Final: AA Metabolism (Ben) Flashcards
What is transamination?
Which AAs can be transaminated?
- Reversible interconversion of amino acids and keto acids
- Uses PLP bound to Lys (via Schiff base) of aminotransferase enyzme to exchange -oxo and -amino groups
- All AAs except Lys, Thr, Pro and HydroxyPro
What are the rxns catalyzed by the enyzmes Alanine Aminotransferase and Aspartate Aminotransferase?
And what can their plasma elevation indicate?
-
Glutamate + Pyruvate α-Ketoglutarate + Alanine
- indicates liver damage (enyzme AKA ALAT/GPT)
-
Glutamate + Oxaloacetate α-KG + Aspartate
- indicates MCI (enzyme AKA ASAT/GOT)
What is oxidative deamination in terms of AA metabolism?
Which AA is deaminated most and by what enyzme?
What inhibits/activates this (in the forward direction)?
- removal of nitrogen from an AA in the form of ammonia
- most often Glu** is deaminated by **Glu Dehydrogenase
- Glu + H2O + NAD+ α-KG + NH4+ + NADH + H+
- ATP/GTP or NADH inhibit
-
ADP/GDP activate
- b/c ADP means low E + thus need for Krebs intermediates
What are the first two steps of the urea cycle and where do they take place?
-
Carbamoyl Phosphate Synthase I (RATE-LIMITING)
- CO2 + NH3 —> Carbamoyl Phosphate
- uses 2 ATP –> 2 ADP + 1 Pi
- N-acetyl glutamate enhances ATP affinity
- deficiency = hyperammonemia type I
-
Ornithine Transcarbamoylase
- Ornithine + Carbam. Phosph. —> L-citrulline + Pi
- deficiency = hyperammonemia type II
- both are in mitochondria of liver cells
After movement of L-citrulline into the cytosol…
what are the cytosolic reactions of the urea cycle?
-
Arginosuccinate Synthase
- Citrulline + Aspartate + ATP —> Arginosuccinate + AMP + PPi
- (b/c 2 macroergic bonds broken, 2 “ATP equivalants” used)
-
Arginosuccinate Lyase
- Arginosuccinate —> Fumarate + Arginine
- (fumarate goes to Krebs, makes more Asp via oxaloacetate + ASAT to refuel cycle)
-
Arginase
- Arginine + H2O —> Urea + Ornithine
- (urea transported to kidney, ornithine re-enters cycle at OTC step)
Urea Cycle Overview…
How much ATP is used?
What are the enyzmes?
Where do the products go?
-
3 ATP used, 4 ATP “equivalents”
- 2 by CPT-I and 1 by ASS (but 2 bonds broken, so 2 equivalents)
- Urea Nitrogens:
- 1st from ammonia and 2nd from Aspartate
- Enzymes:
- Carbamoyl Phosphate Synthase I –> Ornithine Transcarbamoylase –> Arginosuccinate Synthase –> Arginosuccinate Lyase –> Arginase
- Products:
- Urea goes to kidneys for excretion
- Fumarate goes to TCA for Asp regeneration
- Ornithine re-enters cycle
What are the 3 main routes for regulation of the urea cycle?
- Enzyme Induction - via high protein diet/starvation
- Allosteric Activation - of CPT-I via N-acetyl-Glu
- Intermediate Concentration - Arg from diet, Orn from synth
Other than transamination and oxidative deamination…
what 3 types of deamination can occur?
(via what enyzmes, etc.)
-
Catabolic α-Amino Removal
- via Serine/Threonine Dehydratases forming pyruvate/α-KG respectively and using PLP as a cofactor ( to remove H2O and add =O )
-
Flavoprotein α-Amino Removal
- via L-amino oxidases** with an **FMN prosthetic group
-
β/γ-Amino Removal
- via glutaminase** or **asparaginase hydrolyzation of -R group amines
What are 3 routes for incorporation of free ammonia into larger molecules?
- Glutamate Dehydrogenase - alpha-KG + amm. = Glu
- CPT-I - incorporates NH3 into urea cycle
-
Gln Synthetase
- Glu + NH3 + ATP —> Gln + ADP + Pi
How can Asn be synthesized from other AAs?
Asparagine Synthetase
Gln + Asp + ATP —> Glu + Asn + AMP + PPi
How can glutamine also contribute to urinary nitrogen excretion?
Gln acts as a sort of transport form of NH3 in the blood (b/c of its two amine grps).
Once in the kidney, glutaminase** can remove the -R group amide (making Glu) and **transaminases can remove the alpha-amino group (making alpha-KG), both of which nitrogens can be excreted as ammonia.
Which AAs are ketogenic, glycogenic and both?
-
Both Glyco- and Ketogenic:
- Ile, Thr, Phe, Tyr, Trp
- I’ll Throw (a) Phew Tyres (and you’ll) Trip
-
Ketogenic only:
- Leu, Lys
- (AAs starting with L make products usually made from Lipids)
-
Glycogenic only:
- Gly, Cys, Glu, Gln, Asp, Asn, Ser, Pro, Val, Met, His, Ala
- (all others…)
What B-vitamin, along with single-carbon units donated by amino acids, contributes to the formation of various nucleotides?
What is the 1st step in the utilization of this vitamin?
Folic Acid / Folate (B9)
-
Dihydrofolate Reductase
- uses 2 NADPH to reduce folate in two steps
- 1st to H2F, then to H4F
After formation of H4F…
What two amino acids can contribute single-carbon units directly to H4F?
What results from this?
-
Tryptophan
- makes N10-Formyl-H4F which can go into purine synthesis (or lose H2O to become methenyl-H4F)
-
Histidine
- makes N5-formimino-H4F which continues to form other precursors
After addition of a carbon via His…
what happens to N5-formimino-H4F next?
- It can lose NH4 to become N5,N10-methenyl-H4F
- Then Glycine or Serine can contribute to the molecule to form N5,N10-methylene-H4F w/ use od NADPH
- methylene**-H4F can go on to **TMP synthesis (from dUMP)
What is the last step in the modification of folic acid to form biosynthetic intermediates?
- N5,N10-Methylene-H4F is reduced via Methylene-H4F Reductase (MTHFR)to form N5-Methyl-H4F (using NADH)
- Methyl-H4F can enter the SAM cycle
- remember motherfucker for MTHFR enzyme!
Describe the SAM cycle and its 4 enyzmes.
-
Homocysteine Methyltransferase (w/ methyl-B12)
- HCys + Methyl-H4F –> Methionine + H4F
-
Met Adenosyl Transferase
- Met + ATP –> S-Adenosyl-Met + PPi + Pi
-
SAM-dependent Methylases
- SAMe –> S-Adenosyl-Hcy + -CH3
-
S-A-Hcy Hydrolase
- S-A-Hcy + H2O –> Adenosine + Hcy
How can Cysteine be synthesized?
Where do the precursors come from + whats the prosthetic grp?
Cystathionine Synthase
plus PLP
- Homocysteine + Serine —> Cystathionine + H20
- Cystathionine + H2O –> Cysteine + α-ketobutyrate + NH4+
- the HCys comes from the SAM cycle
Which AAs can form succinyl-CoA?
What must they first become to enter the succinyl-CoA forming pathway?
(And how do they get there?)
Met, Thr, Val, Ile
(Met Three Valiant Islanders)
- all first become α-ketoacids
- Met –(SAM cycle)–> HCys –(Cysta. Synthase)–> α-ketobutyrate
- Thr –(Thr Dehydratase)–> α-ketobutyrate
- Val, Ile –> α-ketoacids
Once Met/Thr/Val/Ile have been converted to α-ketoacids…
how are they converted to succinyl-CoA?
4 steps
-
Branched-Chain α-Ketoacid DH** (TPP, lipoic acid, FAD, NAD+, CoA)**
- α-ketoacid –> propionyl-CoA + CO2 + NADH + H+
-
Propionyl-CoA Carboxylase (+ biotin)
- propionyl-CoA + ATP –> D-methylmalonyl-CoA + ADP + Pi
- Methylmalonyl-CoA Racemase (switches D -> L)
-
Methylmalonyl-CoA Mutase (+ 5-adenosyl cobalamine B12)
- L-methylmalonyl-CoA –> succinyl-CoA
If Ile and Val both can eventually form succinyl-CoA…
why is one only glycogenic while the other is both glyco- and ketogenic?
- Isoleucine has more carbons in its R group and thus must give off an acetyl-CoA on the way to becoming succinyl-CoA
- acetyl-CoA can go on to form ketone bodies
What kind of products can Leu form (glyco-/keto-) ?
How?
Leu is Ketogenic
- like other BCAAs it forms an α-ketoacid via BCAA Transaminase
- then BC-αKA-DH turns it into isobutyryl-CoA which undergoes beta oxidation-like rxns to form HMG-CoA and then Acetyl-CoA and Acetoacetate
- produces NADH and FADH2 (like B-oxidation)
Which AAs form oxaloacetate?
How?
Asp and Asn
- Asn + H2O –(asparaginase)–> Asp + NH4+
- —(ASAT)—> Oxaloacetate
- remember ASAT uses PLP
Which AAs form pyruvate?
Two of them do so in one reaction… which?
Ala, Ser, Gly, Cys
Al + Sarah Glide (into the) Cysteine (chapel)
-
Alanine becomes pyruvate directly via ALAT
- remember ALAT uses PLP
-
Serine Dehydratase
- Serine —> Pyruvate + NH4+
- also uses PLP, water is removed but then re-used
