phosphates from deoxynucleoside
• decreases dNTPs needed for DNA replication

Question 13

What is the mechanism of action of AZT?

Answer 13

Phosphorylase by cellular kinases + incorporated into DNA where it acts as a chain terminator

• given SAMHD1 decreases dNTPs, this increases the relative concentration of AZT-triphosphate so viral polymerase more likely to incorporate it into DNA

–> therefore SAMHD1 enhances inhibition of the virus

Question 14

What is the mechanism of action of the Vpx protein?

Answer 14

Targets SAMHD1 for degradation