Conversion of AA 21 Nucleotide synth 22 Flashcards
Porphyrins
Heme
cyclic compounds that readily bind metal ions
- usually Fe2+ or Fe3+
- formed by linkage of 4 pyrrole rings
Heme is one Fe2+ (ferrous) ion corrdinated in center of ring of protoporphyrin IX
The major site of heme biosynthesis is in the ____ and ____.
liver, erythrocyte producing cells of bone marrow
bone marrow active in Hb synthesis
The formation of ALA (d-aminolevulinic acid)
- requires what as a coenzyme
- why is this step imp
All Cs and Ns of the porphyrin molecule are provided by glycine(AA) and succinyl coA (TCA) via ALA synthase (ALAS)
Requires PLP as a coenzyme
1st step of Heme biosynthesis
Rate committed and RLS in porphyrin biosynthesis
Administration of drugs (increases/decreases) ALA synthase activity?
increase significantly
Drugs metabolized by P450 system
- (a hemeprotein oxidase system found in liver)
- In response to drugs, the synthesis of cytochrome P450 proteins ↑
- Enhanced consumption of heme
- Causes decrease in the [ heme ] in liver cells
- Lower intracellular [ heme ] → increase in ALAS1 → ALA synthesis
Why is ALA elevated and anemia seen in lead poisoning?
Condensation of two molecules of ALA → porphobilinogen is extremely sensitive to inhibition by heavy metal ions
Last 2 steps of heme synthesis
Copro-porphyrin-ogen III → Proto-porphyrin IX
Fe2+ (ferrous) ion binds spontaneously, but enhanced by ferrochelatase
Proto-porphyrin IX → Heme
Porphyrias
- common feature
- most common one (describe)
common feature: ↓ synthesis of heme
Accumulation of ALAS1 and toxic intermediates to compensate
Most common porphyria: Porphyria Cutanea Tardia
- deficiency in uroporphyrinogen decarboxylase (cant convert Uro-porphyrinogen III→ Co-proporphyrinogen III)
- patients are photosensitive
Degradation of heme → yellow urobilin + brown stercobilin
- Senescent (120 day old) RBCs are taken up and HEME is broken down to BILIRUBIN by macrophages of reticuloendothelial system (tissue macrophages, spleen, liver)
- BILIRUBIN binds to albumin and gets transported to liver, where it dissociates and enters hepatocytes
- BILIRUBIN is conjugated with glucuronic acid → BILIRUBIN DIGLUCORONIDE (more soluble)
- It is then transported into bile then intestine
- Intestinal bacteria removes glucuronic acid → UROBILINOGEN
- It can then go to the kidney → yellow urobilin (yellow pee)
Or it can stay in intestine → brown stercobilin
*this is why high levels of unconjugated bili is bad
PRPP
- what is it
- what does it do?
- how is it made
- Does Pi and purine ribonucleotides activate or inactivate it?
an “activate pentose” that participates in the synthesis and salvage of purines and pyrimidines.
PRPP is made from ATP and Ribose 5 P (HMPP)
Pi activates PRPP synthesis
Purine ribonucleotide inhibits
Committed step of purine nucleotide biosynthesis
- Which on activates and inhibits this step:
AMP, GMP, PRPP
Synthesis of 5’-phosphoribosylamine from PRPP + glutamine
(amide group of glutamine replaces pyrophosphate group of PRPP)
Activator: PRPP
Inhibitor: AMP, GMP (end products in pathway)
Nucleotide biosynthesis of IMP
the “parent” purine nucleotide
PRPP +4 ATP→ → → IMP
Folic acid analogs and what do they do
Methotrexate
inhibit the reduction of dihydrofolate to tetrahydrofolate
(competitive inhibitor to hihydrofolate reductase)
- they limit the amt of THF available for use in purine synthesis and slow down DNA replication in mammal cells.
- Useful in treating rapidly growing cancers!
- INTERFERES WITH PURINE SYNTHESIS
PABA anologs and what they do
- what does it require as a coenzyme?
- do they interfere with purine synthesis?
Sulfonamides
Competitively inhibit bacterial synthesis of folic acid.
Purine synthesis requires THF as coenzyme, and sulfa drugs slow it down.
- DO NOT INTERFERE WITH HUMAN PURINE SYNTHESIS
Conversion of IMP to AMP and GMP. What E sources do each of them need?
IMP →AMP : needs GTP
IMP → GMP : needs ATP
Both AMP and GMP are the end products that inhibit the first steps in both pathways
- that way IMP makes more of which ever one is needed more
Mycophenolic acid
The drug is a reversible inhibitor of IMP dehydrogenase which catalyzes the first step in IMP → GMP synthesis
- deprives rapidly proliferating T + B cells of nucleic acid components
- good 4 immunosuppressants
Conversion of nucleoside monophosphates to nucleoside diphosphates involve which enzyme?
AMP → ADP
GMP → GDP
What about diphosphates to triphosphates?
GDP → GTP
CDP → CTP
AMP → ADP
- adenylate kinase
GMP → GDP
- guanylate kinase
GDP → GTP
CDP → CTP
- both: nucleoside diphosphate kinase
APRT and HGPRT
Both enzymes convert purine bases to nucleotides + use PRPP as source of ribose 5-phosphate
Irreversible rxns
Hypoxanthine → IMP
Guanine → GMP
- both use HGPRT
Adenine → AMP
- uses APRT
Lesch-Nyhan syndrome
Complete deficiency of HGPRT
Inability to salvage hypoxanthine or guanine and end up excessive uric acid (end products of purine degradation)
- heritable cause of hyperuricemia
- also causes ↑ PRPP levels, ↓ IMP and GMP levels
(PRPP isnt used up to make IMP and GMP), which causes increased de novo purine synthesis
- Hyperuricemia
- Uroacid stones in kidney
- gouty arthritis
- motor dysfxn
- cognitive deficits
- behavioral distubances (self mutilation)
- biting lips/fingers
main enzyme required for reduction of purine and purimidine nucleoside diphosphates
- what does it do
- how is it inhibited? Activated?
ribonucleotide reductase
- responsible for maintaining balanced supply of DNA required for DNA synthesis
Binding of nTPs to substrate specificity sites (2nd site on E), regulates conversion of ribonucleotides to deoxyribonucleotides:
- binding of dATP to enzyme inhibits activity + prevents reduction of any of the 4 nDPs
- binding of ATP activates enzyme
Adenosine deaminase ADA deficiency
ADA deficiency causes a type of severe combined immunodeficency (SCID), involving T + B cells, and NK cell depletion (lymphocytopenia)
- adenosince cannot make inosine
- untreated: can die b4 2 yrs
tx: gene therapy
Gout can be dx by?
presence of NEGATIVELY birefringent monosodium urate crystals in aspirated synovial fluid
- examined by polarized light microscopy
Which of these, purine synthesis or pyrimidine synthesis is the ring synthesized first, before attaching the phosphate (which is donated by PRPP)?
Pyrimidine: ring is synthesized before being attached to ribose 5-P
(Purine : constructed on a preexisting ribose 5-phosphate)
Regulated step of pyrimidine synthesis
synthesis of carbamoyl phosphate from
Glutamine and CO2 + 2 ATP via carbamoyl phosphate synthetase II
CPS I cell location pathway involved source of N regulators
CPS1
cell location: mitochondria
pathway involved: urea cycle
source of N: ammonia
regulators: activator (N-acetyl-glutamate)
CPS II cell location pathway involved source of N regulators
CPS II
cell location: cytosol
pathway involved: pyrimidine synthesis
source of N: amide group of glutamine
regulators: activator (PRPP), inhibitor (UTP)
Both purine and pyrimidine synthesis requires what 3 things as essential precursors?
glutamine
aspartic acid
PRPP
UMP is necessary for pyrimidine synthesis. How is UTP made?
Glutamine + 2 ATP + carbamoyl phosphate synthetase II →→ OMP → UMP → UDP → UTP
(note that UDP is a substrate for ribonucleotide reductase, which generate dUDP, which can be P to dUTP)
First step in purine synthesis vs first step in pyrimidine synthesis
purine: - committed step uses PRPP + N from glutamine (PRPP is used as an "activated pentose" that provides the ribose-phosphate group for de novo purine and pyrimidine synthesis and purine salvage) - end products: AMP and GMP
Pyrimidine: - regulated step - production of carbamoyl phosphate by carbamoyl phosphate synthetase II (activated by PRPP) - end products: UMP →TMP
Gout results from what?
build up of uric acid, which is an end product of purine degradation.
- if gout is present, then there could be an overproduction of the end products of purine degradation
(pyrimidine met is NOT assoc. w/ uric acid production) Purine salvage (not degra) decreases uric acid production
