Proteins/Amino Acids
Preliminary Concepts
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Protein is important in building of organs
and soft structures of the animal body
A continuous supply is needed from feed
sources throughout life for growth/repair
Food protein  body protein
Food protein: plant or animal
Unique proteins found in each animal
Roles of Protein
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bulk composition (structural parts of the cell)
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oxidative metabolism (used as energy source)
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enzymes (regulate and influence metabolism)
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plasma proteins (circulating immune bodies)
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hormones
Proteins within Cell Wall
Elementary Composition of
Proteins
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Chemical composition: primarily carbon,
hydrogen, oxygen
additional difference: nitrogen in amino
groups (~17%)
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may contain sulfur, phosphorus and iron
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structure is complex
Structure of Protein
Molecule
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Proteins are sequences of amino acids
hooked together by the amino group
(NH3)of one to the carboxyl group (COOH)
of another called a peptide bond.
Protein chains of AA have typically 100200 residues
Many proteins have more than one chain
The Peptide Linkage
Protein Structure
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primary: the sequence of AA’s forming the protein
secondary: forces generated by the close proximity
of one AA residue to another (e.g.,  helix design or 
pleated sheet)(i.e., certain amino acids can form bonds
with others, if close enough, cysteine)
tertiary: bending of one AA chain due to attraction
of individual AA’s distant from each other
quaternary: packing of chains together
Protein Structure
Amino Acids (AA)
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As mentioned, proteins are polymerized
residues of amino acids
the number and proportion of AA vary
from protein to protein
when proteins are denatured, the AA
remain
to study protein, you must study AA
at least 30 different AA, some essential
others non-essential
Characterizing AA
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Most AA are made using short-chain fatty
acids (FA; such as acetic, proprionic or
butyric acid)
naturally-occurring have L-configuration
synthetic have large proportion of D
configs
“D” vs. “L”
Configuration
Aliphatic Amino Acids
Aromatic Amino Acids
Chemical Determination of
Protein
The direct determination of protein in
tissue is impractical due to
quantity/variation
 nitrogen, however, occurs at fairly
constant levels:
 [N] x 6.25 = protein level
 some proteins have well-known nitrogen
levels (e.g., milk @ 15.7% N)
 determined by Kjehldal N methodology
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AA Quality
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Amino acids divided into two groups:
essential: those the animal cannot synthesize
in sufficient quantity to support maximum
growth, typically dietary in nature
nonessential: synthesized by animal body,
typically non-dietary in nature
Essential AA
Exceptions
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lysine (LYS)
arginine (ARG)
methionine (MET)
histidine (HIS)
isoleucine (ILE)
leucine (LEU)
threonine (THR)
tryptophan (TRY)
phenylalanine (PHE)
valine (VAL)
ser/gly essential for chicks
pigs don’t need ARG, HIS,
LEU for maintenance
no big problem for
ruminants, why?
All essentials are in “L” form
only humans really need HIS
Do we need protein?
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A protein requirement is really an EAA
requirement (why?)
To say protein “requirement” for fish: 25-50%
says nothing about requirement: it doesn’t
measure intake.
Why? Variance due to culture conditions
Unfortunately, not all sources of protein are
“balanced”, not all are digestible.
Factors Affecting Protein
“Requirement”
Size of fish/shrimp
 Water temperature
 Feed allowance/feeding rate
 Amount of non-protein energy sources
 Quality of protein (AA)
 Availability of extrinsic sources of nutrition
 Salinity (affects digestibility)
 Physiological/nutritional state
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Additional Protein
Requirement Info
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3 g catfish require up to 4x more protein
intake on daily basis vs. 250 g catfish
pond sources of protein are typically protein
dense (over 50% protein on DM basis)
protein “requirement” can be reduced by
feeding more frequently w/attractant
(why?)
Net Protein Utilization (NPU) for most
aquatics is around 40%
could vary with enzyme activity, molt status
in crustaceans
Requirements for Amino Acids?
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Somewhat variable due to “apparent” nature of
determinations
no standardized methodology can be applied due
to differences in feeding behavior, treatment
system design, way in which EAA is presented,
etc.
for fish, the EAA requirements are similar to
those of other animals (all similar???)
major difference is with ARG (Table 2.4, Lovell)
EAA Requirements of Several
Fishes, Chickens and Swine
Amino
Channel
Tilapia
Acid
Catfish
nilotica
Chicken
Swine
ARG
4.3
4.2
5.6
1.2
HIS
1.5
1.7
1.4
1.2
ILE
2.6
3.1
3.3
3.4
LEU
3.5
3.4
5.6
3.7
LYS
5.1
5.1
4.7
4.4
MET+ CYS
2.3
3.2
3.3
2.3
PHE + TYR
5.0
5.7
5.6
4.4
THR
2.0
3.6
3.1
2.8
TRY
0.5
1.0
0.9
0.8
VAL
3.0
2.8
3.4
3.2
Requirement for Lysine by
Fish
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Sciaenops ocellatus
Oreochromis aurea
Oncorhyncus tshawytscha
Ictalurus punctatus
Dicentrarchus labrax
Morone saxatilis
Cyprinus carpio
4.43%
4.30%
5.00%
5.00%
4.82%
3.4-4.0%
5.70%
Sparing EAAs
Requirement for one EAA can be
partially mitigated by a NEAA
 example: CYS sparing of MET
 CYS replaces about 60% of MET
 often reported as MET-CYS
requirement
 example: TYR sparing of PHE (about
50%)
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EAA Requirements for
Shrimp?
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the quantitative requirement for only two essential amino
acids has been determined for shrimp: ARG, LYS
difficulty: crustaceans are sloppy eaters, and they don’t
effectively use crystalline* sources, experimental
conditions allow cannibalism, extrinsic sources of EAA
production too (bacteria)
*Purified amino acids from bacterial culture.
Crystalline Amino Acids (CAAs)
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Most EAA requirement studies have used CAAs
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CAAs produced by bacteria means pure!
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can help reduce formulation cost of feeds because
they are 99% digestible (few; 75-85% in others)
problems: reduced palatability, leaching, rapid
uptake
Amino Acid Metabolism:
protein synthesis
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Complex process occuring in most animal tissues
involving DNA, RNA and ribosomes
chromosomal DNA is storeplace of genetic
information, transmission from one generation to
the next
DNA = 4 nucleotides: adenine, guanine, cytosine,
thymine
Protein Synthesis
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DNA controls formation of RNA
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tRNA transfers amino acids to ribosomes
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ribosomes are the source of protein synthesis
(anabolism)
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protein synthesis (about 50 seconds/protein) (fast!!)
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amino acids also catabolized for energy
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transamination or oxidative deamination
Protein Digestibility
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Dietary protein quality is determined by its
bioavailability to the animal
“bioavailability” is not simply digestibility, it also
includes assimilation and incorporation of the AA into
protein
most common index of protein bioavailability is
apparent protein digestibility (APD)
APD = % of protein not rejected as feces
Protein Digestibility
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APD depends upon degree of purity of proteins
involved
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purified: gelatin, casein, soy-isolate
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semi-purified: hi-pro soybean meal, glutens
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practical: fish meal, squid meal, peanut meal,
rice bran, etc.
Protein Digestibility
Contrary to popular beliefs, animal
protein is not more “digestible” than
plant protein
 digestibility really determined by level
of purification and degree of
interaction (competition for absorption
sites) between one nutrient and another
 factors: salinity (indirect), size/age
(indirect)
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Amino Acid Digestibility
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apparent amino acid digestibility (AAAD) is directly
related to protein digestibility
proteins vary in APD, but amino acids don’t in terms of
AAAD (proteins compete with other nutrients, AA’s
don’t)
amino acids are typically absorbed in the gut (fish) and
midgut/midgut gland (shrimp)
6 transport mechanisms: 1) neutral AA’s (mono’s), 2)
basic (diamino’s), 3) acidic (dicarboxylic’s), 4) aromatics,
5) alanine and 6) glycine
Amino Acid Assimilation
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“assimilation” is not transport, it involves the
appearance of AA’s in various tissues (blood,
hemolymph, muscle, etc.)
appearance OK for intact-sourced AA’s, but
rapid and unsynchronized for CAA’s (too
much, too quickly)
CAA’s possibly used with increased feeding
frequency
Amino Acid Toxicity/Antagonism
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Toxicity/antagonisms are result of dietary
imbalances in EAA
when one EAA is fed in excess it can also
increase the requirement for another,
structurally-similar EAA
toxicity = overfeeding of one EAA and negative
effects not mitigated by increasing other EAA
antagonism = one EAA regulates uptake of
another
LEU/ILE in catfish (Robinson, 1984)
LYS/ARG in shrimp (Fox, 1992)