Pyrimidine Nucleotide Metabolism

Question 1

Order of events

Answer 1

Making the ring, , then attach the sugar

Question 2

First step in denovo. Biosynthesis of Pyrimidines

Answer 2

Synthesis of carbamoyl phosphate using CO2, glutamate (donates N), and ATP
-use enzyme carbamoyl phosphate synthetase II

Question 3

Second step

Answer 3

Formation of N-carbamoyl aspartate from aspartate and carbamoyl phosphate.

• use aspartate transcarbomylase
• committed step

Question 4

Rate limiting steps

Answer 4

1-3

Question 5

4th step

Answer 5

Forms orotic Acid, first pyrimidine base

Question 6

Steps 5 and 6

Answer 6

Form pyrimidine nucleotide UMP.

-uses orotate phosphoribosyl transferase to add PRPP and orotidylic acid decarboxylase in 6th step

Question 7

UMP

Answer 7

2x phosphorylated to UTP. UTP aminated (glutamine) to CTP

Question 8

Thymine nucleotide final steps

Answer 8

UDP converged dUDP by ribonuclease reductase

• dUDP dephosphorylated to dUMP
• dUMP converged to dTMP by thymidylate synthetase (uses N5 N10 methylene tetrahydrofolate (THF)

Question 9

Regulation of pyrimidine synthesis

Answer 9

1st step main regulated step. By expression of CAD gene

• carbamoyl phosphate synthetase II feedback inhibited by UTP stimulated by PRPP
• high UTP shuts pathway down
• high ATP activates

Question 10

Orotic Acid

Answer 10

First pyrimidine ring compound.

• attach sugar phosphate using PRPP and make OMP
• convert OMP to UMP (first nucleotide)

Question 11

Orotic aciduria

Answer 11

Give uridine to treat by salvaging uridine to UMP and make CTP and TMP

• one enzyme that catalyzes both reactions depending on site that is knocked out.
• retardation, apathy, failure to thrive, infections, urinalysis obstructions
• type 1 more severe than type 2

Question 12

Thymidylate synthetase

Answer 12

Converts dUMP to dTMP.

• uses THF derivative to add methyl group to UMP to make TMP
• methotrexate blocks this
• only reaction to yield DHF from THF
• THF must be regenerated from DHF by DHFR, THF then converted to N5 N10 THF by serine hydroxymethyl transferase

Question 13

Suicide substrates

Answer 13

Molecules used to inhibit thymidylate synthetase

-kills ability of methylene THF and thymidylate synthetase to detach

Question 14

Salvage

Answer 14

Add phosphate from ATP

• thymidine creation from salvage important for prep for cell division
• uracil can be salvaged from UMP
• uses uridine cytidine or thymidine kinase
• deosyridine is a substrate for uridine phosphorylates

Question 15

Herpes virus

Answer 15

Makes own thymidine kinase which can be targeted without interfering with own body
-use Acyclovir (guanosine analog) which will be salvaged by kinase to make active compound incorporated into herpes DNA killing it

Question 16

Acyclovir

Answer 16

Guanosine analog treats heroes by being a partial chain and ring replaced with open chain and ring. Selectively converted to acycloGMP by TK to cGMP.
Only Tk of virus can do this conversion, terminated DNA chain

Question 17

Aminopterin

Answer 17

Inhibits dihydrofolate reductase, interferes with folate metabolism, no THF derivatives made
No effect on U and C, only T

Question 18

Reduction of nucleoside diphosphates

Answer 18

• Start with A G C or U
• Take reducing power from thioredoxin
• reduce nucleoside diphosphate to dNDP
• reduced form of thioredoxin can be regenerated by thioredoxin reductase by taking reducing power from NADPH