L28- Protein Digestion

Question 1

What are the two most significant sources of nitrogen in our body?

Answer 1

Approximately 3/4 of nitrogen comes from the breakdown of the body’s own proteins, and the remaining 1/4 comes from the breakdown of dietary proteins.

Question 2

In which organ is the breakdown of dietary proteins initiated?

Answer 2

Stomach.

Question 3

What class of proteolytic enzymes hydrolyze internal peptide bonds?

Answer 3

Endopeptidases.

Question 4

What class of proteolytic enzymes hydrolyze peptide bonds at the terminal end of peptides?

Answer 4

Exopeptidases.

Question 5

What is the optimum pH for pepsin activity?

Answer 5

1.5

Question 6

Where is the enzyme pepsin synthesized?

Answer 6

In the gastric mucosa.

Question 7

How is pepsinogen converted into active pepsin?

Answer 7

Pepsinogen contains a prosegement that sits in the active site at normal cellular pH. The low pH of the stomach causes dissociation of the prosegment, leading to the activation of pepsin.

Question 8

Where does the majority of proteolysis of dietary proteins occur in the gastrointestinal tract?

Answer 8

Duodenum.

Question 9

Partially digested proteins that enter the duodenum from the stomach trigger the release of which peptide hormones from the intestinal mucosa?

Answer 9

Secretin and cholecystokinin (CCK).

Question 10

What are the major endopeptidases present in pancreatic juice?

Answer 10

Trypsin, chymotrypsin and elastase.

Question 11

What are the major exopeptidases present in pancreatic juice?

Answer 11

Carboxypeptidase A and B.

Question 12

Which enzymes hydrolyze di- and tripeptides into amino acids?

Answer 12

Aminopeptidases and dipeptidases.

Question 13

What is the function of the enzyme enteropeptidase?

Answer 13

It cleaves trypsinogen into its active trypsin form.

Question 14

Enteropeptidase is released from the luminal side of the small intestine under the influence of which hormone?

Answer 14

Cholecystokinin (CCK).

Question 15

Which condition can be caused by blockage of the pancreatic duct?

Answer 15

Pancreatitis.

Question 16

Which amino acids form the catalytic triad in trypsin?

Answer 16

Aspartate, serine and histidine.

Question 17

Name the most abundant amino acids in plasma.

Answer 17

Alanine and glutamine.

Question 18

Name the most abundant amino acids inside cells.

Answer 18

Alanine, glutamine, glutamate and glycine.

Question 19

What is the cause of cystinuria?

Answer 19

Cystinuria is caused by a genetic defect affecting a membrane transporter in the kidney glomeruli. It is associated with excretion of high levels of cysteine, arginine, citrulline and lysine.

Question 20

Which disease is caused by a single defective transporter in epithelial cells, resulting in excretion (and poor uptake) of several neutral amino acids such as tryptophan?

Answer 20

Hartnup’s disease.

Question 21

What is the function of the ubiquitin/proteasome pathway?

Answer 21

It degrades proteins in healthy cells to provide amino acids for the synthesis of new proteins.

Question 22

What is the major pathway for the degradation of intracellular protein?

Answer 22

The ubiquitin/proteasome pathway.

Question 23

What is the location and function of cathepsins in the cell?

Answer 23

Cathepsins are proteases found in lysosomes. They make a minor contribution to the degradation of intracellular proteins.

Question 24

What marker polypeptide binds to proteins as a means to target them for degradation?

Answer 24

Ubiquitin.

Question 25

Which portion of the proteasome permits selectivity of proteins destined for degradation?

Answer 25

The regulatory portion (RP).

Question 26

Which portion of the proteasome contains proteases that degrade a wide variety of proteins?

Answer 26

The barrel-like core (CP).

Question 27

What is the average size of the digestion products of proteasomes?

Answer 27

7-10 residues.

Question 28

What are the functions of the regulatory portion (RP) of the proteasome?

Answer 28

The RP recognizes and selects ubiquinated substrates and removes ubiquitin before the substrate enters the CP.

Question 29

How do periods of inadequate caloric intake affect the activity of the ubiquitin/proteasome pathway (UPP)?

Answer 29

During periods of inadequate caloric intake, there is an increase in the levels of mRNA that encode ubiquitin and proteasome activity increases so that intracellular proteins can be broken down to provide amino acids for gluconeogenesis and energy production. This results in higher UPP activity.

Question 30

What standard of measure compares the composition of essential amino acids in a protein with that of a reference protein (such as egg whites).

Answer 30

The chemical score of a protein.

Question 31

List the essential amino acids.

Answer 31

Phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine and lysine.

Question 32

What is the minimum amount of protein that adults should consume daily to prevent the net breakdown of body protein?

Answer 32

Approximately 55 grams per day.

Question 33

What family of enzymes selectively recognizes and ubiquinates intracellular proteins destined for degradation?

Answer 33

E3 ubiquitin-conjugating enzymes.

Question 34

Name a sequence of four amino acids that can sometimes target proteins for rapid degradation.

Answer 34

PEST sequence: proline (P), glutamic acid (E), serine (S), and threonine (T).

Question 35

Mutation of which ubiquitin-conjugating ligase can lead to the accumulation of Lewy bodies in Parkinson’s disease?

Answer 35

Parkin.