Amino Acid Metablism
1
Ex Biochem c8-AA metabolism
Overview of AA metabolism

Principal source of AA from food protein




AA pool: AA in blood and extracellular fluids




No ability to store AA, extra AA used as fuels
Very small compared to total protein in body
AA and protein turnover very quickly
Liver responsible for much of AA metabolism


Broken down to free AA, absorbed into blood
>50% dietary AA taken up by liver
Essential AA: can not synthesized by body, arg, his,
Isoleu, leu, lys, met, phenylala, threonine, tryptophan, val
Kidney in smaller extent
Skeletal muscle the largest repository of free and
protein-bound AA in body
2
Ex Biochem c8-AA metabolism
3
Ex Biochem c8-AA metabolism
Overview of AA metabolism
4
Ex Biochem c8-AA metabolism
AA transporters


AA have charged groups, they need protein
transporters to transfer between extracellular and
intracellular compartments
2 broad categories of AA transporters



Na-dependent: move into cell down Na concentration
gradient, can be moved against AA concentration
gradient
Na-independent
AA transporters may have broad specificity,
recognizing several AA


Some have narrow specificity, recognizing only 1-2
closely related AA
Competition for the same transporters
5
Ex Biochem c8-AA metabolism
Degradation of AA

Balance among AA can be achieved by conversion
reactions


18 AA are glucogenic: provide all or part of their
carbon atom for gluconeogenesis


One AA changed into another by transfer of amino group
Ketogenic: leucine, lysine
AA undergo constant oxidative degradation:




Normal synthesis and degradation, not immediately used
for protein synthesis
Ingest more AA than body can use to make proteins
Starvation
Overtaining, imbalance in protein turnover
(testosterone/cortisol ratio)
6
Ex Biochem c8-AA metabolism
Transamination reactions

Transfer of amino groups in all AA except thr, lys




Freely reversible, net direction depend on relative
concentration of 4 reactants
Alanine aminotransferase (glutamate pyruvate
transaminase, GPT)


Aminotransferase enzyme, transaminase
Most transfer to a-ketoglutarate, making glutamate
Alanine + a-KG < pyruvate + glutamate
Aspartate aminotransferase (glutamate oxaloacetate
transaminase, GOT)

Asp + a-KG < oxaloacetate + glutamate
7
Ex Biochem c8-AA metabolism
Transamination reactions
8
Ex Biochem c8-AA metabolism
9
Ex Biochem c8-AA metabolism
Deamination reactions

Nitrogen from amino groups in liver in the form of
glutamate can be released as ammonia



Production of NH4 and its release from muscle
proportional to exercise intensity



Glutamate + H2O + NAD+  a-KG + NADH + H+ +
NH4+
Glutamate dehydrogenase
Glutamate DHase reactions
Deamination of AMP by AMP deaminase
NH4+ play a role in peripheral and central fatigue


Increased acidity in muscle
Cross blood brain barrier, increased NH4+ uptake by
brain
10
Ex Biochem c8-AA metabolism
Glutamine

Special AA even though not essential


Free glutamine concentration high in variety
of cells and in blood



Important fuel for gut and immune system
(macrophages, lymphocytes)
~60% AA pool
Mostly synthesized from glutamate by glutamine
synthetase
Deamination of glutamine

Glutaminase: glutamine + H2O  glutamate +
NH4+
11
Ex Biochem c8-AA metabolism
Glutamine synthesis
12
Ex Biochem c8-AA metabolism
Branched-chain amino acids, BCAA


Most common essential AA in proteins
Metabolized mainly in skeletal muscle





Increased BCAA oxidation during exercise, used as fuel
or provide carbon backbone for CAC intermediates
BCAA aminotransferease, branched chain ketoacid
dehydrogenase (BCKAD), acyl-CoA DHase
BCKAD inhibited by BCKAD kinase, response to
exercise
Glucose-alanine cycle: transfer amino group from
muscle to liver for urea synthesis
Leucine can enhance protein synthesis by
stimulating initiation of translation

mTOR
13
Ex Biochem c8-AA metabolism
14
Metabolism
of BCAA
Ex Biochem c8-AA metabolism
15
Ex Biochem c8-AA metabolism
Glucose-alanine cycle
16
Ex Biochem c8-AA metabolism
17
Ex Biochem c8-AA metabolism
Urea cycle

NH4+ very toxic, especially to brain



Muscle release alanine, glutamine







N from BCAA  glutamate  ala or gln
Nitrogen in liver


Temporary safe forms: glutamate, glutamine
Converted to urea, secreted in urine
Ala  glu by alanine aminotransferease
Gln  glu by glutaminase
NH4+ taken up from blood
The above 3 provide NH4+ for urea synthesis
The other NH3 in urea from aspartate
Regulation point: carbamoyl phosphate synthetase
Metabolically expensive: 4 ATP for 1 urea
18
Ex Biochem c8-AA metabolism
19
Ex Biochem c8-AA metabolism
20
Ex Biochem c8-AA metabolism
The Urea Cycle -Overview
CO 2 + N H 4 +
2 A TP
2 A DP + 2 H2 O
+
O
H 2 N -C- OPO 3 2 -
H 3 N -CHCH2 COO -
Carbamoyl phosphate
O
H2 N -C- NH2
Aspartate
COO-
H
Urea
cycle
C C
-
Urea
COO-
OOC
H
Fumarate
21
Ex Biochem c8-AA metabolism
The Urea Cycle
O
H2 N -C- OPO 3 2 -
N H3 +
N H2
C O
NH
( CH2 ) 3
( CH2 ) 3
CH- NH3 +
COOOrnithine
CH- NH3 +
COOCitrulline
COO
+
H3 N -CHCH2 COO -
Aspartate
N H2 COOC N -CHCH2 COO NH
( CH2 ) 3
CH- NH3 +
COOArgininosuccinate
22
Ex Biochem c8-AA metabolism
The Urea Cycle
O
H2 N -C- NH2
Urea
N H3 +
( CH2 ) 3
CH- NH3 +
COOOrnithine
N H2
C N H2 +
NH
N H 2 COOC N -CHCH2 COO NH
( CH 2 ) 3
CH- NH 3 +
COOArginine
( CH2 ) 3
COO-
H
-
C C
OOC
H
Fumarate
CH- NH 3 +
COOArgininosuccinate
23
Ex Biochem c8-AA metabolism
Fate of AA carbon skeletons



18 AA can be source for gluconeogenesis
Leucine and lysine only form acetoacetyl
CoA and acetyl CoA: ketogenic
Many AA have carbon skeletons as CAC
intermediates or substances directly related
to CAC
24
Ex Biochem c8-AA metabolism
25
Ex Biochem c8-AA metabolism
26
Ex Biochem c8-AA metabolism
AA metabolism during
moderate-intensity exercise

Study of AA metabolism during exercise is complex




During ex in postabsorptive state, skeletal muscle is in net
protein catabolic state


Measurement of AA differences between arterial and venous blood
Muscle biopsy
Only see the equilibrium between AA synthesis and breakdown,
unless use stable isotope
Most AA produced by net protein catabolism released into blood,
except glutamate and alanine
Net uptake of glutamate at rest and even more during
exercise, glutamate used as precursor for glutamine


Release of ala and gln far out of proportion to their content in
skeletal muscle, synthesized in skeletal muscle at accelerated rate
during exercise
Ala release decline with exercise duration, less glucose  pyruvate
27
Ex Biochem c8-AA metabolism
AA metabolism during
moderate-intensity exercise

Exercise in low-glycogen, protein breakdown
greater, corresponding increase in release of most
AA from muscle


Decrease in total adenine nucleotide content during
prolonged exercise



Induce protein breakdown to release BCAA, use BCAA
as fuel
Prevent AMP accumulation, AMP  IMP by adenylate
deaminase
Can reduce TAN by up to 50%, need to regenerate
adenine
purine nucleotide cycle, use aspartate, cost GTP

Predominantly after exercise, activities of the enzymes
involved too long to produce appreciable AMP during ex
28
Ex Biochem c8-AA metabolism
29
Ex Biochem c8-AA metabolism
Purine nucleotide cycle
30
Ex Biochem c8-AA metabolism
Purine nucleotide cycle
31
Ex Biochem c8-AA metabolism
Purine nucleotide cycle
32
AA metabolism during
high-intensity exercise

Only modest increase in glutamine and
alanine release from muscle, when exercise
intensity is high



Ex Biochem c8-AA metabolism
Glutamine synthesis require ATP and glutamate
Glutamate is source of a-KG, CAC intermediate
Increased adenylate deaminase reaction
during high-intensity exercise


Recruitment of type II muscle fiber, in which
adenylate deaminase activity is high
Increase release of NH4+, IMP is trapped within
muscle
33
Ex Biochem c8-AA metabolism
Central fatigue theory

voluntary maximal work of the muscle < the work
when motor nerve was electrically stimulated




Tryptophan as precursor for serotonin synthesis



Fatigue in central nervous system
Increased production of serotonin in brain
Increased ammonia entry to brain
Most tryptophan bind to albumin
↑ FFA during exercise compete for albumin, ↑free Trp
BCAA compete with trp for blood brain barrier


BCAA supplementation helpful? Endurance exercise?
most animal studies support the theory, but most human
studies failed to show benefit effect
34
Ex Biochem c8-AA metabolism
Central fatigue theory
35
Central fatigue –
supplementation of CHO and BCAA
Ex Biochem c8-AA metabolism
36
Ex Biochem c8-AA metabolism
37
Ex Biochem c8-AA metabolism
38
Additional roles for AA


Precursors for many biologically active
compounds
Neurotransmitters




Tyrosine: dopamine, norepinephrine, epinephrine
Histidine: histamine
Tryptophan: serotonin, role in central fatigue
Maintain cellular redox state: glutathione
Ex Biochem c8-AA metabolism
Glutathione
-
N H3 +
O
O
H
N
O
N
H
O
SH
Glutathione, GSH
(reduced form)
O - 2e oxidation
2e - reduction
O
-
N H3 +
O
O
O
-
H
N
O
O
O
N
H
S
N
H
O
A dis ulfide
bond
S
O
O-
H
N
N H3 +
O
Glutathione, GS-SG
(oxidized form)
O
O-
39
Ex Biochem c8-AA metabolism
Some Small Peptides
+
O
O
H3 N -CH- C-N H-CH -C-OCH3
CH2
CH2
COO-
+
O
H3 N -CH2 - CH 2 -C-N H- CH-COO CH2
C6 H5
L-As partyl-L-phenylalanine
methyl ester
(Aspartame)
NH
-Alanyl-L-histidine
(Carnos ine)
N
40
Ex Biochem c8-AA metabolism
41
Ex Biochem c8-AA metabolism
Tamaki et al, 1992
Aguiar et al, 2013
42