Nucleotide Anabolism Flashcards
In addition to the salvage pathways, how else can purine and pyrimidines be synthesized? Where are the enzymes located in purine and pyrimidine synthesis?
de novo - from starting materials like CO2, NH3, and some of the AA Enzymes are in the cytosol
What are the sources of C and N atoms in pyrimidines?
Aspartate (donates 3C and 1N) Bicarb (donates 1C) Glutamine amide (donates 1N)
What are the steps in pyrimidine synthesis?
Amine from Glutamine Add CO2 Carbomyl Phosphate Add Aspartate Close the ring Orotic Acid Use PRPP to attach ribose-5-phosphate Remove CO2 to make Uridine (UMP) Cytosine or Thymidine From UMP, you can make Cytosine or Thymidine: UMP -> UDP -> UTP -> Cytosine UMP -> dUDP -> dUTP -> dUMP -> dTMP -> Thymidine
What vitamin do you need to make Thymidine, and why?
Folate, because it is the methyl source (N5, N10-methylene H4 folate is the source of CH3)
What does the drug 5-fluorouracil do to pyrimidine synthesis?
Prevents the cells from replicating. Used as a chemotherapy drug
How do pyrimidine and purine synthesis differ?
Pyrimidine: first we make the single ring, then add sugar, then add phosphate Purine: start with PRPP, add N, add and build up double rings from multiple sources
What are the sources of C and N atoms in purines?
Glutamine amide (2N) Glycine (1N, 2C) Aspartate amide (1N) Bicarb (1C) H4 Folate (2C)
What is the generic purine ring called?
IMP
How much ATP is required to synthesize a double ring purine?
4ATP (not including cost of PRPP)
What is the rate limiting step in pyrimdine synthesis?
CPS-2 is the rate-limiting step -glutamine + CO2 + 2ATP -> carbomyl phosphate + 2ADP + PPi
What happens if there is a defect in nucelotide anabolism?
Death (in utero)
What does GMP (Guanosine) require?
NAD, ATP, and NH2 from Gln
What does AMP (Adenosine) require?
GTP and NH2 from Asp
What comes from IMP?
GMP or AMP
What is the difference between Adenosine and Guanosine?
The placement of the amino group
How are the concentrations of AMP and GMP regulated?
The concentration of one affects synthesis of the other because you have to have A to make G and have to have G to make A. Keeps concentrations balanced
What does the antivrial drug Acyloguanosine (Acyclovir) do?
It impairs purine synthesis
How do mono-, di-, and triphosphate nucleotides relate?
They are all interconvertible: CMP + ATP CDP + ADP GDP + ATP GTP + ADP getting to triphosphate form traces back to oxidative phosphorylation because all ADP go back to ETS to get rephosphorylated
What is the only version of Thymidine created?
It only exists in the oxidative form, ie. DEOXYribonucelotide
HMPS generates NADPH. How is NADPH used?
In anabolic/synthetic pathways: fat synthesis as well as to make deoxy forms of nucleoides
How is PRPP formed?
Ribose-5-P + ATP –> AMP + PRPP
Where does the Ribose-5-P come from?
HMPS
What enzyme catalyzes the synthesis of PRPP?
ATP phosphoribosyl transferase
What inhibits ATP phosphoribosyl transferase?
The accumulation of purines and pyrimidines (product feedback inhibition)
What happens if there is a deficiency in HGPRTase (as in Lesch Nyhan)?
Hypoxanthine and Guanine will accumulate because breakdown of nucleotides is occurring instead of recycling The accumulation blocks more PRPP synthesis, so all nucleotide synthesis is shutdown
What is PRPP used for?
Purine and Pyrimidine de novo synthesis, NAD, and salvage pathways
If ATP phosphoribosyl transferase is inhibited, what happens?
Accumulation of purines and pyrimidines –> Increase in purines causes an Increase in uric acid –> Gout
What is the relationship between Glucose 6 Phosphatase deficiency (Type 1, von Gierke’s) and Gout?
G-6-phosphatase deficiency (Type I, von Gierke’s) –> increase G6P –> Glucose can’t be released from the liver cells –> Glucose gets pushed down HMPS pathway –> increase ribose-5-P –> increase PRPP –> increase nucleotide synthesis –> increase purines –> Gout (secondary)
If you make more purines than you need, what happens?
Purines can’t be broken down for energy so you break them down. The waste products (uric acid) accumulate, and cause Gout
Folate (vit B9)
What is Folate’s involvement in Purine (Thymidine in particular) synthesis?
n5,10-methyleneH4folate is methyl donor in purine metabolism (in dUMP –> dTMP)
Folate is also required for Glycine <–> Serine and Homocysteine –> Methionine
What clinical signs would you see in a folate deficicency?
Megaloblastic (pernicious) anemia
What can interfere with folate absorption?
Antiseizure drugs
Bacteria make their own folate. How can this be blocked?
Sulfonamide antibiotics target PABA
What is needed to convert N5-methyl-H4 to H4-folate in mammals?
Cobalamin (Vit B12)
What is B12 required for?
Folate absorption and Methionine synthesis
What is needed to absorb B12?
Intrinsic Factor and Cobalt
What drugs that inhibit dihydrofolate reductase (DHFR) and folate?
Certain Chemotherapy drugs, Antiprotozoal drugs, and Trimethorpim (an antibiotic used in combo with a Sulfonamide)
-they all mimic DHFR
What clinical symptoms would appear in a patient with folate deficiency?
Low blood cell count (Megaloblastic anemia, Leukopenia, Thrombocytopenia, Pancytopenia)
Atrophic glossitis
Inappetance and weight loss
Muscular weakness/fatigue
Neurological/behavioral signs
Elevated levels of homocysteine in blood and urine (because can’t metabolize/breakdown/recycle homocysteine back to methionine)
What diseases are correlated with folate deficiency?
Cardiovascular and Neural
If Carbamyl phosphate could not be be synthesized into UMP, what clinical signs would arise?
Orotic acid would accumulate causing Orotic aciduria, decrease pyrimidine, not making DNA, low CTP and TTP, megaloblastic anemia from not making purines
How do you treat Orotic aciduria?
Feed Uridine to produce pyrimidines which will push UMP –> UTP, which inhibits CPSII, decreasing orotate production
A deficiency in Orotate phosphoribosyltransferase and/or orotidine-5’-P decarboxylase will cause what?
Orotic aciduria
