Nucleotide Metabolism II

What are the characteristic structure of the major pyrimidines?

What are the origins of the carbons and nitrogens on the pyrimidine ring?

Describe the overview of de novo pyrimidine synthesis.

The de novo synthesis of pyrimidines is a six-step pathway to UMP

Steps 1-3 are catalyzed by a multifunctional protein (dihydroorate synthase)

Steps 5 and 6 are catalyzed by a bifunctional enzyme (UMP synthase)

Bicarbonate is added by CPSII (the cytosolic one)

While CPSI in the mitochondria uses NH3 as a source of nitrogen, CPSII uses glutamine as the nitrogen donor to carbamoyl phosphate

How is the de novo synthesis of pyrimidines regulated?

CPSII is allosterically activated by PRPP and IMP.

CPSII is inhibited by several pyrimidines.

Aspartate transcarbamoylase (ATCase)

Important regulatory point of prokaryotes.

Catalyzes first committed pathway step.

Allosterically regulated by CTP (negative), CTP and UTP (negative), and ATP (positive).

Describe the synthesis of CTP from UMP. What regulates this?

UMP is converted to UDP and then UTP (2 ATP req).

UTP is converted to CTP by CTP synthetase (req glutamine).

Describe the process of pyrimidine synthesis.

What is orotic aciduria?

inability to convert orotic acid to UMP (de novo pyrimidine synthesis) because of defect in UMP synthase (bifunctional enzyme). Can also be caused by defects in mitochondrial CPS I.

How can orotic aciduria be differentiated between it being caused by UMP synthase or OTC deficiency?

Blood ammonia levels; OTC deficiency means that the urea cycle is not functioning, and therefore can be determined by analysis of blood ammonia levels.

What is trimethoprim?

A drug inhibitor of DHFR, like methotrexate

What is pyrimethamine?

A drug inhibitor of DHFR, like methotrexate

Compare the structure of tomudex to methotrexate.

Describe the process of pyrimidine salvage.

Hydrolyzed to the nucleosides and inorganic phosphate

Then T, U, and deoxyribose-1-phosphate are produced

Catabolism of thymine and uracil bases end with intermediates of central metabolism (acetyl CoA from uracil and succinyl CoA from thymine)

Describe the formation of deoxynucleotides and the enzyme that catalyzes allat.

Deoxyribonucleotides are synthesized by the reduction of ribonucleotides by RDR (ribonucleotide diphosphate reductase)

What are the two routes of conversion of dUDP to dUMP

direct and hydrolysis

Describe the reduction of ribonucleotides diphosphate.