Lecture 63

Question 1

structure of nucleotides and nucleosides

Answer 1

• nucleoside: ribose sugar and nitrogenous base (NO phosphates)
• nucleotide: ribose sugar, nitrogenous base, and 1-3 phosphates

pg 1601

Question 2

purine synthesis de novo: part 1

Answer 2

• ribose-5-P (from g-6-P via PPP) converted to PRPP (5-phosphoribosyl 1-pyrophosphate) via PRPP synthetase
• PRPP is a high-energy molecule
• committed step: PRPP converted to 5-phosphoribosyl 1-amine using glutamine and water (enzyme = glutamine phosphoribosyl amidotransferase)

pg 1604

Question 3

purine synthesis de novo: part 2

Answer 3

• building of purine ring
• molecules come from glycine (+ATP), N10-formyl-FH4 (1-C donor), glutamine (+ATP), CO2 (+ATP), aspartate (+ATP), and N10-formyl-FH4
• forms inosine monophosphate (IMP)

pg 1605

Question 4

role of one-carbon donors

Answer 4

N¹⁰-formyl-FH₄ is one-carbon donor used for purine synthesis → donates a single carbon to form new bonds; important for AAs and nitrogenous bases

pg 1606

Question 5

one carbon metabolism: FH₄ and folate

Answer 5

• folic acid converted to active forms to function as 1-C donor, requires dihydrofolate reductase
• different one-carbon donors used for pyrimidine and purine synthesis

pg 1607

Question 6

folic acid deficiency and anemia

Answer 6

• inadequate serum levels of folate
• caused by: increased demand (pregnancy and lactation), poor absorption (pathology of SI, alcoholism), treatment with drugs that are dihydrofolate reductase inhibitors (methotrexate)
• folate-free diet can cause a deficiency within a few weeks
• primary result of folic acid deficiency is megaloblastic anemia caused by diminished synthesis of purine nucleotides and TMP, which leads to an inability of cells to make DNA and an inability to divide

pg 1608

Question 7

one-carbon metabolism inhibitors: PABA analogs

Answer 7

• para-aminobenzoic acid (PABA) is part of the folate molecule
• sulfonamides are structural analogs of PABA that competitively inhibit bacterial synthesis of folic acid
• because purine synthesis requires tetrahydrofolate as a coenzyme, sulfa drugs slow down this pathway in bacteria
• humans CANNOT synthesize folic acid and must rely on external sources of this vitamin; therefore, sulfa drugs do not interfere with human purine synthesis

pg 1609

Question 8

one-carbon metabolism inhibitors: folic acid analogs

Answer 8

• methotrexate and related compounds inhibit the reduction of dihydrofolate to tetrahydrofolate, catalyzed by dihydrofolate reductase
• these drugs limit the amount of THF available for use in purine synthesis and slow down DNA replication in mammalian cells
• useful in treating rapidly growing cancers, but are toxic to ALL dividing cells

pg 1610

Question 9

purine synthesis de novo: part 3

Answer 9

• IMP converted to AMP and GMP
• AMP formed from aspartate and GTP
• GMP formed from glutamine and ATP
• ATP and GTP act as energy donors/activators for the conversion
• AMP and GMP then go on to form ADP, ATP, GDP, GTP

pg 1612

Question 10

regulation of purine synthesis de novo

Answer 10

• AMP and GMP (purine ribonucleotides) only produced WHEN and IF they are needed
• AMP and GMP serve as feedback inhibition for their own pathways (in part 3)
• in part 1: free phosphate (indication of low energy) activates PRPP synthetase, purine ribonucleotides inhibits PRPP synthetase (feedback), PRPP activates the committed step (substrate availability) and AMP/GMP inhibit the committed step (PRPP needed for pyrimidine synthesis so only stop further formation of purines)

pg 1613-1614

Question 11

purine synthesis inhibitors: mycophenolic acid

Answer 11

• reversible inhibitor of inosine monophosphate dehydrogenase (converts IMP to GMP and AMP)
• divides rapidly proliferating T and B cells of key components of nucleic acids
• immunosuppressant used to prevent graft rejection
• also used for autoimmune disorders

pg 1615

Question 12

pyrimidine synthesis de novo: part 1

Answer 12

• CO₂ and H₂O combine to form HCO₃^-
• HCO₃^- combined with glutamine to form carbamoyl phosphate (a high-energy molecule)
• occurs in cytosol by carbamoyl phosphate synthetase II (CPS-II) and 2 ATP

pg 1617

Question 13

pyrimidine synthesis de novo: part 2

Answer 13

• aspartate and carbamoyl phosphate join to form carbamoyl aspartate and release a free phosphate
• carbamoyl aspartate undergoes several subsequent reactions to close the ring and form orotic acid (orotate)

pg 1618

Question 14

pyrimdine synthesis de novo: part 3

Answer 14

• orotate converted to OMP by orotate phosphoribosyltransferase
• uses a PRPP and releases pyrophosphate

pg 1619

Question 15

pyrimidine synthesis de novo: part 4

Answer 15

• OMP uses orotidine 5’-P decarboxylase to release CO₂ and become UMP
• UMP converted to UTP
• UTP joined with glutamine and ATP to form CTP

pg 1620

Question 16

regulation of pyrimidine synthesis de novo

Answer 16

• PRPP activates CPS-II
• UTP provides feedback inhibition for CPS-II

pg 1621

Question 17

pyrimidine synthesis deficiencies: orotic aciduria

Answer 17

• orotate phosphoribosyltransferase and OMP decarboxylase are separate catalytic domains of a single polypeptide, UMP synthase
• low activity of either or both domains results in poor growth, megaloblastic anemia, and the excretion of large amounts of orotate in the urine
• administration of the uridine results in improvement of the anemia and decreased excretion of orotate
• excess carbamoyl phosphate from mitochondria leaks into cytoplasm leading to pyrimidine production

pg 1622