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Amino Acid Catabolism
Amino Acids
1. essential - excess are converted to metabolic intermediates such as:
pyruvate, acetyl-CoA and are precursors for fatty acids, glucose, ketone
bodies
2. nonessential - biosynthesis
Amino Acid Breakdown (3 Stages)
1. Deamination – remove an amino group
2. Incorporation of ammonia and aspartate nitrogen atoms into urea for
excretion
3. Conversion of amino acid carbon skeletons to common metabolic
intermediates
-ketoglutarate and glutamate play central roles in
amino acid catabolism
Stage 1 – Deamination - Remove -amino group
Purpose: excrete excess nitrogen and degrade the remaining carbon
skeleton or convert it to glucose
Main way = Transamination (2 Stages)
i.
AA + enzyme → -keto acid + enzyme-NH2
ii.
-ketoglutarate + enzyme-NH2 → enzyme + glutamate
Enzyme requires PLP which acts as a temporary store of amino groups until the
next substrate comes in. (“ping-pong” mechanism)
Amino transferases
Now…
How do we get the amine group off of glutamate?!?!?
Oxidative deamination via Glutamate dehydrogenase (GDH)
-first committed step on the pathway to nitrogen excretion
- instead of just “swapping” amino groups, there is a NET LOSS of nitrogen
***GDH is unique because it is one of the only enzymes that does not discriminate
between NAD+ and NADP+
Stage 2 of Amino Acid Breakdown
Incorporation of ammonia and aspartate nitrogen atoms into urea for excretion
(The Urea Cycle)
Living organisms excrete excess nitrogen in 1 of 3 ways.
1. as ammonia – ex) aquatic animals
2. as urea – ex) people
3. as uric acid – ex) birds and reptiles
Urea is synthesized in your liver, secreted into the bloodstream and sequestered
by the kidneys for excretion into the urine. Urea is very soluble, but still
requires an appreciable amount of water to remove it from the kidneys. Why
is it so vital to get rid of? Ammonium salts are toxic and can cause vomiting,
convulsions and ultimately coma and death when the blood concentration
exceeds 0.25 mM.
2
Net Reactions
1Glutamate + H2O + NADP+ ---> -KG + NH4+ + NADPH + H+
Aspartate + NH3 + 3ATP + H2O + HCO3- --->
2ADP + AMP + PPi + 2Pi + fumarate + urea
fumarate + NAD+ ---> oxaloacetate + NADH
1Glutamate + oxaloacetate ---> -KG + aspartate
2 glutamate (NH3) + HCO3- + 3ATP + NAD+ --->
2 -KG + urea + 2ADP + AMP + 4Pi + NADH
NADH = 3ATP
Stage 3: Recycling the skeletons
Alanine: the simplest case
COOH
C
CH3
NH3+
Ala is glucogenic and
pyruvogenic
-ketoglutarate
Alanine
aminotransferase
UREA CYCLE
glutamate
O
-OOC
C
Pyruvate
CH3
Per 2NH3
4 ATP are
consumed by
the urea cycle
Degradation of alanine costs 2ATP!!
How then do we get energy out of pyruvate?
Pyruvate ---> acetyl-CoA + CO2 = 1 NADH
Isocitrate ---> -ketoglutarate + CO2 = 1 NADH
-ketoglutarate ---> succinyl-CoA + CO2 = 1 NADH
Succinyl-CoA ---> succinate = 1 GTP
Succinate ---> fumarate = 1 FADH2
Malate ---> oxaloacetate = 1 NADH
So: Pyruvate = 4 NADH + 1 FADH2 + GTP
1 NADH = 3 ATP
1 FADH2 = 2 ATP
1 GTP = 1 ATP
1 NH3 = -2 ATP
Then: Pyruvate = 15 ATP
If: Ala = pyruvate + NH3
Then Ala = 15 ATP - 2 ATP = 13 ATP
Asparagine
COOH
C
CH2
O
C
H
A
O
C
C
NH2
B
NH3+
H
H
CH2
-ketoglutarate
aspartate
aminotransferase
COO-
O
C
CH2
C
NH3
+
Aspartate
O-
NH3+
O-
asparaginase
H
COO-
O
O-
+ NH3
glutamate
-OOC
C
O
CH2
C
O-
Oxaloacetate
Asp = oxaloacetate + NH3
Asn = oxaloacetate + 2NH3
How then do we get energy out of oxaloacetate? It
is not degraded by the TCA cycle!
Run malate dehydrogenase in reverse
H
COO-
H
Malate
H
C
O
-OOC
H
H
C
H
O
A
O
Oxaloacetate
COO-
H
O
-OOC
H
C
H
C
NH2
NH2
N
Malate Dehydrogenase
(reverse rxn)
R
Loss of one NADH
N+
R
Malic enzyme: malate dehydrogenase, decarboxylating
B
H
O
COO-
O
C
C
H
NH2
O
COO-
O
C
C
CH2
Malate
NH2
N+
C
O
CH2
O-
R
N
C
NADP+
O-
O
NADPH
R
COOCOOO
Pyruvate
C
-O
C
CO2
CH2
H
CH3
A
Malate dehydrogenase + Malic enzyme = Net conversion of one
NADH to one NADPH
Oxaloacetate + NADH + NADP+ ---> pyruvate + CO2 + NAD+ + NADPH
So…oxaloacetate = pyruvate - NADH + NADPH
If: pyruvate = 4 NADH + 1 GTP + 1 FADH2
Then: oxaloacetate = 3NADH + GTP + FADH2 + NADPH
If: Asp = oxaloacetate + NH3 and
Asn = oxaloacetate + 2 NH3
Then: Asp = 3 NADH + GTP + FADH2 + NH3 + NADPH and
Asn = 3 NADH + GTP + FADH2 + 2 NH3 + NADPH
If: NADH = 3 ATP
GTP = ATP
FADH2 = 2 ATP
NH3 = -2 ATP
Then: Asp = 10 ATP + NADPH and Asn = 8 ATP + NADPH
Glutamine
H
COO-
O
C
C
CH2 CH2
H
A
NH2
H
B
NH3+
H
C
CH2 CH2
C
C
CH2 CH2
O-
NADP+
O-
Glutamate
dehydrogenase
COO-
O
NH3+
H
Glutaminase
COO-
NADPH + NH3
O
C
NH3+
Glutamate
O
O-OOC
C
O
CH2 CH2
C
-ketoglutarate
O-
How do we get energy from -ketoglutarate?
If: -ketoglutarate ---> succinyl-CoA + CO2 = 1 NADH
If: Succinyl-CoA ---> succinate = 1 GTP
If: Succinate ---> fumarate = 1 FADH2
Then: -ketoglutarate to malate = 1 NADH, 1 GTP, 1 FADH2
If: malate to pyruvate converts NADH to NADPH
If: Pyruvate = 4 NADH + 1 GTP + 1 FADH2
Then: -ketoglutarate = 4NADH + 2 GTP + 2 FADH2 + NADPH
So… -ketoglutarate = 4NADH + 2 GTP + 2 FADH2 + NADPH
If: Glu = -ketoglutarate + NH3 + NAPDH and
Gln = -ketoglutarate + 2 NH3 + NAPDH
Then: Glu = 4 NADH + 2GTP + 2FADH2 + NH3 + 2 NAPDH and
Gln = 4 NADH + 2GTP + 2FADH2 + 2NH3 + 2 NAPDH
If: NADH = 3 ATP
GTP = ATP
FADH2 = 2 ATP
NH3 = -2 ATP
Then: Glu = 16 ATP + 2 NADPH and Gln = 14 ATP + 2 NADPH
Alanine
Pyruvate + NH3
Cysteine
Pyruvate + S2- + NH3
Serine
Pyruvate + NH3
2 Glycine
Pyruvate + CO2 + 2 NH3 + NADH
Threonine + glycine
Pyruvate + Acetyl-CoA + 2 NH3 + CO2 + 2NADH
Aspartate
Oxaloacetate + NH3
Asparagine
Oxaloacetate + 2 NH3
Glutamate
-ketoglutarate + NH3 + NADPH
Glutamine
-ketoglutarate + 2 NH3 + NADPH
Arginine
-ketoglutarate + urea + 2 NH3 + 2 NADPH
Proline
-ketoglutarate + NH3 + 2 NADPH
Histidine
-ketoglutarate + N5,N10-methenyl THF + 3 NH3 + NADPH
Methionine + serine (Costs 3 extra ATP)
methylated acceptor + 2 NH3 + pyruvate + S2- +
NADH + succinyl-CoA
Isoleucine (Costs 1 extra ATP)
Acetyl-CoA + succinyl-CoA + NH3 + 2 NADH + FADH2
Valine (Costs 1 extra ATP)
Succinyl-CoA + NH3 + 3 NADH + FADH2
Leucine (Costs 1 extra ATP)
3 Acetyl-CoA + NH3 + NADH + FADH2
Lysine
2 Acetyl-CoA + 3 NADH + 2 CO2 + 2 NH3
Tryptophan (Costs 2 NADPH)
Formate + pyruvate + 2 acetyl-CoA + 2 NH3 + 3 CO2 + NADH
Tyrosine (Costs ascorbate)
Fumarate + 2 acetyl-CoA + NH3 + CO2
Phenylalanine (Costs ascorbate and NADPH)
Fumarate + 2 acetyl-CoA + NH3 + CO2
When there is lots of
glucose
When there is a
glucose deficiency
but plenty of protein
High fat diet
The lipostat theory