Protein

Question 1

What is the difference between essential, non-essential and conditionally essential amino acids?

Answer 1

Essential = body cannot produce endogenously or do so at a sufficient rate to support demand

Non-essential = body can produce endogenously via transamination

Conditionally essential = essential under certain conditions. Histidine during infancy. Glutamine and arginine during disease or trauma. Tyrosine in PKU (cannot convert phenylalanine to tyrosine via transamination = results in phenylalanine toxicity = inborn error)

Question 2

What is the peptide bond

Answer 2

Join AA to form dipeptides, tripeptides, oligopeptides and polypeptides

Question 3

Describe the four structures of proteins

Answer 3

Primary = AA sequence
Secondary = 2D structure, H-bonds, alpha helices and beta sheets
Tertiary = 3D structure made from a-helices and b-sheets. Disulfide bridges and other interactions
Quaternary = interactions between multiple polypeptide chains

Question 4

Which structures determine protein function

Answer 4

Primary codes for other structures. Function expressed at the tertiary level

Question 5

What is the fate of dietary amino acids once absorbed?

Answer 5

Synthesis of non-protein N-containing compounds

Ketone production from C skeletons of some AA. Fat production when in excess

Glucose production from C skeletons of some AA via gluconeogenesis

Energy production from AA C skeletons

Synthesis of proteins for cell structure, immune function, enzymes, hormones and other essential functions

Question 6

Why is a regular supply of essential amino acids so important?

Answer 6

Important to keep AA pool full. This ensures adequate protein synthesis

Question 7

How is the quality of dietary protein assessed? (explaining criteria and briefly the methodology)

Answer 7

BV = how efficiently a protein can be digested, absorbed and used for protein synthesis. Takes into account for faecal and urinary excretion. Egg white has a BV of 100 (all N absorbed is retained and matches the body’s protein AA composition)

PER = weight gained (g) / protein consumed. Reflects BV since growth and weight gain is proportional to AA uptake and usage to form body protein

CS = amount of EAA (mg) per g of food / a control. The lowest EAA ratio is the limiting AA and is the CS for the tested protein

PDCAAS = CS x Digestibility. Protein digestibility corrected AA score. Digestibility is a measure of how well the AA can be digested and absorbed. This is the amount that can be used to support growth and maintenance (highest score is 1 = milk proteins, eggs and soy). Digestibility of animal proteins = 90-100%; plant proteins = 70-99%

Question 8

What is meant by a high-quality protein?

Answer 8

High quality protein has all 8 (or 9) EAA in the right proportions used by the body to support growth and maintenance

Question 9

What are food sources of high-quality protein from both animal and plant sources?

Answer 9

Animal proteins, say, quinoa

Question 10

What is meant by the “limiting” amino acid?

Answer 10

Limiting AA is the EAA that is in the lowest concentration or not present at all in the sample

Question 11

What would happen for DNA expression to continue, when low quality protein, or too little protein is consumed in the long term?

Answer 11

Body would break down its own proteins to support itself

Question 12

What is meant by “protein turn-over”? How is the proposed to contribute to protein requirement?

Answer 12

Protein turnover is the reabsorption of AA from cellular waste, debris, mucus and enzymes. Refers to the continual renewal of protein. The balance between protein synthesis and degradation. Proposed that we can reabsorb all EAA, thus, the need for combining plant protein sources in vegan diets has been challenged

Question 13

How is protein implicated in fluid balance?

Answer 13

BP pushes fluid out of the capillary bed into the interstitial space. Blood proteins (albumin and globulins) pull the fluid back in via osmosis. Low blood protein leads to oedema

Question 14

What is gluten? Is it bad and should it be avoided? Why?

Answer 14

Gluten is term used to describe the proteins found in grains. Should only be avoided in people with coeliac disease = gluten consumption leads to flattening of the microvilli (immune response)

Question 15

Compare Marasmus and Kwashiorkor. List the causes, the characteristic signs, and the health consequences in the long term

Answer 15

Kwashikor = severe protein + moderate energy deficiency. Characterised by oedema, slight weight loss, growth impairment (60-80% of normal weight for age), rapid onset, fatty liver

Marasmus = severe protein + energy deficiency. Characterised by severe weight loss, muscle and fat wasting, severe growth impairment (<60%), gradual development

Question 16

How was the RDI for protein determined? Why is the amount so different than the AMDR for protein?

Answer 16

Based on minimum amount to prevent signs of deficiency for 98% of the population. Only takes into account gender, age and weight

The AMDR is more wholistic and takes into consideration EER (BMR x PAL)

Question 17

What is considered a “high” protein diet?

Answer 17

No definition as to what constitutes a high protein diet. >RDI? >AMDR? No UL for protein

Question 18

Summarise the considerations when discussing high protein diets

Answer 18

What is the underlying health and nutritional status of the person?

What are the protein sources in the diet? (high fat/ highly processed/ animal/ plant)

What are the methods of cooking? (deep fried, chargrilled, or little processed sources)

What is the rest of the dietary intake made of in terms of other macronutrients, fibre, nutrient density, quality of foods

Is the diet sustainable long-term form a nutrition and lifestyle point of view?

What are the goals of the high protein diet? (appetite control, weight loss, muscle gain, prevention of muscle loss)

Question 19

Explain why high protein diets are popular in the context of weight loss

Answer 19

Protein is the most satiating of the macronutrients and yields the highest thermic effect (protein intake more costly in energy during digestion and metabolism)

Increases sleeping metabolic rate and fat oxidation

Dietary protein may spare muscle mass used in gluconeogenesis by providing the required AA

If not fully digested, protein fermentation by bacteria will occur in the distal GIT => produces metabolites associated with CV disease, T2DM, chronic kidney disease, inflammatory bowel disease

Question 20

What are some protein-related diseases that require serious nutritional and medical advice?

Answer 20

Coeliac, Kwashikor, Marrasmus

Question 21

Evaluate the myths or facts about high protein diets

Answer 21

Overburden kidneys
Gluten is toxic
High protein = poor everything else
Will acidify blood
Will increase calcium excretion

Question 22

What determines AA essentiality? (why the body cant make them)

Answer 22

Cannot make C skeleton
Cannot attach amine to C skeleton
Cannot do so at a fast enough rate

Question 23

What happens if EAAs are not provided in the diet?

Answer 23

Can lead to a number of negatives. Poor nutrient interconversion, poor immune function etc. Muscle wasting

Question 24

Describe dietary protein digestion

Answer 24

Mouth: teeth perform mechanical digestion

Stomach:
Gastrin secreted by G cells in response to food intake
Gastrin signals secretion of HCl (parietal cells) and pepsinogen (chief cells)
HCl denatures proteins and activates pepsinogen to pepsin (low HCl in stomach results in poor protein digestion and thus poor AA absorption)
Pepsin cleaves peptides into shorter fragments

Protein denaturation:
Altering the proteins 3D structure
Acid/alkaline, heat, enzymes can modify the structure
At acidic pH, the electrical balance necessary to retain the native structure is lost
• Examples:
- Yoghurt production. Bacteria ferment lactose to lactic acid, altering the pH and causing milk proteins (caseins) to coagulate
- HCl denatures protein in the stomach- allows greater exposure to peptidases for breakdown of peptide chain
- Heat in cooking denatures bacterial protein in food

Small intestine:
Chyme-nutrient mixture stimulates secretion of secretin and CCK from S cells and I cells of the duodenum, respectively
Secretin and CCK stimulate secretion of bicarbonate (neutralises acidity) and proteases/peptidases from the pancreas
Proteases/peptidases are secreted as zymogens (inactive enzymes, don’t want active enzymes in pancreas cause it’d break itself down) which are activated in the small intestine
Polypeptides are digested by peptidases into tripeptides, dipeptides and AA

Trypsin, chymotrypsin, carboxypeptidase

Dipeptides and tripeptides are absorbed via co-transport with H+
Amino acids are transported across the basolateral membrane into the blood by amino acid-specific transporters (either sodium-dependent or independent)

Question 25

Describe deamination

Answer 25

• Occurs when AA are converted to pyruvate, acetyl CoA, intermediates of the CAC, or to oxaloacetate for gluconeogenesis

1. Removal of the amine group from an amino acid
2. Amine group converted to ammonia
3. Ammonia converted to urea in the urea cycle
4. Urea filtered by kidneys and excreted via urine

Question 26

Describe transamination

Answer 26

Occurs when additional non-essential AA are required in the AA pool
Occurs principally in the liver
Transfer of an amine group from one AA to a carbon skeleton to form a new AA

Enzymes required = transaminases:

• Alanine aminotransferase (ALT)
• Aspartate aminotransferase (AST)

Cofactor = Vitamin B6
Hence why vitamin B6 is often added to protein supplements or products targeted to athletes/body builders

Question 27

Describe gene expression

Answer 27

Transcription:

	- DNA is transcribed to messenger RNA (mRNA) in the nucleus
	- mRNA moves from the nucleus to the cytosol

Translation:
- mRNA is translated into protein by ribosomes in the cytosol
- In the ribosome, each nucleotide in mRNA is “read” as a codon (3 nucleotides) coding for one AA, to produce a chain of amino acids (=peptide)
- Each codon codes for one specific AA
- tRNA present the correct AA to the ribosome to be attached to the previous AA by a peptide bond
Peptide chains are folded into tertiary and quaternary structures that confer their physiological functions

Question 28

List the functions of amino acids as building blocks of proteins

Answer 28

1. Form rigid structures and movement structures
   
   • Collagen in connective tissue/bond
   • Keratin in fingernails/hair
   • Muscle fibres (actin and myosin), cilia, spindle fibres in mitosis
2. Digestion, absorption, metabolism and transport of nutrients
   
   • Enzymes
   • Transport proteins
   • Membrane transport proteins facilitate cellular uptake: move substances across cell membranes
   • Transport proteins move vitamins/minerals (e.g. vitamin A, iron, copper), and oxygen in the circulation
3. Hormones and neurotransmitters
   
   • Chemical signals/messengers: hormones and neurotransmitters (e.g. thyroxine, insulin; tryptophan is the precursor of serotonin and melatonin, and tyrosine of dopamine, norepinephrine, and adrenaline
4. Acid-base balance
   
   • The pH of the blood and other body tissues must be maintained within a tight range (7.35-7.45 for blood)
   • Proteins act as buffers as they can receive or donate hydrogen ions
5. Immune function
   
   • Antibodies that bind specifically to foreign substances (antigens) to identify them as intruders to the immune system are proteins
   • Insufficient essential amino acids availability may impair antibody formation and immune function
6. Fluid balance
   
   • BP pushes fluid out of the capillary bed at the arterial end into the interstitial space
   • Plasma albumin and globulins attracts water from the interstitial space back into the capillary for venous return
   • If plasma albumin (a protein) is insufficient, fluid remain in interstitial space resulting in oedema
7. Forming glucose
   
   • Gluconeogenesis uses the carbon backbone of glucogenic AA
8. Providing energy (17kJ/g)
   
   • Muscle wasting during starvation, malnutrition or in hypermetabolic medical conditions (e.g. cancer0 => cachexia where muscle protein used for energy production

Question 29

What is the difference between complete, incomplete and complementary proteins?

Answer 29

Cells require a constant pool of EAA for the synthesis of gene products

Complete proteins:

- Contain all 8 (9) EAA in the right ratio
- Classified as complete or high-quality protein
- Include animal proteins (except gelatine), soy and quinoa (but bioavailability may be reduced)

Incomplete proteins:

- Do not contain all 8 (9) EAA, or are low in some of the EAA
- Classified as incomplete or lower quality based on BV
- Plant proteins are generally incomplete. They are limits in sulphur-containing AA (methionine, cytosine, and tryptophan) compared to animal sources
- May contain protease inhibitors, lectins, and other naturally occurring components in plants that may impact the structural components of plant proteins and prevent adequate digestion

Complementary proteins:

- 2 or more incomplete proteins can be combined to form complementary proteins that contain all of the 8 EAAs (for adults) - Complementary proteins do not need to be consumed at the same meal