Lecture 13: Protein nutrition, digestion and absorption

Question 1

What is the role of proteins in the body?

Answer 1

Immune system
Hormones
Structural and mechanical
Transport
Fluid balance
Acid-base balance
Energy
Channels and pumps
Enzymes

Question 2

Describe the constituents of a amino acid;

Answer 2

Side group (variable)
Acid group
Amino group

Question 3

How many amino acids are there?

Answer 3

20 amino acids

9 of which are essential and must be obtained from foods i.e animal products, fish, soy products, quinoa

Question 4

What sort of bonds to proteins have?

Answer 4

Peptide bonds

Di and tri-peptide

Question 5

Whats important about protein folding?

Answer 5

Tertiary folding helps determine structure and function

Any variation in amino acids can change folding and thus function

i.e sickle cell anemia

Question 6

Whats the RDI and AMDR of protein?

Answer 6

RDI; 0.84g/kg/day males and 0.75g/kg/day females

AMDR; 15-25% of energy intake

Question 7

Whats the importance of protein quality?

Answer 7

Digestibility; Animal vs plant protein
Amino acid composition; limiting amino acid

That is if a food lacks one of the amino acids then this may limit how much of a protein can be made.

Question 8

If someone is strict vegeterian or vegan, how can they ensure they get enough of the amino acids?

Answer 8

They can work out which foods are complementary for the amino acids and make sure they are getting all the essentials and not limiting

Question 9

Prior to eating what can aide protein digestion?

Answer 9

Cooking

• Unfolds, denatures proteins
• Softens food
• Easier to chew and swallow
• Safer to eat

Question 10

Describe steps 1-2 of protein digestion;

Answer 10

1) The mouth, Mechanical digestion, chewing and crushing

2) Saliva, lubricates foods

Question 11

Describe how protein is digested in the stomach (step 3)

Answer 11

Acid activates pepsinogen to pepsin which breaks down large proteins into polypeptides and amino acids

Question 12

Describe steps 4-5 of protein digestion

Answer 12

Polypeptides and amino acids enter the small intestine, the pancreas has released;

Trypsinogen
Chymotrpsinogen
Procarboxypepetidase

Enteropeptidase on the lumen surface of enterocytes activates trypsinogen-> trypsin (breaksdown peptide bonds, inhibits trypsinogen synthesis and converts;

Chymotrpsinogen -> chymotrypsin. (Breaks peptide bonds).

Procarboxypeptidase -> Carboxypeptidase. (Breaks apart peptide bonds at the carboyxl end)

Question 13

Describe the further breakdown of polypeptides in the small intestine;

Answer 13

Luminal surface of enterocytes also contains enzymes;

• Amino peptidase
• Dipeptidase
• Tripeptidase

These all release AA which are absorbed by the SI

Question 14

How are AA transported across the apical surface of the enterocytes?

Answer 14

Primary active transport with Na+ (ATP)
Secondary active transport with Na+ (gradient, not ATP)
Di/tri-peptides with Na+ secondary active transport.

Question 15

How do AA and di/tri-peptides enter circulation from enterocytes?

Answer 15

Di/Tri-peptides are broken down to AA

AA then undergo facilitated diffusion via various transporters

Na/K ATPase create the gradient.

Question 16

What is the end product of AA metabolism?

Answer 16

Nitrogen/ammonia

Question 17

What can happens to the nitrogen pool?

Answer 17

Nitrogen Pool Supplies

Mainly;
NH3-> Urea cycle = Urea
Tissue protein

Some;
Pyruvate acid
Acetyl-CoA
Citric Acid Cycle

Question 18

What is nitrogen balance?

Answer 18

Rate of nitrogen in (Dietary AA) and loss of nitrogen (urea and some fecal)

Thus, energy needs must be met to prevent amino acids from being relocated to become an energy source

Question 19

What can urinary nitrogen be used for? and whats assessed

Answer 19

To assess the rate of nitrogen loss i.e tissue breakdown etc

Urinary nitrogen =

• Urea
• Creatanine
• Uric acid

Indicates excess nitrogen (cus ammonia is toxic thus made into other forms)

Question 20

Whats the nitrogen intake equation?

Answer 20

N x 6.25 = G protein intake

Question 21

What is positive nitrogen balance?

Answer 21

Protein synthesis > Protein breakdown

• Increased protein intake (short term)
• Growth
• Pregnancy
• Recovery from illness or trauma
• Gym

Question 22

What is negative nitrogen balance?

Answer 22

Protein synthesis < Protein breakdown

i.e
starvation
reduced GI function
stress
cancers
lactation

Question 23

What is nitrogen balance?

Answer 23

Protein synthesis = protein breakdown

healthy
protein needs being met

Question 24

What can cause protein energy malnutrition?

Answer 24

• Infections i.e dysentary
• Rehabilitation
• Kwashiorkor (pot belly, ascites), Protein deficit but energy intake
• Marasmus (Diet deficient in protein / starvation, no other foods)