proteins Flashcards
phenylketonuria
- when a person is unable to breakdown Phe into Tyr
- Tyr becomes an essential AA
- solution: limit Phe in diet and supplement with Tyr
liver disease (cirrhosis)
- impairs Phe and Met catabolism
- Tyr and Cys are usually synthesized from Phe and Met
- Tyr and Cys become essential because they cannot be synthesized
Basic AAs
- Lysine: involved in the production of carnitine, limiting in grains
- Arginine: conditionally essential (for infants)
- Histidine: used to produce histamine (inflammation)
Acidic AAs
- Aspartate
- Glutamate
both non-essential
can react with basic amino groups to form Gln and Asn
Asparagine and Glutamine
- both non-essential
- made when amino group reacts with asp and glu
- glutamate is used to make GABA
Glycine
- non-essential
- used to produce porphyrin (component of heme)
what are the essential amino acids
- histidine
- isoleucine
- leucine
- lysine
- methionine
- phenylalanine
- threonine
- tryptophan
- valine
branched chain AAs
leucine, isoleucine and valine
- all are essential
- high levels in circulation (not catabolized in the liver)
- promote protein synthesis
- BCAA levels high in protein suppliments
Methionine
- 1st step in the synthesis of all proteins
- limitting in legumes
- makes cystine
hydroxylated (OH) AAs
Serine and threonine
- OH in their side chains is important for the phosphorylation PTM of proteins
Cystine
- sulfur containing
- non-essential
- made from methionine*
- used in glutathione synthesis in oxidant defence
- “spares” methionine when it is consumed in diet
Aromatic AAs (rings)
- phenylalanine: used to make Phe
- tyrosine: spares “Phe”
- Tryptophan: used for niacin (B3) synthesis
- Proline: important for collagen production, aliphatic side chain
PTM: phosphorylation
- happens on the OH side chains of serine, threonine and tyrosine in peptide chains
- needs phosphate
Hydroxylation of lysine
- produces hydroxylysine
- copper dependent
hydroxylation of proline
- makes hydroxyproline (Pro-OH)
- vitamin C dependent
- important in collagen subunits
- associated with scurvy
PTM: gamma-carboxylation
- vitamin K dependent
- required for calcium homeostasis and blood clotting
- another carboxyl group is added to glutamate to allow protein to bind to calcium
PTM: iodination
- crucial for the formation of thyroid hormones
- crucial for regulation of metabolic rate
PTM: ADP ribosylation
- dependent on Niacin (vit B3)
- adding ADP-ribose to an acceptor protein
- when NAD+ is broken down in the cell, ADP-ribose and nicotinamide are the products
Brief protein digestion
- Mouth: no enzymatic breakdown, just mechanical
- Stomach: HCl in gastric juice is released, denatures proteins and activates pepsin
- Pancreas: pancreatic juice containing zymogens (inactive enzymes)
- Small intestine: activates zymogens to produce small peptides and AAs
protein digestion in the stomach
- HCl is present in gastric juice, it denatures proteins and activates pepsin
- pepsin is an endopeptidase - the first enzymatic part of breakdown
- pepsin is secreted as pepsinogen (inactive) but is activated by HCl in the stomach
- pepsin generates oligopeptides and some free AAs to be further digested in the SI
protein digestion in the small intestine
- trypsinogen travels from the pancreas and is converted to trypsin by enteropeptidase in the brush border
- once activated, trypsin will activate all other zymogens
- the enzymes breakdown oligopeptides from the stomach into free AAs and small peptides
which part of the polypeptide chain does each intestinal enzyme attack
trypsin: basic AAs
elastase: neutral aliphatic AAs
pepsin and chymotrypsin: large neutral AAs
amino peptidase: removes N-terminal end
carboxypeptidase: removes C-terminal end
AA absorption
- most are absorbed by active transport
- uses PEPT1 transporter - sodium dependent (indirect ATP requirement)
- minor amount absorbed by facilitated diffusion
Are all AAs absorbed the same?
- essential AAs are absorbed faster than non-essential AAs
- free AAs (supplements) have no advantage over food AAs
what is glutamine used in intestine cells for
- generate energy for the cell
- stimulate cell proliferation
- increase synthesis of Heat shock proteins
- drive mucus production
how does the liver use amino acids
- uses 20% to make new proteins, enzymes and make peptide hormones
- catabolizes the remaining 80% were NH3 is sent to the urea cycle and the carbon skeleton os sent to the Kreb’s cycle or used for gluconeogenesis or lipogenesis
what is the basis of protein quality
- AA composition: more essential AAs = higher quality
- digestibility: more digestible = higher quality
- presence of toxic factors: less toxic factors = higher quality (e.g. soybeans contain inhibitors that interfere with trypsin thus preventing protein digestion)
- Species consuming the protein: ruminants have different needs than us because bacteria in the rumen can make all AAs
Protein Efficiency ratio
Test - rats fed a diet with adequate nutrients except protein (10% kcal)
- pros: sensitive to AA balance, digestibility and toxic factors
- cons: don’t know WHY the protein is poor quality
PER = gain in body mass / total protein intake
Chemical score (CS)
Test: test protein is digested into free AAs then quantified and compares to the AA composition of whole egg protein (perfect protein)
- whichever AA gives the lowest % (test/egg) is the CS
- pros: identifies limiting AA
- cons: doesn’t account for digestibility and toxins
CS = (amount of limiting AA in test protein/amount of same AA in egg) x 100
Nitrogen balance
NB = N intake - N loss
- most people NB = 0
- if you don’t have enough protein NB < 0
- during growth and pregnancy NB > 0
consequences of high protein diets
- short term weight loss
- improved insulin sensitivity compared to CHO diets
- moderate increase in protein is cardio productive
- high protein may be a concern longterm
Marasmus
- protein AND calorie deficiency
- body switches to starvation mode
- characterized by complete loss of body fat and muscle, peeling skin, uneven pigmentation
Kwashiorkor
- sufficient calories but very deficient in protein (1-2% protein)
- comes with high CHO foods
- characterized by enlarged abdomen, burns on skin and diarrhea
- enlarged abdomen due to decreased plasma proteins causing osmotic imbalance (swelling of gut)
- liver is enlarged due to inability to export fat from liver (can’t make VLDL)
