Protein Digestion

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Protein Digestion
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Monogastric Protein Digestion

• Whole proteins are not absorbed
• Too large to pass through cell membranes intact
• Digestive enzymes
• Break peptide bonds
• Secreted as inactive pre-enzymes
• Prevents self-digestion

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Monogastric Protein Digestion

• Initiated in stomach
• HCl from parietal cells
• Stomach pH 1.6 to 3.2
• Denatures 40, 30, and 20 structures
• Pepsinogen from chief cells
• Cleaves at phenylalanine, tyrosine, tryptophan
• Protein leaves stomach as mix of insoluble
  protein, soluble protein, peptides and amino acids

HCl
Pepsinogen
Pepsin
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Protein Digestion Small Intestine

• Pancreatic enzymes secreted
• Trypsinogen
• Chymotrypsinogen
• Procarboxypeptidase
• Proelastase
• Collagenase

Zymogens
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Monogastric Digestion Small Intestine

• Zymogens must be converted to active form
• Trypsinogen Trypsin
• Endopeptidase
• Cleaves on carbonyl side of Lys Arg
• Chymotrypsinogen Chymotrypsin
• Endopeptidase
• Cleaves carboxy terminal Phe, Tyr and Trp
• Procarboxypeptidase Carboxypeptidase
• Exopeptidase
• Removes carboxy terminal residues

Enteropeptidase/Trypsin
Trypsin
Trypsin
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Protein Digestion

• Small intestine (brush border)
• Aminopeptidases
• Cleave at N-terminal AA
• Dipeptidases
• Cleave dipeptides
• (Enterokinase or enteropeptidase)
• Trypsinogen ? trypsin
• Trypsin then activates all the other enzymes

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Trypsin Inhibitors

• Small proteins or peptides
• Present in plants, organs, and fluids
• Soybeans, peas, beans, wheat
• Pancreas, colostrum
• Block digestion of specific proteins
• Inactivated by heat

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Protein Digestion

• Proteins are broken down to
• Tripeptides
• Dipeptides
• Free amino acids

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Free Amino Acid Absorption

• Free amino acids
• Carrier systems
• Neutral AA
• Basic AA
• Acidic AA
• Imino acids
• Entrance of some AA is via active transport
• Requires energy

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Amino Acid Transporters Brush Border Membrane
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Peptide Absorption

• Form in which the majority of protein is absorbed
• More rapid than absorption of free amino acids
• Active transport
• Energy required
• Metabolized into free amino acids in enterocyte
• Only free amino acids absorbed into blood

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Absorption of Intact Proteins

• Newborns
• First 24 hours after birth
• Immunoglobulins
• Passive immunity
• Adults
• Paracellular routes
• Tight junctions between cells
• Intracellular routes
• Endocytosis
• Pinocytosis
• Of little nutritional significance...
• Affects health (allergies and passive immunity)

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In the Enterocytes

• First cells that can use the amino acids
• Transport into portal blood
• Protein synthesis
• Digestive enzymes
• Structure and growth
• Energy

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Basolateral Membrane

• Transport of free amino acids only
• Peptides are hydrolyzed within the enterocyte
• Transport mainly by diffusion and Na-independent
  carriers

Groff Gropper, 2000
Whole proteins are nutritionally insignificant...
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Protein Transport in the Blood

• Amino acids diffuse across the basolateral
  membrane
• Enterocytes ? portal blood ? liver ? tissues
• Transported mostly as free amino acids
• Liver
• Breakdown of amino acids
• Synthesis of non-essential amino acids

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Overview of Protein Digestion and Absorption in
Monogastrics
Groff Gropper, 2000
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Ruminant Protein Digestion

• Ruminants can exist with limited dietary protein
  sources due to microbial protein synthesis
• Essential amino acids synthesized
• Microbial protein is not sufficient during
• Rapid growth
• High production

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Protein in the Ruminant Diet

• Types of protein
• Dietary protein contains amino acids
• Rumen Degradable Protein (RDP) available for
  use by rumen microbes
• Rumen Undegradable Protein (RUP) escapes rumen
  fermentation enters small intestine unaltered
• Varies with diet, feed processing
• Dietary non-protein nitrogen (NPN) not true
  protein provides a source of nitrogen for
  microbial protein synthesis
• Relatively CHEAP - decreases cost of protein
  supplementation

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Ruminant Protein Feeding

• Feed the rumen microbes first (RDP)
• Two counteractive processes in rumen
• Degradation of (dietary) protein
• Synthesis of microbial protein
• Feed proteins that will escape fermentation to
  meet remainder of animals protein requirements
• Escape protein, bypass protein, or rumen
  undegradable protein (RUP)
• Aldehydes increase inter-protein cross-linking
• Heat treatment
• Utilization depends on
• Digestibility of RUP source in the small
  intestine
• Protein quality

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Protein Degradation in Rumen
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Rumen Protein Utilization

• Factors affecting ruminal degradation
• Rate of passage
• Rate of passage ? ? degradation ?
• Solubility in water
• Must be solubilized prior to degradation
• Heat treatment
• Degradation ?
• N (and S) availability
• Energy availability (carbohydrates)

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Protein Fractions

• Dietary proteins classified based on solubility
  in the rumen
• A
• NPN, instantly solubilized/degraded
• B1 B2 B3
• Potentially degradable
• C
• Insoluble, recovered in ADF, undegradable

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Ruminant Protein Digestion

• Rumen microbes use dietary protein
• Creates difference between protein quality in
  feed and protein actually absorbed by host
• Microbes break down dietary protein to
• Amino acids
• NH3, VFAs, and CO2
• Microbes re-synthesize amino acids
• Including all the essential amino acids from NH3
  and carbon skeletons
• No absorption of protein or amino acids from
  rumen (or from cecum or large intestine!)

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Rumen Protein Hydrolysis

• Process with multiple steps
• Insoluble protein is solubilized when possible
• Peptide bonds of solubilized protein are cleaved
• Microbial endo- and exo-peptidases
• Amino acids and peptides released
• Peptides and amino acids absorbed rapidly by
  bacteria
• Bacteria degrade into ammonia N (NH3)
• NH3 used to produce microbial crude protein (MCP)

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Microbial Crude Protein (MCP)

• Protein produced by microbial synthesis in the
  rumen
• Primary source of protein to the ruminant animal
• Microbes combine ammonia nitrogen and
  carbohydrate carbon skeleton to make microbial
  crude protein
• Diet affects the amount of nitrogen entering the
  small intestine as microbial crude protein

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Factors Limiting Microbial Protein Synthesis

• Amount of energy
• ATP
• Available nitrogen
• NPN
• Degraded feed intake protein nitrogen (RDP)
• Available carbohydrates
• Carbon residues for backbone of new amino acid
• Microbial crude protein synthesis relies on
  synchronization of carbohydrate (for carbon
  backbones) and nitrogen availability (for amino
  group)

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Microbial Protein Synthesis

• Synchronization of carbohydrate and N
  availability
• NPN supplementation
• Carbohydrates used for carbon skeleton of amino
  acids

Carbon backbone (from CHO fermentation)
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Microbial Protein Formation
Dietary Starch Sugar
Dietary Cellulose Hemicellulose
rapid
slow
Dietary NPN
Carbon Skeletons
Sulfur
Other Co-factors
rapid
Microbial Proteins
NH3
ATP
Amino Acids
slower
Dietary Insoluble RDP
very slow
Dietary Soluble RDP
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Nitrogen Recycling

• Excess NH3 is absorbed through the rumen wall to
  the blood
• Quickly converted to urea in the liver
• Excess NH3 may elevate blood pH
• Ammonia toxicity
• Costs energy
• Urea (two ammonia molecules linked together)
• Relatively non-toxic
• Excreted in urine
• Returned to rumen via saliva (rumination
  important)
• Efficiency of nitrogen recycling decreases with
  increasing nitrogen intake

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Nitrogen Recycling

• Nitrogen is continually recycled to rumen for
  reutilization
• Ability to survive on low nitrogen diets
• Up to 90 of plasma urea CAN be recycled to rumen
  on low protein diet
• Over 75 of plasma urea will be excreted on high
  protein diet
• Plasma urea enters rumen
• Saliva
• Diffuses through rumen wall from blood

Urease
Urea
Ammonia CO2
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Feed Protein, NPN and CHO
RUP
Feed Protein
AA
Feed Protein
RDP
NH3/NH4
NH3
SMALL INTESTINE
Feed NPN
Bacterial N
MCP
MCP
AA
NH4 loss
RUMEN
Salivary N
Liver
ATP
Blood Urea
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Ruminant Digestion and Absorption

• Post-ruminal digestion and absorption closely
  resembles the processes of monogastric animals
• However, amino acid profile entering small
  intestine different from dietary profile

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Overview of Protein Feeding Issues in Ruminants

• Rumen degradable protein (RDP)
• Low protein quality in feed ? very good quality
  microbial proteins
• Great protein quality in feed ? very good quality
  microbial proteins
• Feed the cheapest RDP source that is practical
  regardless of quality
• Rumen undegradable protein (RUP)
• Not modified in rumen, so should be higher
  quality protein as fed to animal
• May cost more initially, but may be worth cost if
  performance boosted enough

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Recycled urea
Salivary Urea
NH3 UREA
LIVER
NPN
Non-utilized Ammonia
Dietary Nitrogen
NH3
PEPTIDES
AMINO ACIDS
LEVEL TO PROVIDE FOR MAXIMUM MICROBIAL GROWTH
POOL
AMINO ACIDS
65 OF PROTEIN
AMINO ACIDS
RDP
PROTEIN
MICROBIAL PROTEIN
SMALL INTESTINE
35 OF PROTEIN
RUP
Reticulo-rumen