Lecture 75

Question 1

What are the enzyme and substrate,energetics and regulation of PRPP synthesis?

Answer 1

Enzyme: PNPP synthase (ATP –> AMP)
Substrate: Ribose 5-phosphate (from pentose phosphate pathway)
Regulation: Inorganic phosphate is a strong allosteric activator of PNPP synthase. Purine nucleotides inhibit PNPP synthase.

Question 2

What is the nitrogen source for purine biosynthesis?

Answer 2

Aspartate, glycine and glutamine

Question 3

What is the carbon source for purine biosynthesis?

Answer 3

N1- formyl tetrahydrofolate, CO2,

Question 4

What is the common precursor for GMP and AMP synthesis?

Answer 4

IMP (inosine monophosphate)

Question 5

What is the salvage pathway reaction?

Answer 5

phosphoribosyltransferase: purine base + PRPP
- -> base-ribose-phosphate + PPi

*note: there are separate phosphoribosyltransferases for adenine and guanine.

Question 6

How many high energy bonds are required for salvage and de novo purine synthesis?

Answer 6

de novo: 6

salvage: 2

Question 7

Lesch-Nyhan Syndrome

Answer 7

• Deficiency in hypoxanthine guanine phosphoribosyl transferase, or HGPRT, which is the enzyme for guanine salvage.
• x-linked disorder
• excess uric acid formation, mental retardation

Question 8

Which enzymes convert monophosphorylated purine nucleotides to di and triphosphorylated purine nucleotides?

Answer 8

guanylate kinase and adenylate kinase:
GMP + ATP –> GDP + ADP
AMP + ATP –> ADP + ADP

nucleoside diphosphate kinase:
NDP(dNDP) + ATP –> NTP (dNTP) + ADP

Question 9

In de novo pyrimidine biosynthesis, what is the product of the attachment of PRPP to the base?

Answer 9

UMP (uridine monophosphate)

Question 10

What compounds are used to form the ring in de novo pyrimidine biosynthesis?

Answer 10

Carbamoyl phosphate (synthesized by CPSII)
Aspartate

Question 11

What happens to UMP after it is synthesized from PRPP and the pyrimidine ring?

Answer 11

UMP is converted to UTP by uridyl kinase and nucleoside diphosphate kinase.

Question 12

How is CTP formed?

Answer 12

CTP is formed by transferring an amide group from glutamine to the 4 position of UTP

Question 13

What is the regulated step in pyrimidine synthesis?

Answer 13

CPSII (CTP is a negative allosteric effector, ATP is a positive allosteric activator, and PRPP is an activator)

Question 14

Which enzyme converts nucleoside diphosphates to deoxyribonucleotides?

Answer 14

ribonucleotide reductase

Question 15

Describe the allosteric regulation of ribonucleotide reductase.

Answer 15

• Activated by ATP
• Inhibited by dATP
• dATP stimulates pyrimidine deoxynucleotide synthesis
• dTTP stimulates purine deoxynucleotide synthesis
• dGTP stimulates formation of dADP

Question 16

How are electrons transferred to NDPs during the formation of dNDPs?

Answer 16

NADPH–> Thioredoxin –> ribonucleotide reductase

Question 17

Which enzyme converts UMP to TMP? Where does the methyl group come from?

Answer 17

• Thymidylate Synthase

- N5,N10 methylene THF

Question 18

How is uric acid formed?

Answer 18

Degradation of purine nucleotides

Question 19

What is the treatment for gout? How does it work?

Answer 19

Allopurinol. It is a uric acid analog, and it functions as a competitive inhibitor for the enzyme that makes uric acid (xanthine oxidase), resulting in the production of xanthine, which is more soluble.

*Works for overproducers, not underexcretors!

Question 20

Describe the 2 chemotherapeutic agents that inhibit thymidylate synthase.

Answer 20

1. Fluorouracil: Suicidie inhibitor of thymidylate synthase

2. Methotrexate: Competitive inhibitor of dihydrofolate reductase

Question 21

Which chemotherapeutic agent targets deoxynucleotide synthesis?

Answer 21

Hydroxyurea: inhibits ribonucleotide reductase

*Also used to treat patients with Sickle Cell Anemia

Question 22

Name two drugs used in the control of herpesvirus and HIV.

Answer 22

• Acyclovir and AZT
• phosphorylated to yield inhibitory forms
• Acyclovir is phosphorylated by a kinase encoded in the HSV genome, providing specificity to infected cells
• DNA replication terminators