Amino acid digestion and catabolism

Question 1

two objectives of protein digestion

Answer 1

1. Digest dietary protein
2. Reclaim digestive enzymes (for reuse)

Question 2

What percentage of endogenous protein is included in the digested protein?

Answer 2

30-50%
-if endogenous protein is lost in excess than high negative balance will exist even though dietary protein is adequate

Question 3

Protein digestion overview

Answer 3

Question 4

What is occurring in the stomach or proventriculus?

Answer 4

Acidic environment (HCl= pH 1-3)
-solubilizes and denatures proteins
-disrupts H bonding/secondary structure
-protein denaturation assisted by pepsin
-peristalsis mixes the gastric juices to aid breakdown of proteins

Question 5

What is included in chemical vs mechanical digestion?

Answer 5

Chemical= acid and pepsin

Mechanical= peristalsis

Question 6

What happens in the duodenum and jejunum?

Answer 6

1. Acidic gastric digesta is buffered by HCO3- ion to a slightly basic pH

2.Pancreatic proteases are released
*enzymes either endolytic or exolytic

Question 7

Endolytic enzymes

Answer 7

Hydrolyze peptide bonds in the middle of protein
-trypsin, chymotrypsin, elastase

Question 8

Exolytic enzymes

Answer 8

Hydrolyze peptide bonds at the carboxy or amino ends of the protein
-carboxypeptidases A and B, Amino peptidase

Question 9

Endolytic and exolytic synergy

Answer 9

Endolytic enzymes create more carboxy and amino acids, and exolytic enzymes then have more ends to work with

Question 10

Protein absorption

Answer 10

Question 11

Amino acid composition in jejunum

Answer 11

-2/3 AA are present as di and tri peptides while 1/3 are free AA

Question 12

How are peptides and amino acids absorbed?

Answer 12

Diffusion and active transport
-diffusion= when concentration of AA is high
-Active transport when AA concentration is low

Question 13

Active transport of AA

Answer 13

-requires energy
-many different transporters and each of them has a preference for certain AA

eg. Lys and Arg use system y+
-excess of Arg may restrict Lys absorption by competition for AA transporter
**need a balance

Question 14

What % of gut is body weight?

Answer 14

Gut is 2-6% body weight

Question 15

Gut metabolism responsibilities

Answer 15

1. 10-20% of whole body CO2 production
2. 50-75% of maintenance energy requirements

Question 16

Glutamine and glucose use in the gut

Answer 16

Gut used 90% of glutamine

Gut uses 8-15% of glucose

Question 17

What happens to absorbed peptides and AAs?

Answer 17

-Once in mucosal cells, peptides broken down by proteases to free AA
-Free AAs transported across mucosal membrane into protal vein to liver

Question 18

What are the absorbed AAs used for?

Answer 18

1. Anabolism: protein synthesis
2. Catabolism: excess AA are oxidized for energy

Question 19

Removal of ammonia

Answer 19

Switch from glutamate to alpha-ketoglutarate, which is then used in TCA cycle

Question 20

What are the conditions for amino acid catabolism?

Answer 20

1. Normal synthesis and degradation of cellular proteins
2. Dietary protein excess
3. Starvation or in diabetes mellitus

Question 21

What happens when dietary protein in excess?

Answer 21

When ingested amino acids exceed need in the body, the surplus is catabolized
-AA cannot be stored!

Question 22

Amino acid catabolism during starvation or diabetes mellitus

Answer 22

When carbohydrates are unavailable or not properly utilized, the proteins are used for fuel

Question 23

Normal amino acid catabolism

Answer 23

Some AAs released during protein breakdown are oxidized if not needed for new protein synthesis

Question 24

Catabolism of AAs

Answer 24

-Extremely toxic; so once it happens, need to convert to less toxic forms in mammals

Question 25

Conversion of AA into less toxic forms

Answer 25

1. Transamination
2. Oxidative deamination
3. ammonia transport
4. Urea cycle

Question 26

Transamination

Answer 26

AA acid has its amino group removed and transferred to alpha-ketoglutarate
*need energy

RESULTS= glutamate and an alpha-ketoacid

Question 27

Oxidative deamination

Answer 27

The release of the NH3 from alpha-ketoglutarate by L-glutamate dehydrogenase

Question 28

Ammonia transport

Answer 28

The free amino group is added to glutamate molecule giving glutamine which is then transported to the liver

Question 29

Urea cycle

Answer 29

1.The amino group is released from the glutamine reforming glutamate

2. Amino group is combined with CO2, and enters the urea cycle and leads to urea synthesis

Question 30

Why can’t we excrete the ammonia?

Answer 30

Ammonia is toxic
-keep amino group attached to amino acids to avoid toxicity

Urea is non toxic and easy to excrete in concentrated form

Question 31

Ruminants ammonia intoxication

Answer 31

Caused by the absorption of excess ammonia from the rumen
-insufficient glucose to form alpha-ketoglutarate and therefore glutamine

Question 32

Cats ammonia intoxification

Answer 32

Low arginine diets can cause ammonia intoxication
*can be life threatening

Question 33

Symptoms of ammonia intoxification

Answer 33

-tremors
-blurred vision
-coma death

Question 34

What happens to the carbon skeletons left once urea formed?

Answer 34

Source of energy and glucose for TCA cycle
-AAs are ketogenic or glucogenic or both

Question 35

What are ketogenic AAs for?

Answer 35

Form ketone bodies (acetone, acetoacetate, beta-hydroxybutyrate) which can then be used to synthesize fatty acids

Question 36

What are glucogenic AAs for?

Answer 36

Form glucose
**essential during starvation

Question 37

Kwashiorkor in humans

Answer 37

Condition results from protein deficiency and since serum protein is severely decreased= edema/swollen abdomen
*often occurs in kids after weaning in famine areas

Question 38

Protein deficiency symptoms in animals

Answer 38

-weight loss
-muscle loss and weakness
-poor digestion
-poor coat
-fluid build up in chest and abdomen