Download Amino Acids

Metabolisme Protein
Lecture 9

Amino acid digestion cont…

Amino acids classification

Intestinal absorption and utilization of amino
acids
Trypsin Inhibitors in the Diet

Diets can contain protease inhibitors
–

Trypsin inhibitors in soybeans
Resistance of the soy protein to digestion
also contributes significantly to increases in:
–
–
–
–
Pancreatic protein
Trypsin
Chymotrypsin
Pancreatic enlargement
Soy Protein

The major soy storage protein glycinin and
trypsin inhibitors are rich in disulfide bonds

Dietary soy bean trypsin inhibitor causes the
release of cholecystokinin (CCK)
–
Stimulates pancreatic enzyme secretion and
pancreatic enlargement
Enteropeptidase
Trypsinogen ---------------------------- Trypsin
 Trypsin
 Chymotrypsin
 Carboxypeptidases

Disruption of the disulfide bonds may change
the three-dimensional structure of the
proteins so that they lose their bioactivity
(such as trypsin inhibition activity or potential
allergenicity)

Become more susceptible to digestive
enzymes
Chicken Study with Treated Soy
Protein
ADG (g/d)
Pancreas (g)
25
2
20
1.5
15
1
10
0.5
5
0
0
Treat Cont
Treated Soy
Treat Cont
G:F
Treated Soy
Protein Efficency Ratio
0.5
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.4
0.3
0.2
0.1
0
Treat Cont
Treated Soy
Treat Cont
Treated Soy
PER =
Body weight gained (g)
Protein consumed
Amino Acids

Amino acids can be classified by side its side
groups or by the presence pr absence of net
charge
–
–
–
–
–
–
Neutral such as Alanine, Glycine, Valine etc…
+ve such as Arginine, Histidine, Lysine
-ve such as Aspartic acid, Glutamic acid
Aromatic such as Tyrosine and Tryptophan etc…
Acidic such as Glutamine, Asparagine etc…
Basic such as Lysine and Histidine etc…
Essential vs Non-essential amino acids


What is essentiality?
–
Amino acids that must be supplied by the diet
–
Cannot be synthesized at a rate sufficient to meet
the nutritional requirements of the animal
20 different amino acids commonly
incorporated into proteins, but not all are
essential
Non-essential Amino Acids

Some amino acids can be synthesized by
the body and don’t need to be provided via
the diet
–
Non-essential
Alanine
Asparagine
Glutamine
Glycine
Aspartic acid
Cysteine
Glutamic acid
Proline
Serine
Tyrosine
Essential Amino Acids
Arginine
Histidine
Isoleucine
Methionine
Phenylalanine
Threonine
Leucine
Lysine
Tryptophan
Valine




MATT HILL VP
Histidine and Arginine not essential for maintenance of most
adult mammals
Arginine essential for birds
Poultry also require glycine and proline for feathers and rapid
growth
Dietary Limiting Amino Acids

The first limiting amino acid (essential amino acid) is
the amino acid present at the lowest amount in the
diet relative to the animal’s amino acid requirement

The amount of this amino acid will determine the
level of production

All or nothing attitude for amino acids
First Limiting Amino Acid
Corn-soybean meal based diets
–
–
Lysine is usually the first limiting amino acid
Followed by tryptophan, threonine, and
methionine
140
% of requirement

120
100
80
60
40
20
0
His
Ile
Leu
Lys
Met
Phe
Thr
Trp
Val
H.H. Stein
Cats

Higher crude protein requirement
–

Inability of nitrogen catabolic enzymes in the cat’s
liver to adapt to changes in dietary protein intake
–
–

Increased needs for maintenance of normal body tissue
Low conservation of nitrogen under low CP diets
Liver enzymes function at high rates of activity  high
catabolism of protein
Dietary Arginine, Taurine, sulfur amino acids and
aromatic amino acids (eg. phenylalanine, tryptophan,
and tyrosine) are in high demand
Protein Deficiency

Reduced appetite and feed intake

Reduced birth weights and growth

Reduced colostrum and milk production

Decreased hormonal production

Decreased fertility
Intestinal Absorption and Amino Acid
Transport

After digestion, the resulting amino acids and
peptides (di and tri) are efficiently absorbed by
enterocytes

Occurs along the entire small intestine
–

Most proximal (upper) small intestine
Absorption requires carriers (active transport) to
bring the amino acid across the brush border
membrane, but some passive absorption occurs
–
Active transport is important when lumen amino acid levels
are low
Amino Acid Transport Systems

These carrier systems traditionally been
designated using a lettering system
–
Upper case for sodium-dependence
–
Lower case for sodium-independence (more
passive transport)
–
Some exceptions (eg. System L, does not require
Na+)

Competition between transporter systems
–
The affinity (Km) of a carrier is influenced both by
the hydrocarbon mass of the side chain of the
amino acid and by the net electrical charge



Branch chain amino acids absorbed faster than smaller
amino acids
Neutral amino acids absorbed faster than basic or acidic
amino acids
Essential absorbed faster than non-essential
Amino Acid
Transport System
Sodium
Required
Amino Acids Carried
L
No
Leucine, other neutral
B
Yes
Phenylalanine, tyrosine, tryptophan,
isoleucine, leucine, valine
IMINO
Yes
Proline, glycine
y+
No
Basic
X-AG
Yes
Aspartate, glutamate
B0,+
Yes
Most neutral and basic
b0,+
No
Basic and neutral
y+L
No/Yes
Basic and neutral
ASC
Yes
Alanine, serine, cysteine
t
No
Tryptophan, phenylalanine, tyrosine
asc
No
Alanine, serine, cysteine
N
Yes
Glutamine, asparagine, histidine
ag
No
Aspartate, glutamate
Sodium
Coupled Amino
Acid Transport
1.
Sodium binds
to the amino
acids
transporter
2.
Binding
increases
carrier affinity
for amino acids
which bind to
the carrier
Sodium
Coupled Amino
Acid Transport
3.
4.
5.
Co-transporter
forms
Carrier complex
conformation
changes and
the sodium/
amino acids
enters the
cytosol
Sodium is
pumped out of
the cell by a
Na+, K+ATPase
Peptide Absorption (ie. PEPT1)
1.
Peptides are coupled to
H+ and transported into
the intestinal cell
2.
H+ pumped back into the
intestinal lumen in
exchange for Na+
3.
Na+, K+ ATPase pumps
Na+ out of the cell in
exchange for K+ across
the basolateral
membrane
Gropper et al 2008
Intestinal Basolateral Membrane
Transport of Amino Acids



Amino acids use similar
transporters to cross
the basolateral
membrane and into the
interstitial fluid
Free amino acids only
Many amino acids
absorbed into
enterocytes are not
transported out
Intestinal Basolateral Membrane
Transport of Amino Acids
Amino Acid
Transport
System
Sodium
Required
Amino Acids Carried
L
No
Leucine, other neutral
y+
No
Basic
X-AG
Yes
Aspartate, glutamate
b0,+
No
Basic and neutral
A
No/Yes
Alanine, neutral, short chain, polar
ASC
Yes
Alanine, serine, cysteine, other 3-4 carbon
amino acids
t
No
Tryptophan, phenylalanine, tyrosine
GLY
Yes
Glycine
asc
No
Alanine, serine, cysteine
Protein Metabolism in the Enterocyte

Many amino acids absorbed following
protein digestion are used by the intestinal
cells for energy or protein synthesis
–
–
–
–
–
Structural proteins
Nucleotides
Apoproteins
New digestive enzymes
Hormones

Amino acids may be partially metabolized to
other amino acids or compounds, that may
be released into portal blood

Intestines use 30-40% and internal organs
use up to 50% of the essential amino acids
absorbed from the diet
–
90% of glutamate absorbed is thought to be used
by the intestines
Intestinal Metabolism- Glutamine

Glutamine not used for energy, may be
partially catabolized to generate ammonia
and glutamate
–
–
Ammonia enters the portal blood for liver uptake
or may be used by the intestine for carbamoyl
phosphate synthesis
Glutamate may undergo transamination (removal
on amino group) to form alpha-ketoglutarate


Intermediate in the Tricarboxylic acid (TCA) cycle
Amino group is transferred to pyruvate to form alanine
 portal blood and to the liver