L20: Introduction to Protein Metabolism

Question 1

LO1: Define protein turnover and nitrogen balance

Answer 1

PROTEIN TURNOVER

• no storage form of protein, so they are constantly being degraded vs. synthesized
• 75% of AAs released during turnover are reutilized for protein synthesis, while the rest are used largely for gluconegenesis (especially during starvation)

NITROGEN BALANCE
-compareison between the intake of nitrogen (protein) and the excretion of nitrogen (loss through urea in urine)

Question 2

LO2: Describe how nitrogen balance differs in health vs. disease

Answer 2

HEALTHY NITROGEN BALANCE
-nitrogen equilibrium: nitrogen losses are balanced by nitrogen intake

DISEASE STATE NITROGEN BALANCE

• negative nitrogen balance: dietary protein intake is inadequte to meet body’s needs (inadequate diet, metabolic stress, sepsis, trauma, surgery, AA deficiency)
• positive nitrogen balance: net increase in body proteins (growth, pregnancy, recovery from surgery or trauma)

Question 3

LO3: What’s the minimum dietary protein requirement for healthy adults?

Answer 3

• minimum requirement is 0.8g/kg body weight/day (calculated based on average obligatory nitrogen loss, population variation, and average biological value of American diet (70%))
• dependent on age, growth status, pregnancy/lactation, illness, anything causing negative vs. positive nitrogen balance, caloric level/digestibility of diet, (if diet is balanced)
• this is just the minimum!

Question 4

LO4: Describe the ultimate metabolic fate of dietary AAs in healthy adults

Answer 4

-dietary AAs used to replace AAs lost through irreversible pathways, maintain growth and homeostasis of proteins, provide energy (during starvation)

Question 5

LO5: Explain why AAs are classified as essential, non-essential, or conditionally essential

Answer 5

ESSENTIAL-must be received in diet (multiple essential AAs gives a substance high biological value)

P- Phenylalanine
V- Valine
T- Threonine

T- Tryptophan
I- Isoleucine
M- Methionine

H- Histidine
A- Arginine*
L- Leucine
L- Lysine

NON-ESSENTIAL-can be synthesized/made from other AAs

G(3)A(3)S(1) (except arginine) CPT

G- Glutamate
G- Glutamine
G- Glycine
A- Alanine
A- Aspartate
A- Asparagine
S- Serine

C- Cysteine
P- Proline
T- Tyrosine

CONDITIONALLY ESSENTIAL-essential in certain instances

• arginine*
• only essential in infancy/early childhood until body is able to synthesize enough of it on its own

Question 6

LO6: List the essential amino acids

Answer 6

Phenylalanine
Valine
Threonine

Tryptophan
Isoleucine
Methionine

Histidine
Arginine
Leucine
Lysine

Question 7

LO7: Define biological value of proteins and list properties of dietary proteins that determine their biological value

Answer 7

BIOLOGICAL VALUE-depends on content of its essential AAs; egg white used as reference (assigned value of 100)

PROPERTIES THAT DETERMINE BIOLOGICAL VALUE-digestibility, amount and number of essential AAs

Question 8

LO8: List good sources of animal and plant proteins

Answer 8

1. Whey protein (104)
2. Egg (100)
3. Milk (91)
4. Beef (80)
5. Casein (in dairy)

Question 9

LO9: Explain why a mixed diet of plant proteins is important for vegetarians

Answer 9

• mixture of plant proteins important to ensure that no deficiency of any AA occurs and that diet overall has high biological value
• plant proteins have lower biological value than animal proteins

Question 10

L10: Explain the relationship between a swollen abdomen, hypoalbuminemia and protein malnutrition

Answer 10

• inadequate intake of high quality proteins leads to reduced serum albumin, which leads to edema in abdomen (can’t collect fluids from tissues; fatty liver as proteins needed to transport fat out of liver)
• seen in marasmus (malnutrition of calories and protein) and kwashiorkor (adequate calories but inadequate protein)