Protein

Question 1

what is the average nitrogen content?

Answer 1

Average nitrogen of proteins = 16%
Convert nitrogen cont into protein content:
1/0.16 = 6.25
N x 6.25 = protein content

Question 2

What is the significance of protein?

Answer 2

Acceptable Macronutrient Distribution Range: 10-35%
Protein in the human body:
- 40% in skeletal muscle
- 25% in body organs
- 35% in skin and blood
Proteins Functions:
1. structural
2. regulatory
3. energy

Question 3

What are the structural functions?

Answer 3

• contractile proteins => actin, smooth muscle
• fibrous protein => collagen, elastin, certain
• globular proteins=> spherical shape e.g. myoglobin

Question 4

What are the regulatory functions?

Answer 4

Enzymes/catalysts: changing rate of reactions
- cofactors => minerals (metalloprotein), folate, B-vitamins

Hormones/Messagners: acting as chemical messengers

Transporters:
- in blood - hemoglobin, albumin (fatty acids), prealbumin = transthyretin (thyroxine and RBP), transferring (iron), ceruloplasmin (copper), lipoprotein

Immunoglobins/antibodies

Buffers=> amino acids act as acids or bases

Fluid balance:
- ascites => fluid balance is impaired in protein deficiency - abnormal accumulation of fluid in the abdominal

Acute Phase Reactant Proteins:
- formed in the liver - in response to infection, injury/inflammation
functions:
- stimulate the immune system, wound healing, chelate and remove iron from circulating
C-reactive protein = clinical indicator for inflammatory status

Question 5

What are other functions?

Answer 5

Storage proteins:

Conjugated proteins: Glycoproteins

Question 6

What is the storage protein for glucose storage?

Answer 6

glycogenin

Question 7

Which glycoprotein has a role in clinical diagnosis?

Answer 7

glycated hemoglobin (A1C, blood glucose level)

Question 8

What are the sources of energy at excessive intake?

Answer 8

Deaminination of amino aids; enters into:
- TCA cycle -> formation of ATP
- glucogenesis -> energy storage
- fatty acid synthesis -> energy storage

Question 9

Describe the protein structures.

Answer 9

Primary: polypeptide backbone does not differ btw polypeptide chains but side chains differ

Secondary: alpha (cylindric) and beta-pleated sheets (stretched out backbone) => both stable

tertiary: interaction among the amino acids residues or side chains

Quaternary: oligomers = 2 or 4 polypeptides chains => held together by hydrogen bonds and electrostatic attraction

Question 10

What are the essential amino acids?

Answer 10

phenylalanine, valine, threonine, methionine, tryptophan, histidine, isoleucine, leucine, lysine

Question 11

What is phenylketonuria (PKU)?

Answer 11

missing phenylalanine hydroxylase => converts phenylalanine to tyrosine
- instead into phenylpyruvic acid
- interferes with neurotransmitter synthesis saying retaration
Is an essential amino acid
- classic diet therapy

Question 12

what are exogenous sources of amino acid?

Answer 12

Animal-sourced foods: meats, poultry, fish, dairy
plant-sourced foods: almond milk, tofu, grains, legumes
Protein supplements/protein bars
amino acids supplements

in GI tract:
- desquamated mucosal cells
- digestive enzymes and glycoproteins

Question 13

In the case of proteins, the small intestine can only absorb

Answer 13

amino acids, dipeptides, tripeptides

Question 14

How is protein digested in the stomach?

Answer 14

1. release of hydrochloric acid (HCl) from parietal cells, which is stimulated by: gastrin, acetylcholine, histamine
   - protein degradation => protein unfolding; disrupts hydrogen and electrostatic bonds (quaternary, tertiary, secondary)
2. Pepsinogen
   - released from chief cells => pepsin
   - hydrolysis of peptide bonds
   - protein -> large polypeptides, oligopeptides, free amino acids

Question 15

How is protein digested in the small intestine?

Answer 15

the major site of protein digestion, absorption

acidic chyme enters the duodenum
- stimulation of secretion of secretin and cholecystokinin
- stimulate pancreatic juice secretion from acinar cells -> containing digestive enzymes (zymogens)
- stimulates the secretion of enteropeptidases released from brush border
trypsinogen => trypsin
chymotrypsinogen => chymotrypsin
proelastase => elastase
procollagenase => collagenase
procarboxypeptidases A and B => Carboxypeptidases A and B -> only exopeptidase => release of amino acids from C-terminal end of a peptide chain

Question 16

Aminopeptidases

Answer 16

hydrolyze AA from the N-terminal end of oligopeptides

Question 17

Dipeptideylaminopetidases

Answer 17

hydrolyze AA from the N-terminal and of dipeptides

Question 18

tripeptidase

Answer 18

hydrolyze specific AA to yield dipeptide and 1 free AA

Question 19

How much amino acids aborbed in different parts of the body?

Answer 19

60% => form of di- or tripeptides
pept1 => transporter into enterocytes
40% of absorbed AA as free amino acids

30-50% of AA used by intestine
50-70% of amino acids transported vis basolateral membrane to portal blood

Question 20

Amino acid transport

Answer 20

y+ => passive
ASC => active transport; binding of Na+
- amino acids need to be transported out of the enterocytes -> reach blood

Question 21

How is the uptake of polypeptides or proteins directly into the bloodstream bad?

Answer 21

• leaks btw epithelial cell junctions
• transport by vesicles to the submucosa
  potential consequences:
• acute immune response
• development of sensitivity to polypeptides
• food allergy, asthma
• idiosyncratic drug reaction

vulnerable population:
- atopic heredity
- infants => feed formula solely -> resolution breastfeed for 6 months or use partially or extensively hydrolyzed infant formula (smaller protein fragments)
- GI tract diseases

Question 22

How are amino acids used in enterocytes?

Answer 22

synthesis of proteins, nitrogen-containing compounds and energy
- new digestive enzymes
- hormones
-structural proteins and nucleotides for new intestinal cells

Retained amino acids include: glutamine, glutamate, aspartate, arginine, methionine

Question 23

What is the role of glutamine?

Answer 23

• the primary source of energy
• formation of alanine
• trophic effects -> stimulates GI mucosal cell proliferation
  -> prevents atrophy of gut mucosa
  -> creates a barrier against bacterial translocation
  w/ threonine => synthesis of mucin
  -> produced in skeletal muscles => taken up by intestinal cells
  -> Enteral and parental nutrition products are enriched with glutamine

Question 24

What is the role of glutamate?

Answer 24

90% of absorbed glutamate used by the intestine
- energy
- formation of alanine
- formation of proline => released into portal blood for delivery to the liver
- with aspartate -> ornithine -> citrulline (urea cycle)
- with glycine and cysteine -> glutathione (antioxidant)

Question 25

What is the role of arginine?

Answer 25

- used in intestinal cells; source of energy - little found in portal blood - transaminated to => oxaloacetate (TCA cycle); ornithine (urea cycle)

Question 26

what is the role of arginine?

Answer 26

- critulline (formation) and urea - intestinal injury => conditionally indispensable amino acid; arginine or citrulline supplementation in the diet

Question 27

what is the role of methionine?

Answer 27

50% -> metabolized in the intestine - precursor for S-adenosylmethionine (SAM) -> one-carbon donor => formation of neurotransmitter, DNA methylation - transformed to cysteine via tran-sulfuration pathway (vit B-6 dependent) => used for formation of glutathione and taurine (bile salts)

Question 28

Role of the liver

Answer 28

- monitor the passage of AA from the gut to the peripheral circulation - monitors AA concentration in plasma - uses AA for protein synthesis - primary site of AA catabolism

Question 29

Which hormone regulates protein anabolism?

Answer 29

Insulin - promotes cellular AA uptake ' - simulates transcellular movement of AA transporter to membrane - inhibition of AA oxidation - stimulates protein synthesis After/during meal: inc. protein synthesis and dec. degradation of proteins

Question 30

Protein formation in liver

Answer 30

For release into plasma: Albumin - most abundant plasma proteins - critical for fluid balance - indicator of visceral protein status - transports proteins: nutrients (Ca, Zn, B-6, FA) and drugs & hormones Transthyretin: - transports thyroid hormones (T4) and vit A - a more sensitive indicator of visceral protein status -> rapid protein turnover More transport proteins: VLDL -> lipid transport HDL-> reverse cholesterol transport ceruloplasmin -> transport of Fe others: prothrombin -> blood coagulation immunoglobins -> immune response acute phase proteins -> infection, injury, inflammation purines and pyrimidines are synthesized in the liver

Question 31

interorgan amino acid transport

Answer 31

provide amino acids for protein synthesis and other functions to all tissues - happens in all states

Question 32

Dietary proteins: whey vs. casein

Answer 32

whey protein: - 'fast protein' => digested faster, amino acid reached, quicker - fast, high, transient - inc, protein synthesis and amino acid oxidation - no change in protein breakdown casein protein: - 'slow proteins' => slow digestion; amino acids in blood deposited slowly - slow, lower, prolonged - protein synthesis slightly inc, - amino acid oxidation is moderately stimulated - protein breakdown is marked inhibited by 30%

Question 33

When do protein and AA catabolism occur?

Answer 33

1. During starvation 2. low CHO => TCA cycle intermediates, brain glucose Protein intake > needs for growth and maintenance

Question 34

hormones involved in catabolism

Answer 34

glucagon catecholamines glucocorticoids thyroxin

Question 35

Transamination

Answer 35

transfer of -NH2 group from one amino acid to alpha-keto acid (carbon skeleton of amino acids) - involved in alanine, glutamate, aspartate

Question 36

Deamination

Answer 36

release of -NH2 group form of ammonia serine to pyruvate

Question 37

Deamidation

Answer 37

loss of an amide functional group -NH2 in the side chain of amino acids glutamine and asparagine in the form of ammonia glutamine to glutamate

Question 38

Pathways for ammonia removal in the skeletal muscle:

Answer 38

glutamate and glutamine formation alanine-glucose cycle

Question 39

Glutamine and glutamate formation:

Answer 39

disposal pathway in extrahepatic tissues - glutamate is generated in the muscle -> deaminate by glutamate dehydrogenase to alpha-ketoglutarate and ammonia -> in the form of glutamine carried out of the cells to the liver, kidney, and intestine; - glutamine-> also made in the brain and adipose tissues; greated formation in muscle and lungs

Question 40

Alanine-glucose cycle

Answer 40

in the fed state: glutamate => form glutamine for disposal of ammonia in fasting state: - between meals and excessive glucose needs of the muscle - glutamate -> alanine -> released into blood -> liver takes up alanine and catabolizes it and releases ammonia to urea cycle

Question 41

Hyperammonemia [NH3]

Answer 41

impaired urea cycle - liver damage, liver disease - urea cycle enzyme defects Consequences -> brain malfunction and coma Treatments: - nitrogen-restricted diet, drugs that acidify the GI tract, antibiotics

Question 42

What can the carbon skeleton be used for?

Answer 42

energy, glucose (TCA cycle), ketone bodies (pyruvate acetyl-CoA, acetoacetate), cholesterol (directly from leucine), fatty acids

Question 43

Dietary proteins are required for:

Answer 43

- supply of nitrogen for the synthesis of non-essential AA - supply of essential AA

Question 44

What factors do the quality of protein depend on:

Answer 44

1) amount and proportion of essential amino acids 2) Protein digestibility 3) amino acid availability

Question 45

high-quality or complete proteins

Answer 45

proteins containing all essential AA in approximate amounts as needed by humans - animal sources, soy proteins, quinoa

Question 46

Low-quality or incomplete proteins

Answer 46

proteins having too low amounts of one or more essential AA than needed by humans - plant sources

Question 47

limiting amino acid

Answer 47

the essential amino acid present in the lowest amount in the food relative to the requirement for this AA in the human body - mutual supplementation ex. legumes are high in lysine but low in methionine and cysteine grain products are low in lysine but high in methionine and cysteine

Question 48

Protein digestibility

Answer 48

amount of amino acids being absorbed after ingestion of given proteins

Question 49

Amino acid availability

Answer 49

diminished during food processing because of: - extensive heating => creates protein side chains that cannot be hydrolyzed during protein digestion - Maillard reaction => presence of sugars -> reduced availability of lysine - oxidation of amino acids

Question 50

How can you evaluate protein quality?

Answer 50

1. chemical or amino acid score 2. protein digestibility corrected amino acid score 3. protein efficiency ratio 4. Biological value 5. net protein utilization

Question 51

Chemical or amino acids score

Answer 51

analytical determination of AA composition of the test protein Procedure: 1. Quantification of essential AA conc. in the protein 2. comparison to the value of reference proteins considered to have a value of 100 the essential amino acid with the lowest score (in %) in the test protein = limiting amino acid of the test protein determines the amino acid score

Question 52

protein digestibility corrected amino acid score

Answer 52

= amino acid score (in %) x true digestibility (in %) Nitrogen => indirect method to quantify protein content

Question 53

protein efficiency ratio

Answer 53

= gain in body weight/grams of protein consumed measurements: body weight before and after diet total grams of test proteins consumed

Question 54

Biological value

Answer 54

measure for the amount of retained amino acids => part of amino acids that were absorbed + being utilized = Nitrogen retained/nitrogen absorbed x 100 high biological value -> provides amino acids in amounts consistent w/ the individual amino acid requirements

Question 55

net protein utilization

Answer 55

measure how well the ingested proteins are digested and absorbed and how well the amino acids absorbed are utilized = nitrogen retained/nitrogen consumed x 100