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Proteins and amino acids

Essential part of living organisms
Synthesized in all cells
Must come form diet unless synthesized from nonprotein N source
% required by diet decreases with age
% required increases with production and lactation


Variable in many aspects:
Chemical composition
Physical properties
Biological function

Made up of?

Protein is made up of amino acids
More than 200 aa, but 20 are most common
10 essential amino acids
carboxyl group and amino group


sequence of aa held by peptide bonds


twists of aa into coilded structure


rigid structure of layers
H and disulfide bonds


large proteins, several tertiary linked, harder to digest complex proteins


Shape, solubility, other
Conjugated proteins


Albumins, globulins, glutelins, prolamines, histones, protamines
Soluble in water or dilute acid/base


Insoluble in water or digestive enzymes
Collagens, keratins, elastins

Conjugated proteins

Conjugated proteins
Wide array of compounds and nonprotein structure


Stable through electrostatic, hydrophobic and H bonds
Membrane protein in animal cells


Chondroitin sulfates (cartilage, tendon, skin)
Mucoproteins (mucous, ovalbumin)

Synthesis of protein

Chain of amino acids
Linked by peptide bonds
Elongation - add more aa to form tri or polypeptides
Feedstuff may have many proteins, but need adequate composition of amino acids

Essential vs. Non-essential AA

Essential - need in the diet
Non-essential - can by synthesized by tissues in the body

Ruminants and herbivores AA

can use non-protein N to supply amino acids.

Cats require


Poultry require


Functions of proteins

1. Structural unit in animal body
Collagen, elastin, contractions, blood, keratins
2. body metabolism
Enzymes, hormones, antibodies
3. source of energy after deamination

Examples of tissue proteins...



content increases with age, contributes to toughness in meat, insoluble in water and digestive enzymes


found with collagen, elastic, insoluble in water and digestive enzymes


proteins - in sarcoplasm

Contractile proteins

muscle contractions ,actin, tropomyosin B, myosin


hair, wool, feathers, hooves, claws, beaks

Blood proteins

albumin and globulins


100s in each cell with specific function


carried in blood, not all are proteins


Proteolytic enzymes
Efficiency determines what is absorbed(enzyme avaliability)
AA composition important
Enzymes - hydrolyze peptide bonds


pepsinogen pepsin

Small intestine

from pancreas
Trypsinogen trypsin
Chymotrypsinogen chymotrypsin
Procarboxypeptidase carboxypeptidase


Intestinal epithelium acts as barrier
AA - active transport
Di- and Tripeptides
Whole protein

AA - active transport

Neutral, dibasic, acidic, imino
Requires energy

Di- and Tripeptides

Actively absorbed by peptide transporter (PEPT1)
Mostly in growing ruminants

Whole protein

Engulfed by mucosal cells
(imunoglobulins from colostrum)

Post absorption of AA

1. Tissue protein synthesis
2. Synthesis of enzymes, hormones, and metabolites
3. Deamination or transamination - use C skeleton for energy


remove amine group and then enters urea cycle. c skeleton can then be used for energy


transfer amino group from one AA to C skeleton of keto acid

Synthesis and Degradation of Proteins

Protein synthesis requires nucleic acids
DNA - deoxyribonucleic acid
Chromosomes, genetic info
RNA - ribonucleic acid
Controlled by DNA
3 types - ribosomal, transfer, messenger
mRNA controls synthesis of proteins


phosphate-linked deoxyribose and 4 nitrogenous bases
Adenosine - Thymine
Cytosine - Guanine
Double helix structure


transfer info from DNA to protein
Ribose vs. deoxyribose
Uracil vs. thymine
Codons determine AA and thus protein

Protein Turnover

Synthesis and degradation occur at the same time
"Protein turnover"
Continuous and simultaneous
Degraded by cathepsins and calpains in muscle
Balance between the 2 determines net protein accretion
Measure by use of radioisotopes (N and C)
Precursors to endpoint

Can measure protein

Can measure this by using radio isotopes that have precursors to protein and track

Protein Turnover

Growth occurs in young and old
Regulated by polypeptides which influence protein metabolism, development and maintenance at cellular, tissue and organ level
GH and I

Urea Cycle

Key metabolic phenomenon in protein metabolism
Ammonia from deamination and ultimately forms urea
5 enzymes
Mitochondrial AA transporters
Urea excreted in urine.

Synthesis of AA

Three main sources in GI tract
1. diet
2. recycled
3. synthesized by microorganisms

synthesized by microorganisms

Ruminants and herbivores (horse, rabbit)
Most AA derived from microbial production
Bacteria and protozoans in rumen and lower GI tract
Synthesize AA in presence of ammonia, S, C source

Rumen Microbial Protein

Microbial crude protein (MCP) - N from:
dietary protein
nonprotein nitrogen (NPN)
Recycled N
MCP amino acids then flow through omasum, abomasum to SI
40% of nonammonia N in SI
NPN is useful from ruminal bacteria synthesis of amino acids and protein
Diet formulated to meet needs of bacteria

Rumen undegraded protein (RUP)

Quality of protein from microbial synthesis?
RUP allows protein to pass through the rumen
Digested in small intestine

Degradation of AA

Intestinal tract by microbes
Major products - ammonia, S, FA, CO2
Liver is primary organ
Also in SI
Various enzymatic reactions and pathways

N cycling in intestine

N from aa, di- and tripeptide and recycled
Endogenously produced (urea)
Urea - remove ammonia
Ammonia incorporated into AA for microbial protein

Requirements and deficiencies

Primarily for AA
Monogastric vs. ruminant
Most common nutrient deficiency
energy sources low in protein
protein supplements are expensive
Greater requirement with growth
Ratio of protein:calories is important
Protein only used for energy when:


AA deficiency similar
One feedstuff will be deficient in some AA

Signs of protein deficiency

Reduced growth rate
Reduced N balance
Reduced efficiency of feed utilization
Reduced birth weight

Measures of nutritive value of proteins

True protein = only AA
NPN (non protein N) = contains N, but not true protein
CP (crude protein) = total protein (true protein + any other N source)
%N x 6.25
Digestible protein = portion of CP that can be digested
BV (biological value) = evaluate protein source
% of N absorbed from GI tract and available for body functions

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