PROTEIN DEGRADATION AND SYNTHESIS IN THE RUMEN

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PROTEIN DEGRADATION AND SYNTHESIS IN THE RUMEN

• Reference
• Church Chapter 12, pp. 227-249

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PROTEIN SOURCES FOR RUMINANTS

• True protein
• Nonprotein nitrogen (NPN)
• In feeds
• Free amino acids,
• Nucleic acids
• Ammonia
• Nitrates
• Urea
• Biuret
• Recycled N
• Urea
• No N2 fixation

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PROTEIN DIGESTION IN RUMINANTS

• Rumen
• True protein
• NPN
• 
  Undegraded
  Small intestine
• 
  
  Metabolizable
• Degraded
  
  protein
• Recycled via
• saliva
  
• (20 of dietary N) NH3
  Microbial
• 
  protein
• NH3
• 
  Liver
• Urea
  Kidney
  Excreted

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• Ruminal degradation of true protein
• Two steps
• Proteolysis-slow
• Deamination-rapid
• By ruminal bacteria and protozoa
• All proteases are cell-bound
• Bacterial proteases are extracellular require
  attachment
• Protozoal proteases are intracellular engulf
  feed and bacterial proteins
• Endproducts
• C-chains
• Volatile fatty acids
• Straight chain (Acetic acid)
• Branched chain (Isovaleric, Isobutyric, 2-methyl
  butyric acids)
• Ketones
• Ammonia
• CO2

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• Effects of carbohydrate fermentation on
  deamination
• Fermentable carbohydrate adequate for growth
• Most amino acids incorporated into microbial
  protein directly
• Fermentable carbohydrate limiting growth
• Most amino acids deaminated to C-chains and NH3
• Ruminal protein degradation is not totally
  desirable
• There is always some loss of NH3
• Reduces efficiency
• Increases N excretion
• Valuable to have protein escape ruminal
  degradation in animals with high protein
  requirements

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• Factors affecting ruminal protein degradation
• Protein source
• 
  Ruminal degradation,
• Corn
  60
• Soybean meal
  
• Solvent processed 75
• Expeller processed
  50
• Corn gluten meal
  40
• Corn gluten feed
  80
• DDG
  55
• Alfalfa
  80
• Urea
  100
• Protein sources that resist ruminal degradation
  are referred to as undegraded, escape or
  bypass proteins

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• Reasons for differences in ruminal degradability
  between protein sources
• Structure
• Intermolecular disulfide bonds inhibit
  degradation
• Solubility
• Soluble proteins tend to degrade faster, but
  there are insoluble proteins that are degradable
  at different rates
• Therefore, solubility does not always
  degradability

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• Physical barriers in feed reduce protein
  degradation
• Forage cell walls
• Cross-linking of proteins
• Tannins in birdsfoot trefoil
• Increasing rate of passage decreases ruminal
  protein degradation
• Increased feed intake
• Feed processing
• Grinding or pelleting
• Heat treatments
• 100 C for 4
  hours
• Soybean meal
  Reduced protein
• 
  degradation
• Excessive heat will reduce protein digestion
• Coating proteins with materials resistant to
  degradation
• Feeding monensin
• Inhibits growth of bacteria that only use amino
  acids as energy sources

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• Ruminal degradation of NPN compounds
• Urea CO2 2NH3
• Very rapid
• Urease primarily found in bacteria near rumen
  wall
• Biuret 2CO2 3NH3
• Slow
• Needs adaptation
• Allows synchronization on NH3 and energy
  availability for microbial protein production
• NO3 NO2 NH3
• Slow

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• N recycling to rumen
• Primarily as urea from the blood
• Blood urea sources
• Directly from urea synthesis in the liver from
• Excess NH3 absorbed from rumen or large intestine
  
• NH3 produced inefficient tissue metabolism of
  amino acids
• Resorption of urea from the kidney
• Increases on low protein diets

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• Routes of N recycling
• Saliva
• 15 to 50 of total recycled N
• Factors
• Blood urea concentration
• Saliva flow
• Gut wall
• Major route
• Factors
• Low ruminal NH3
• Upregulates a urea transporter which increases
  
• transfer of urea from blood to
  epithelium or vice
• versa
• Increases microbial urease activity of
  microbes
• adhered to the rumen wall
• increases conversion on urea to NH3 at rumen
  wall
• Decreased ruminal pH
• Converts NH3 to NH4 in the rumen
• Only NH3 can cross the rumen wall

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Urea Diffusion into RumenUpdate
Rumen wall Urea transporter Blood urea
Urea High NH3 inhibits NH3
Bacterial population
Urease
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• Effects of dietary protein concentration on
  N-recycling (Marini et al. 2003)
• CP N recycled (saliva) N
  recycled (Gut wall)
• g/d of
  total g/d of total
• 9.1 0.8 3.0
  25.1 97.0
• 11.8 1.5 3.6
  39.6 96.4
• 15.7 3.8 10.4
  32.7 89.6
• 18.6 5.4 13.7
  33.9 86.3
• Importance of N recycling
• Supplies N on low protein diets
• Infrequent supplementation of protein
• Minimizing N excretion

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Bacterial ProteinSynthesis in the Rumen
NH3 Amino acids Peptides VFA Amino
acids Microbial Fermentation proteins CHOH
VFA
Microbial protein synthesis related to 1.
Available NH3 and amino acids (DIP) 2.
Fermentation of CHOH - Energy
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• Requirements for bacterial protein synthesis
• N
• Cellulolytic bacteria
• NH3
• Starch digesting bacteria
• Amino acids and peptides
• Branched-chain volatile fatty acids
• Cellulolytic bacteria
• Energy from carbohydrate fermentation
• Minerals
• S
• P

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• Factors affecting microbial protein production in
  the rumen
• Ruminal NH3-N concentration
• Microbial
  Ruminal NH3-N
• protein
• ( of Max)
  5 mg
• 12
• Crude protein in diet,
• Rate of ammonia release
• Urea
• NH3
  Treshold
• 
  Biuret
• 2
• Time after feeding, hours

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• Energy level of the diet
• Energy and C-skeletons from carbohydrate
  fermentation are needed by rumen bacteria to
  produce microbial protein from ruminal NH3
• Standard relationship
• Bacterial CP (g/d) 0.13 x TDN consumed (g/d)
• Generally results in greater bacterial protein
  synthesis on high energy diets
• However, bacterial protein synthesis efficiency
  coefficient varies with
• Passage of liquid digesta from the rumen
• High feed intake gt Low feed intake
• High quality forage diet gt Very high
  concentrate diet
• High quality forage diet gt Very low quality
  forage diet
• Rumen pH
• High rumen pH gt Low rumen pH

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Efficiency of Microbial Growth
Slow Low rumen passage pH
Bacteria Low quality use energy to
forages slow pump protons passage
G BCP/100 g TDN 8 13
TDN, feed DM
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• Sulfur level of diet
• Sulfur needed to synthesize S-containing amino
  acids
• Particularly important when NPN fed
• Required NS ratio
• 101

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• Advantage of ruminant CP digestion
• Ability to utilize NPN
• Limitations of ruminant CP digestion
• Loss of ruminal NH3
• Inefficient
• Increases environmental N loading
• Increases energy requirements
• Urea toxicity
• Occurs when blood NH3gt60 mg
• Causes
• Feeding excess urea (gt 1 of ration dry matter)
• Inadequate energy fed with urea
• Poor mixing of urea in diet
• High blood NH3 toxic to brain cells

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• Protein digestion in the abomasum
• Abomasum secretes hydrochloric acid and
  pepsinogen
• Hydrochloric acid converts pepsinogen into the
  active protease, pepsin
• Peptides hydrolyzes ruminal undegraded protein
  and microbial protein into peptides
• Small intestinal protein digestion in ruminants
• Pancreas secretes proenzymes of proteases
• Trypsinogen converted to the protease, Trypsin
• Chymotrypsinogen converted to the protease,
  Chymotrypsin
• Procarboxypeptidase converted to the protease,
  Carboxypeptidase
• Pancreatic proteases degrade proteins to peptides
  and amino acids
• Intestinal mucosa secretes peptidases that
  degrade peptides to amino acids

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• Composition and digestibility of microbial and
  ruminal escape proteins
• Microbial protein
• Composition
• 80 amino acids
• 15 nucleic acids
• 5 other NPN
• Digestibility of protein
• 80
• Ruminal escape protein
• Digestibility
• 80