Final: Rate-Limiting/Regulatory/Important Steps (Ben) Flashcards
What is the rate limiting step of urea cycle ?
Regulation?
Deficiency?
Carbamoyl Phosphate Synthase I
CO2 + NH3 —> Carbamoyl Phosphate
uses 2 ATP –> 2 ADP + 1 Pi
N-acetyl glutamate enhances ATP affinity
deficiency = hyperammonemia type I
What is the committed step in de novo purine synthesis?
(plus regulation)
Gln:PRPP Amidotransferase
- PRPP + Gln + ATP –> Phosphoribosyl-amine + Glu + ADP + Pi
- inhibited by AMP/GMP/IMP
- stimulated by PRPP
How is ribose-5-P “activated”?
And how is this step regulated?
PRPP Synthetase
- R-5-P + ATP –> Phosphoribosyl phosphoamine + AMP
- inhibited by AMP/GMP
- activated by Pi… (allosteric)
In pyrimidine synthesis, there are two multi-action polypeptides that together perform 5 of the 9 enzymatic reactions which eventually result in CTP.
What are these two peptides called and which enzymatic actions does each of them have?
-
CAD
- has Carbamoyl-P Synthetase II, Aspartate Transcarbamoylase and Dihydro-orotase actions (rxns 1-3)
-
UMP synthase
- has Orotate Phosphoribosyl Transferase and OMP decarboxylase actions (rxns 5 + 6)
At what two steps is pyrimidine synthesis regulated and how?
-
CAD
- inhibited by UMP
- activated by PRPP and ATP
-
UMP synthase
- inhibited by UMP
What is the enzyme responsible for formation of deoxynucleotides from nucleotides?
How does it do this?
Ribonucleotide Reductase
- heterodimeric (R1/R2 subunits)
- reduces nucleotide diphosphates (UDP, GDP, ADP, CDP)
- reduces the 2’ OH of ribose using its -SH groups
- -SH groups oxidize to become S-S which is then reduced back to -SH via Thioredoxin Reductase (also has -SH grps which ox. to S-S)
- TRase is then reduced by FADH2 and FADH2 is reduced by NADPH
How is ribonucleotide reductase regulated?
2 ways
- Induction - is an S phase enzyme only induced in dividing cells
-
Allosteric
- the different deoxynucleoside triphosphates (not the direct diphosphate products of the enzyme) inhibit the synthesis of some dNTPs while enhancing the synthesis of others
- the specific way this inhibition works regulates the ratio of dNDPs made by RReductase
- remember the CUGA order of the nucleoside diphosphates substrates… starting with dTTP (2nd product), each product inhibits its own synthesis plus that of all products above it, while stimulating synthesis of the product just below it + ATP enhances synthesis of all products
What is the first and rate-limiting/committed/regulatory step in synthesis of protoporphyrin IX for heme?
ALA Synthase
- Succinyl-CoA + Glycine –> δ-aminolevulinate + CoA + CO2
- has a 6C intermediate (alpha-amino-beta-keto-adipate)
- is mitochondrial
How is the rate-limiting step in Heme synthesis regulated?
ALA Synthase has two isoforms
-
ALAS-L (liver)
- Heme inhibits transcription, mRNA export, mitochondrial uptake AND enzyme activity (allosterically)
-
ALAS-E (marrow)
- Erythropoietin stimulates transcription
- heme does not inhibit ALAS-E but does inhibit iron release from ferritin + stimulates globin synthesis
What is the rate-limiting step of catecholamine synthesis?
In what 3 ways is it regulated?
Tyrosine Hydroxlase
(with H4-Biopterin co factor)
- Noradrenaline - negative feedback inhibition
- Ca++ - stimulates the enzyme
- Phosphorylation - stimulates the enzyme (PKA, PKC, calmod-dependent K) by increasing H4-biopterin affinity
What is the enzyme responsible for glucuronidation?
Where is it?
What molecule does it use for glucuronic acid?
Notable substrates?
UDP-glucuronyl transferase (UGT) in the inner layer of ER membrane uses UDP-glucuoronate
- Acetaminophen (on the phenolic -OH)
- Ibuprofen (on a -COOH group)
- p-Aminosalicylic Acid (on the amino group)
What is the 2nd most important kind of conjugation?
Via what enzyme + where is it found?
Co-factor?
Substrates?
Sulfation via sulfotransferases in the cytosol usings PAPS (from PAPS synthase).
- estrone
- coumarin
- acetaminophen
- methyl-DOPA
- generally phenols, alcohols + aromatic amines
What are the enzymes involved in glutathion conjugation?
Substrates?
- Glutathione-S-transferases transfer Glu-Cys-Gly tripeptride (glutathione)
- Glutamyl transferase removes Glu
- Cysteinyl-glycinase removes Gly
- Acetyltransferase acetylates Cys to form mercapturic acid derivative
- Leukotrienes are glutathione conjugated… each successive numbered leukotriene is just a product of each of the above steps, in order
What 2 enzymes catalyze amino acid conjugation?
Substrates?
- Acyl-CoA Synthetase - adds CoA to molecule to be conjugated
- N-Acyltransferase - adds “activated” molecule to an AA (Gly/Tau)
Substrates: acyl-CoA derivatives of carboxylic acids
- benzoic acid
- salicylic acid
- bile acids
- nicotinic acid
