1. THFA is an important contributor to the synthesis of both purines and a specific pyrimidine. Which pyrimidine? Why does a block in the enzyme dihydrofolate reductase (DHFR) have negative consequences on nucleotide biosynthesis? What other negative consequences does this have: i.e. what other pathways might be influenced? Why?
2. What distinguishes the carbamoyl phosphate synthetase used for the urea cycle (CPS I) from that used for de novo pyrimidine biosynthesis (CPS II)? This should include the following characteristics: pathway, cellular location, nitrogen source, and regulation.
3. What is meant by substrate channeling? How is CPS II an example of an enzyme that utilizes this?