Digestion and Absorption of Amino Acids

Question 1

What is the first place digested amino acids go after being absorbed by enterocytes?

Answer 1

the hepatic portal vein to be transported to the liver

Question 2

Once in the liver, what are the major fates of absorbed amino acids?

Answer 2

1) protein synthesis
2) synthesis of N-containing compounds

3/4) split into carbon and nitrogen- carbon can be broken down for energy or stored; nitrogen can be broken down to urea

5) biosynthesis of amino acids

Question 3

What are essential amino acids?

Answer 3

AAs that the body cannot synthesis. Thus, they are requires in your diet. Non-essential AAs can be made by the body and, thus, are not needed exogenously

Question 4

Which non-essential AAs require input from essential AAs to be made?

Answer 4

cysteine and tyrosine synthesis depend on adequate amount of methionine (sulfur) and phenylalanine, respectively

Question 5

What is an AA that is essential in children but not really necessary in adults (although still considered ‘essential’)

Answer 5

arginine

Question 6

What is the recommended daily allowance of protein?

Answer 6

0.8g of high quality protein per kilogram of ideal body-weight (50g/day for women and 60g/day for men)

Question 7

What is ‘high-quality’ protein?

Answer 7

just means that the protein contains all of the essential AAs in adequate amounts (generally, protein of animal origin-milk, egg, meat)

Question 8

What is ‘low-quality’ protein?

Answer 8

low in one or more essential AA (generally protein from plant origin- easy to compensate with mixtures of low-quality protein)

Question 9

What is nitrogen balance?

Answer 9

Means that the dietary nitrogen intake is equal to the excreted nitrogen. Someone who has a positive nitrogen balance has more dietary N than excreted N, while someone who has a negative nitrogen balance excretes more nitrogen than they ingest

Question 10

When would someone be nitrogen positive and it be normal?

Answer 10

growing children or pregnant women (steroid users would too)

Question 11

What is Kwashiorkor?

Answer 11

a disease caused by a deficiency of protein in a diet that is adequate in calories; common in Africa and third world countries

clinically: muscle wasting and decreased concentrations of muscle plasma

Question 12

Why is there muscle wasting associated with Kwashiorkor? What happens when you consume none or insufficiency quantities of one or more essential amino acid?

Answer 12

non-essential AAs are going to be used to synthesis the essential ones, resulting in muscle wasting

Question 13

What would you see in a patient that has decreased plasma proteins?

Answer 13

distended/bloated stomach

Question 14

Where does proteolysis begin?

Answer 14

in the stomach, indicating that none occurs in the mouth or esophagus

Question 15

What are the two phases of proteolysis in the stomach?

Answer 15

1) denaturation in acidic conditions that unfold the protein to make them a bettie substrate for breakdown by protease
2) Pepsin- a protease- breakdown

Question 16

What are zymogens?

Answer 16

(Aka proenzymes) proteases that are produced as inactive precursors

require some change to become active form (eg. cleavage or conformational change)

Question 17

How is pepsin activated?

Answer 17

pepsin is made as a zymogen called pepsinogen (secreted by chief cells). Under acidic conditions, the structure of pepsinogen changes and it cleaves itself (auto cleavage) generating pepsin

NOTE: Pepsin is not denatured by the pH in the stomach

Question 18

What happens after proteolysis is initiated in the stomach?

Answer 18

the products of the initial breakdown continue on to the small intestine where they continue to be broken down by proteases secreted from the pancreas (secreted as zymogens)

Question 19

What are the main zymogens/proteases secreted by the pancreas?

Answer 19

1) trypsinogen
2) chymotrypsinogen
3) proelastase
4) procarboxypeptidases

Question 20

What is trypsinogen activated by?

Answer 20

enteropeptidase cleaves it to become trypsin, which acts as a digestive protease itself AND activates the other three pancreatic proteases

Question 21

How does the pancreas regulate the activity of trypsin?

Answer 21

1) trypsin is primarily found in its zymogen form

2) the pancreas also makes an inhibitor as a safety net

Question 22

Why are there so many proteases in the pancreas?

Answer 22

they have different specificities for where they cleave

Question 23

How does cystic fibrosis affect proteolysis?

Answer 23

defective chloride channels cause drying and thickening of pancreatic exocrine secretions, leading to obstructed ducts so proteases can’t be released

Question 24

How are single amino acids transported from the intestinal lumen into cells for transport to the liver?

Answer 24

Na+-dependent active transport by carriers (secondary active transport) into the intestinal cell, followed by facilitated passage into the hepatic portal vein from the basal side of the enterocyte

Can also occur by the gamma-glutamyl cycle (not covered in class)

Question 25

How are single amino acids transported from the liver cells into peripheral tissue?

Answer 25

mostly Na+-dependent active transport by carriers (secondary active transport) out of the bloodstream with some facilitated passage

Question 26

T or F. Most AAs are transported by more than one transporter

Answer 26

T

Question 27

What is Hartnup Disease (neutral amino aciduria)?

Answer 27

relatively rare autosomal recessive disorder caused by the inability of intestinal and/or kidney epithelial cells to absorb NEUTRAL amino acids (Leu, Val, The, Ile, Ala, Ser) and aromatic amino acids (Phe, Trp, Tyr)- just know the classes

Question 28

What happens to the kidney as a consequence of Hartnup Disease?

Answer 28

increased AAs in the urine (hyperaminoaciduria)

Question 29

What happens to the intestine as a consequence of Hartnup Disease?

Answer 29

malabsorption -> deficiency of essential AAs

Question 30

What are the clinical manifestations of Hartnup’s Disease?

Answer 30

“Pellagra-like” symptoms:

1) photosensitive rash
2) ataxia
3) neuropsychiatric symptoms

Pellagra results from deficiency of Trp or niacin which impairs synthesis of NAD+ or NADP+

Question 31

What is the treatment for Hartnup’s Disease?

Answer 31

Exogenous Niacin (vitamin B3)

Question 32

What is Cystinuria?

Answer 32

A rare autosomal disorder caused by the inability of intestine and/or kidney to absorb cystine (two fused cysteine AAs by sulfide bond) and BASIC amino acids (Arg, Lys, Ornithine)

Will also cause associated hyperaminoaciduria

Question 33

What does cystine cause if not absorbed?

Answer 33

the insolubility of cystine can generate kidney stones that block the ureter and can cause urinary bleeding and severe pain (renal colic)

I

Question 34

What could be used to treat cystinuria?

Answer 34

can’t do much since cysteine are non-essential amino acids but you can treat with high fluid intake and oral alkalinizing agents