Nucleotide and deoxynucleotide metabolims- Lecture 77-78 Flashcards
What are the functions of purines and pyrimidines?
active precursors of DNA and RNA activated intermediates in carb (UDP) and lipid (CDP) metabolism, and methylation (SAM) high energy intermediates metabolic regulators part coenzymes
What is the structure of a nucleoside?
base + sugar
What is the structure of a nucleotide?
nucleoside + Pi
How is the activated form of ribose synthesized?
ribsose 5-P + ATP –> 5-phosphoribosyl-1-pyrophosphate (PRPP) + ADP
via PRPP synthase (ribose P pyrophosphokinase)
Describe the second step of purine ring synthesis.
PRPP + glutamine –> 5-Phosphoribosyl-1-amine + glutamate
via amido phosphoribosyl transferase
rate limiting
Describe the final steps of purine ring synthesis.
IMP + GTP + Asp –> GDP + adenylosuccinate
adenylosuccinate –> adenylate (AMP) + fumarate
OR
IMP + NAD+ –> xanthylate + NADH
xanthylate + Gln + ATP –> guanylate (GMP) + Glu + AMP
What is the composition of a purine ring?
N1 from Asp C2 from N10 formyl THF C8 from N5,N10 methenyl-THF C4, C5, N7 from Glycine C6 from CO2 N3, N9 from Glutamine
Where does purine ring synthesis occur?
in the cytosol of all cells
What regulates purine synthesis?
AMP inhibits step 1
GMP inhibits step 2
Describe the process of nucleoside phosphate interconversion.
Any XMP –> XDP via nucleoside mono P kinase + XTP
(GMP + ATP < —- > GDP + ADP)
(CMP + GTP < —- > CDP + GDP)
Any XDP –> XTP via nucleoside di P kinase + XTP
(GDP + ATP < —- > GTP + ADP)
(CDP + GTP < —- > CTP + GDP)
What are the fates of RNA degraded into free nitrogenous bases?
excreted as uric acid
salvaged and returned to the purine nucleotide pool
Why is the purine salvage pathway important?
because purines that are salvaged don’t need to be synthesized by energetically expensive de novo pathways
What are the two major purine salvage enzymes?
adenine phosphoribosyl transferase (APRT)
hypoxanthine guanine phosphoribosyl transferase (HGPRT)
What is the function of adenine phosphoribosyl transferase (APRT)?
adds PRPP to adenine to make AMP
What is the function of hypoxanthine guanine phosphoribosyl transferase (HGPRT)?
adds PRPP to guanine or hypoxanthine to make GMP or IMP
What is Lesch-Nyhan syndrome?
deficiency of HGPRT resulting in compulsive self-mutilation, aggressiveness, spacisticity, intellectual disability
Describe the breakdown of GMP.
GMP –> guanosine –> guanine –> xanthine –> uruic acid
via xanthine oxidase
excretion in urine
Describe the breakdown of AMP.
AMP --> IMP via adenosine deaminase IMP --> inosine --> hypoxanthine --> xanthine via xanthine dehydrogenase xanthine --> uric acid via xanthine oxidase excretion in urine
What is gout?
excess uric acid produces crystalline, insoluble deposits of calcium and urate in the joints –> pain and inflammation
associated with high purine diet, alcohol, renal disease, deficiencies in salvage enzymes, and ketoacidosis
How do you treat gout?
low purine diet
ant-inflammatory agents (eg methotrexate/colchicine) decrease immune cell proliferation
allopurinol (suicide inhibitor of XDH/XO, buildup hypoxanthine, more soluble than uric acid)
Describe the pathway of pyrimidine biosynthesis.
- glutamine + CO2 + 2ATP –> Carbamoyl phosphate
via carbamyl synthase II (CPSII) - Asp + carbamoyl phosphate –> N-carbamoylaspartate + Pi
via aspartate transcarbamylase
Carbamoylaspartate –> –> –> orotate + PRPP –> CO2 + UMP
UMP –> UDP –> UTP
via nucleoside mono-/di-phosphate transferase
What regulates the pyrimidine biosynthesis pathway?
product inhibition (UTP inhibits step 1 and CTP inhibits 2nd step)
High levels of orotic acid in blood can be due to _________.
decrease in the urea cycle enzyme ornithine transcarbamylase
There are _____ reactions by which a deoxynucleotide can be converted to ribonucleotide, however, a ribonucleotide can be converted to deoxynycleotide by _______.
no
ribonucleotide reductase
What are the binding sites on ribonucleotide reductase?
active site (where catalysis takes place)
activity site
substrate specificity site (dictates which ribonucleotide diphosphate will be converted to a deoxyribonucleotide diphosphate at the active site)
What are the substrates for ribonucleotide reductase?
ADP, GDP, CDP, UDP
What regulates ribonucleotide reductase?
ATP stimulates synthesis of deoxyribo-nucleotides
dATP prevents more synthesis of deoxynucleotides
What is hydroxyurea?
a free radical quencher for tyrosine free radical (found in the R2 subunit of ribonucleotide reductase)
Describe the flow of nucleotides that bind to the RR complex.
- ATP (most abundant in the cell binds first at high concentrations)
- dTTP (stronger affinity than dCTP and when built up can displace ATP to slow pyrimidine and increase purine production)
- dGTP (increased concentrations displaces dTTP)
- dATP (accumulates to signal that all necessary deoxynucleotides have been produced)
What does binding of ATP to the RR complex do?
increase CDP –> dCDP —-> dCTP
increase UTP –> dUTP —-> dTTP
What does binding of dTTP to the RR complex do?
decrease CDP –> dCDP decrease UDP –> dUDP
increase GDP –> dGDP —-> dGTP
What does binding of dGTP to the RR complex do?
decrease GDP –> dGDP
increase ADP –> dADP —-> dATP
What is SCIDS?
severe combined immunodeficiency is a deficiency in adenosine deaminase –> buildup of dATP inhibits ribonucleotide reductase –> low levels of B and T cells
