Protein Digestion & Absorption

Question 1

What is the importance of proteins?

Answer 1

involved in all these things:

• enzymes & hormones
• structure proteins like contractile proteins (myosin, actin) or fibrous proteins (collagen, elastin, keratin)
• immunoprotein (immunoglobulins)
• transport proteins such as albumin, transthyretin, hemeproteins, transferrin,
• glycoproteins and lipoproteins

Question 2

What is the importance of dietary protein?

Answer 2

• synthesis of new proteins for growth and maintenance
• replacement of exisitng proteins (worn-out)
• synthesis of glucose, ketones, fatty acids (i.e. energy source)
• production of non-protein nitrogen (NPN) containing compounds (ex: urea, ammonia, etc)

Question 3

What are the two classes of protein sources?

Answer 3

1. Endogenous (70g/day)
2. Exogenous = from our diets (>100g/day)

Question 4

What are some exogenous sources of protein?

Answer 4

• animal products (meat, poultry, eggs and dairy products)
• plant products (grains, legumes and vegetables)

Question 5

what are some endogenous sources of protein?

Answer 5

• mucosal cells by desquamation
• digestive enzymes
• glycoproteins

Question 6

where does protein digestion take place?

Answer 6

• stomach
• lumen of small intestine
• brushborder of small intestine
• enterocytes

Question 7

What happens to protein in the stomach??

Answer 7

In the stomach, HCL and pepsin denature the proteins (making them lose their 4th, 3rd and 2nd structures). Then pepsin can more easily access the peptide bonds and break them down into smaller polypeptides.

**some larger polypeptides remain and are broken down in the small intestine

Question 8

Protein digestion requires two groups of enzymes, what are they? What do they do?

Answer 8

1. Endopeptidase: split up polypeptides at interior bonds (inside chain)
2. Exopeptidase: cleave off amino acid at extremity of polypeptide chain (ends of chain)

Question 9

What is the end product of endopeptidases and exopeptidases?

Answer 9

endopeptidases = smaller polypetides, tripeptides or dipeptides

exopeptidases= amino acids

Question 10

What are some examples of endopeptidases?

Answer 10

1. trypsin
2. chymotrypsin
3. elastase
4. pepsin

Question 11

what are some examples of exopeptidases?

Answer 11

1. carboxypeptidase (A or B)
2. aminopeptidase

Question 12

What stimulates secretion of HCL in the stomach ?

Answer 12

1. Gastrin
2. gastrin releasing peptide (GRP)
3. acetylcholine

Question 13

which structures (1st,2nd,3rd,4th) of a protein does HCL break?

Answer 13

HCL denatures 4th, 3rd, 2ndary structures of protein (unfolding)

Question 14

Pepsinogen + HCL = _______

Answer 14

Pepsin

Question 15

what are the endproducts of pepsin on protein ?

Answer 15

• polypeptides
• oligopeptides (2-20aa)
• free amino acids

Question 16

what do gastric chief cells secrete?

Answer 16

pepsinogen

Question 17

does pepsin function in a pH of 3.5 or higher?

Answer 17

NO!

Question 18

what stimulates the intestinal endocrine cells to produce CCK?

Answer 18

food mixed with chyme entering the duodenum

Question 19

What stimulates the pancreas acinar cells?

Answer 19

CCK and the vagus nerve

Question 20

what are zymogens? what produces them? give three examples.

Answer 20

Zymogens are inactive enzymes produced by the pancreas acinar cells ex: trypsinogen, chymotrypsinogen, procarboxypeptidase

Question 21

how are chymotrypsin and carboxypeptidases formed?

Answer 21

1. first trypsinogen is activated to trypsin by an enteropeptidase (enterokinase) secreted by the duodenal enterocytes
2. then trypsin triggers the activation of chymotrypsinogen into chymotrypsin and procarboxypeptidase into carboxypeptidases

Question 22

what are the proteolytic proenzymes secreted by the pancreas in response to CCK?

Answer 22

1. trypsinogen
2. chymotrypsinogen
3. pro-carboxypeptidases A &B
4. pro-elastase
5. collagenase

Question 23

What peptide bonds will trypsin cleave?

Answer 23

Trypsin cleaves peptide bonds with arginine and cysteine at the C terminal (peptide bonds adjacent to basic AA)

Question 24

what other enzymes are activated by trypsin?

Answer 24

Procolipase, Phospholipase A2

Question 25

what peptide bonds will **chymotrypsin** cleave?

Answer 25

chymotrypsin breaks peptide bonds adjacent to **aromatic AA**

Question 26

what peptide bonds will **carboxypeptidase A** and **carboxypeptidase B** cleave?

Answer 26

carboxypeptidase break peptide bonds adjacent to C-teminal **neutral** (A) and **basic** amino acid (B)

Question 27

what are the end products of **carboxypeptidase A** on polypeptides with free carboxyl groups?

Answer 27

they are broken down to **lower peptides**, **aromatic amino acids and neutral amino acids**

Question 28

what are the end products of **carboxypeptidase B** on polypeptides with free carboxyl groups?

Answer 28

they are broken down to **lower peptides and basic amino acids**

Question 29

what peptide bond does **elastase** cleave?

Answer 29

elastase cleaves the peptide bond at **alpheric C-terminal**

Question 30

what are the end products of **elastase** and **collagenase** on polypeptides?

Answer 30

**oligopeptides and tripeptides**

Question 31

what forms of proteins are transported into the **enterocytes** of the small intestine from the brush border?

Answer 31

1. free amino acids 2. tripeptides 3. dipeptides

Question 32

what are the enzymes found in the **brush border** membrane called? what do they hydrolyze?

Answer 32

**Oligopeptidases** hydrolyze larger oligopeptides

Question 33

what are **large oligopeptides** broken down into? **where** are they broken down?

Answer 33

**tripeptides, dipeptides and amino acids** broken down in the **brush border** membrane by **oligopeptidases**

Question 34

what enzymes are found in the **enterocytes**?

Answer 34

* **peptidases** : aminopeptidases (tri and di), **di**peptidylaminopeptidases (di), and **tri**peptidases (tri)

Question 35

**Peptides** are absorbed into the **brushborder** ______ than **free amino acids**

Answer 35

**FASTER**

Question 36

**Pep T1** is found on the \_\_\_\_\_\_

Answer 36

**brushborder**

Question 37

Transport of **peptides** is associated with movement of what ion?

Answer 37

**H+** movement

Question 38

How are amino acids transported across the **basolateral membrane**?

Answer 38

**actively in the upper small intestine (Duodenum) Na+ dependent or independent**

Question 39

How are peptides transported both across the **apical membrane** and the basolateral membrane of the enterocytes?

Answer 39

1. peptides are transported into the cell with **H⁺ (Pep T1)** 2. the H+ are **pumped** back in exchange of a **Na⁺** ion 3. A **pump is required** to pump Na⁺ out of the cells for K⁺ across the basolateral membrane

Question 40

what factors affect amino acid **absorption**?

Answer 40

1. hydrocarbon **mass** of the side chain (**higher mass is fastest**) 2. electrical charge (**neutral=faster** than acidic or basic) 3. essential AA = faster than nonessential

Question 41

absorption of amino acids is \_\_\_\_\_\_

Answer 41

**active transport** (Na+ dependent/independent)

Question 42

How are amino acids used by the intestinal cells?

Answer 42

* use for synthesis of apoproteins for **lipoprotein** formation * use for synthesis of **digestive enzymes** * synthesis of **hormones** * synthesis of **N-contraining compounds** * **Glutamine** used extensively for **energy sources**, catabolize glutamine to **glutamate** and **NH₃** * synthesis of **proline from glutamine**

Question 43

What are the major steps of protein digestion?

Answer 43

1. gastric hydrolysis of peptide linkages 2. digestion of protein to smaller peptides by pancreatic enzymes 3. hydrolysis of peptide linkages into **oligopeptides** by **brush border peptidases** and transport of amino acids and **di and tri peptides** across the membrane of the **enterocytes** 4. further digestion of tri and di-peptides by **cytoplasmic peptidases** in the enterocytes 5. metabolism of amino acids within the **enterocytes** 6. transport of amino acids across the **basolateral membrane** of the enterocyte into the interstitial membrane

Question 44

Which of the following activities does **NOT** occur in the brush border membrane region of enterocytes? A. Endocytosis of polypeptides B. Transport of free amino acids C. sucrose digestion D. glutamine metabolism

Answer 44

**A.** Endocytosis of polypeptides does not occur in the brush border membrane.

Question 45

**carboxypeptidase B** will act on polypeptides that have been split by \_\_\_\_\_\_\_\_

Answer 45

**Trypsin **

Question 46

** carboxypeptidase A** will act on polypeptides that have been cleaved by \_\_\_\_\_and \_\_\_\_\_\_

Answer 46

chymotrypsin and elastase

Question 47

How are amino acids transported across the apical membrane into the enterocytes?

Answer 47

Active transport