Ford (Nucleotide Metabolism) Flashcards
What is the important precursor for all nucleotides?
PRPP
Ribose-5-phosphate –> PRPP (PRPP synthetase)
Pyrimidines
- nitrogenous bases INDEPENDENT of PRPP
- unidirectional pathway
- cytoplasmic
- precursors: NH3 from Gln, Asp, HCO3
CAD
- multifunctional, eukaryotic protein performs functionsL
- Carbamoyl phosphate synthetase II (CPSII)
- has channel for glutamine
- Aspartate transcarbamoylase (ACTase)
- has two allosteric regulators (ATP +/CTP -)
- Dihydroorotase
- closes the ring (hydrolase in reverse)
- dihydroortase dehydrogenase located in mitochondria
Megaloblastic Anemia
- occurs when cells FAIL TO DIVIDE
- common: B9/B12 deficiencies
Hereditary Orotic Aciduria
- excessive orotate in urine
- megaloblastic Anemia that fails to respond to B9/B12 treatment
DEFICIENCY OF UMP SYNTHETASE
Purines
- form nitrogenous base on PRPP
- branched pathway
- cytoplasmic
- precursors: NH3, Gly, Asp, N10-formyl-THF, HCO3
Regulation: feedback inhibition by purines
Purines - Purinosome
6 enzymes perform 10 reactions
- 1,4,8 –> 3 individual proteins
- 2,3,5,6,7,9,10 –> 3 multifunctional proteins
2 additional enzymes provide N10-Formyl-THF
proximity facilitates flow
Purines - Branching off from IMP
- AMP –> uses GTP for energy
- replace carbonyl w/NH3 from Asp
- releases fumarate
- AMP inhibited - GMP –> uses ATP for energy
- redox w/ water to make 2nd carbonyl, then replaces with NH3 from Gln
- GMP inhibited
Ribonucleotide Reductase Regulation
- Activity Site –> allosteric
- ON = ATP
- OFF = dATP (alter subunit contacts)
- Specificity Site
- dATP –> pyrimadines preferred
- dTTP –> GDP (+), pyrimadines (-)
- dGTP –> ADP (+), pyrimadines (-)
Antivirals (Acyclovir)
Viral Thymidine Kinase accepts purines as well as T’s
Acyclovir binds tightly to viral Thymidine Kinase, does not react with human TK
- incorporated into viral DNA, terminates synthesis
Uric Acid
- antioxidant
- uric acid/urate is insoluble
- Urate Oxidase solubilizes urate, at cost of antioxidant potential
- genes mutated in humans and great apes
Gout
- high concentrations = urate crystallizes
- chronic elevated lvls of urate in blood (hyperuricemia) –> uric acid crystals in joints (GOUT)
- inflammation, arthritis, joint degradation
HPRT deficiency (Lesch-Nyhan Syndrome)
SCID
Severe Combined Immunodeficiency Disease
- Adenosine Deaminase deficiency - w/o --> dAMP accumulates (converted to dATP by salvage pathway) - dATP inhibits ribonucleotide reductase
Deoxyribonucleotides NOT produced, DNA NOT synthesized, cells do not proliferate
Ring Structure of Pyrimidines
- Glutamine: provides 1 N
- CO2: provides 1 C
- Aspartate: provides 1 N and 3 carbons
Carbomoyl Phosphate Synthetase II
- makes bicarbonate into carbamoyl phosphate
Aspartate transcarbamoylase (ATCase)
- adds aspartate to carbamoyl phosphate
- creates carbamoylaspartate
Dihydroorotase and Dihydroorotate Dehydrogenase
- dihydroorotase closes ring (hydrolase in reverse) = dihydroorotate
- dehydrogenase changes dihydroorotate into orotate
UMP Synthetase
- removes PPi when orotate added to PPRP
2. decarboxylates orotate to form uracil
Nucleoside monophosphate and diphosphate kinases
monophosphate: specific to each NMP
diphosphate: broad specificity
CTP synthetase
- similar to ATCase (GTP/ATP act, CTP inactivates)
- changes UTP to CTP
Ring Structures of Purines
5 membered ring:
- Glycine: 1 N and 2 Cs
- N-formyl THF: 1 C
- Glutamine: 1 N
6 membered ring:
- Aspartate: 1 N
- N-formyl THF: 1 C
- Glutamine: 1 N
- CO2: 1 C
Ribonucleotide Reductase
- reduction reaction that converts ribose to deoxyribose
- NADPH provides necessary electrons for redox rxns
2 ways to created dUMP
- removal of PPi from dUTP
2. deamination of dCMP
Thymidylate Synthase (dUMP)
adds methyl to dUMP to create dTMP
Pyrimidine Salvage
- use phosphorylases and kinases to salvage in 2 steps
- phosphorylases make NUCLEOSIDES
- kinases make NUCLEOTIDES
Purine Salvage
- use phosphoribosyltransferases to salvage in 1 step
Types of Phosphoribosyltransferases (2)
- Adenine –> make AMP
2. Hypoxanthine-Guanine (HGPRT/HGPT) –> IMP/GMP
Breakdown of Nitrogenous bases (2 end products)
- Uric acid (purine catabolism)
2. B-Ureidopropinoic acid converted to alanine, CO2, NH3 by Ureidopropionase (pyrimidine catabolism)
Endonucleases and Exonucleases
- Endonuclease: cuts nucleic acids from the inside
- makes oligonucleotides - Exonucleases: cuts oligonucleotides from outside
- makes nucleoside monophosphates
