Protein Digestion and Nitrogen Balance

Question 1

In terms of diet, what are the different kinds of amino acids?

Answer 1

Essential (9) and non-essential

Question 2

How are proteins linked and what are the subunits?

Answer 2

Peptide bonds, creating dipeptides and tripeptides

Question 3

What determines protein (from food) quality?

Answer 3

Amount of essential amino acids

Digestibility of the protein (protein from animals is seemingly easier to digest)

Question 4

How is protein digestion begun?

Answer 4

Chewing in mouth breaks down and moistens proteins

Question 5

What happens to proteins in the stomach?

Answer 5

HCl denatures proetins strands, and converts inactive pepsinogen into pepsin
Pepsin breaks down proteins into polypeptdes and AA’s

Question 6

How is protein digested in the SI?

Answer 6

Pancreatic juices containing-
Pancreatic endopeptidases (trypsin, chymotrypsin and elastase) break down polypeptides into tripeptides, dipeptides and some AA's
Tripeptidases, dipeptidase and aminopeptidases break these down further

Question 7

How do membrane bound enzymes on enterocytes affect these proteases?

Answer 7

Membrane bound enteropeptidase will convert trypsinogen into trypsin
Trypsin will convert other inactive proteases to their active forms? But negative feedback so trypsin production ceases trypsinogen production

eg’s are procarboxypeptidases to carboxypeptidases and elastase

Question 8

What ways are protein absorbed?

Answer 8

Co-transport of di and tri peptidases with H+
AA co-transport with Na+ (note that these symporters are powered by a Na+/K+ antiport)
Transcytosis of polypeptides
in the cell some peptidases break down protein

Question 9

What is nitrogen balance?

Answer 9

Nitogen intake = Nitrogen expenditure

g N x 6.25 = g protein
Assessed by urine tests

Question 10

What is and what causes negative nitrogen balance?

Answer 10

When nitrogen degradation > nitrogen synthesis

Caused by: decreased protein intake; reduced GI function; injury, illness, burns and surgery

Question 11

What is and causes positive nitrogen balance?

Answer 11

When synthesis > degradation

Caused by: increased protein intake; growth; pregnancy; recovery

Question 12

What is marasmus?

Answer 12

Chronic PEM (protein energy malnutrition) where there is a severe deprivation of protein/impaired absorption

Presents with no edema, no appetite and severe muscle wasting.

Question 13

What is kwashiokor?

Answer 13

An acute PEM that effects mainly children, and is associated with inadequate protein intake or infection.

Presents with edema, fatty liver no appetite and often skin lesions.