10 – AA Digestion and Catabolism Flashcards
What are the 2 objectives of protein digestion?
- Digest dietary protein
- Reclaim digestive enzymes (they are protein too)
What percent of endogenous protein is of digesta protein?
- 30-50%
- *if endogenous protein is lost in EXCESS = may get a negative N balance even though dietary protein is adequate
o More is going to mucous (pooped out) and digestive enzymes
Stomach or proventriculus: protein digestion
- Acid environment
o Solubilizes and denatures proteins (=becomes linear)
o Disrupts H-bonding (secondary protein structure) - *protein denaturation assisted by PEPSIN
- Peristalsis mixes the gastric juices to aid in breakdown of proteins (mechanical digestion)
Duodenum and Jejunum: protein digestion
- Acidic gastric digesta is buffered by HCO3- ion to a slightly basic pH
- Pancreatic proteases are released
o Endolytic OR exolytic
What are some endolytic enzymes?
- Trypsin
- Chymotrypsin
- Elastase
- *hydrolyze peptide bonds in middle of protein
What are some exolytic enzymes?
- Carboxypeptidase A and B
- Amino peptidase
- *hydrolyze peptide bonds at carboxy or amino ends of proteins
Enzyme synergy
- Endolytic enzymes create more carboxy and amino ends
- Exolytic enzymes have more ends to work on
Proteins in the small intestine: enzymes and transporters in different membranes
- Brush border peptidases
- Brush border AA transporters
- Brush border di and tri-peptide transporters
- Intracellular peptidases
- Basolateral membrane AA transporters
- Basolateral membrane di and tri-peptide transporters
Jejunum and absorption of digestion products
- 2/3 AA are di and tri-peptides
- 1/3 are free AA
What are the 2 processes for how peptides and AA are absorbed?
- Diffusion
o Important when concentration of AA is high - Active transport
o Efficient even at low concentrations of AA
Active transport of AA
- Requires energy
- Many different AA transporters
- Several AA may use the same transporter (ex. Lys and Arg use system y+)
o Excess of Arg may restrict absorption of Lys by competition from AA transporter
o **AA balance is important
AA as energy substrates for the gut: CO2 production and maintenance energy requirements %
- Gut is 2-6% body weight
- Responsible for
o 10-20% of whole-body CO2 production
o 50-75% of maintenance energy requirements - *gut is energetically expensive (even more than the heart and brain=they are smaller)
Glutamine and glucose % utilized by gut for energy
- Glutamine: 90% (so only 10% will be absorbed)
- Glucose: 8-15%
What is the fate of absorbed AA?
- Once in mucosal cells, most peptides are broken down by proteases to free AA
- Free AA are transported across the mucosal membrane into the portal vein to the LIVER
What are the AA used for?
- Anabolism: protein synthesis
- Catabolism: excess AA oxidized for energy
What are the 3 conditions where AA catabolism may occur?
- Normal synthesis and degradation of cellular proteins
- Dietary protein excess
o If ingested excess AA=surplus is catabolized (AA can NOT be stored) - Starvation or in diabetes mellitus
o No CHO available or properly utilized=use proteins for fuel
Catabolism of AA nitrogen in mammals
- Amino groups produced by catabolism are extremely TOXIC
o Need to be converted to less toxic forms in mammals
What are the 4 steps to convert AA nitrogen to less toxic forms?
- Transamination
- Oxidative deamination
- Ammonia transport
- Urea cycle
Transamination
- AA has amino group removed and transferred to alpha-ketoglutarate (makes it glutamate)
- End up with
o Glutamate: ONLY AA that can be oxidative deaminated
o Alpha-ketoacid - **requires energy
Oxidative deamination
- Release of N from glutamate as ammonia which is catalyzed by L-glutamate dehydrogenase
- *forms glutamine and regenerates an alpha-ketoglutarate
Ammonia transport
- Free amino group (ammonia) is added to a glutamate molecule giving GLUTAMINE
o Transported to the liver
Urea cycle
- Glutamine releases amino group (ammonia) and you reform glutamate
- Amino group is combined with CO2
- *enters urea cycle and leads to urea synthesis
Production of urea from arginine
- Arginase converts arginine to ornithine and urea
Why don’t we just excrete ammonia?
- Ammonia is toxic
- By keeping amino group attached to AA=toxicity is avoided
- Urea is relatively non-toxic and easy to excrete in concentrated form
Ammonia intoxication: ruminants
- Caused by absorption of excess ammonia from the rumen (ex. too much in feed)
o Insufficient glucose to for alpha-ketoglutarate and therefore glutamine
Ammonia intoxication: cats (+carnivores)
- Low arginine diets can cause it
o Can be life threatening!
Ammonia intoxication symptoms
- Tremors
- Blurred vision
- Coma
- Death
What is the fate of carbon skeletons from the AA?
- Source of energy and glucose
o Either ketogenic or glucogenic or BOTH
Ketogenic AA
- Form ketone bodies
o Acetone, acetoacetate or beta-hydroxybutyrate
o *can be used to synthesize fatty acids
Glucogenic AA
- Form glucose
- **Essential during starvation
What is Kwashiorkor in humans?
- Condition resulting from protein deficiency
- Serum protein is severely decreased causing edema=swollen abdomen
o Serum proteins normally=have oncotic pressure (without them=body can’t keep the fluid in vasculature) - Often occurs in children after weaning in famine areas
In some animals, what does protein deficiency cause?
- Weight loss
- Muscle loss and weakness
- Poor digestion
- Poor coat
- Fluid build up in chest or abdomen
