Protein Flashcards
essentiality
Non-essential or “dispensable” AAs
Can be produced by the body via
transamination of amino acids as required
Conditionally essential AAs
Essential only at specific times:
• during infancy ( e.g. histidine)
• during disease or trauma to meet the pace of
demand: must come from the diet at these
times to meet requirements
Essential or indispensable AAs
Must be provided by the diet as the body is
unable to produce these at all, or not at a
sufficient rate to meet demand
There are 8 essential amino acids in adulthood
AA are essential (EAA) because:
- Inability to form the carbon skeleton of the EAA (lack of enzymes)
- Inability to attach the carbon skeleton to the amine group (lack of enzymes)
- Inability to perform the above at a fast enough pace to meet the demand
- Muscle mass is a source of EAA: this may be another reason for protein catabolism
when the diet does not provide adequate protein (amount and type of protein)
- Overall protein turn-over (protein synthesis and degradation at the cellular level) is
a dynamic process providing the EAA required in new synthesis
- Protein turn-over accounts for 10-25% of BMR
- Protein turn-over is estimated to represent 1-2% of body protein daily (up to over
300 g). Muscle mass accounts for 20-35% of protein turn-ove
conditionally essential
Glutamine and arginine after trauma and infection
• Needed for repair and recovery
Histidine in infancy
• Increased requirements in growth and development
Tyrosine in phenylketonuria (PKU)
• Conversion of phenylalanine to tyrosine requires phenylalanine hydroxylase
• In PKU there is a mutation in this enzyme and patients cannot convert
phenylalanine to tyrosine via transamination.
Phenylalanine accumulates, resulting in toxicity. Dietary phenylalanine must be
restricted.
Tyrosine becomes ”essential” because it cannot be produced from phenylalanine;
=> must be provided by the diet
• https://medlineplus.gov/genetics/condition/phenylketonuria/
fate of amino acids
Amino acid pool in cells come from
1/ protien ‘tunover which is
1. amino acids from cell breakdown
2. amino acids from diet
AMino acids in pool of cell then go to
Transamination
non essential requored in pool AA transfer of amine group from one AA to a carbon skeleton to form new AA
1. synthesis of protien for cell structure, immune function, enzymes, hormones and other essential functions
2. SYnthesis of non protien nitrogen containing compounds
Deamination
AA converted to puruvate and acetyle coA
- KEtone production from carbon skeletons of some amino acids
- Fat production in excess
- GLucose production from carbon skeletons of some amino acids by gluconeogensieis
- Energy production from amino acid carbon skeletons yeilds on average 4kcal/g
Protein DIgestion
MOUTH
STOMACH - partially digested pepsin HCL
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
PANCREAS- enzymes released further digestion
SMALL INTESTINE - final digestion to aino acids
Chyme-nutrient mixture stimulates secretion of secretin and CCK from S cells and I cells of
the duodenum, respectively
• Secretin & CCK stimulate secretion of bicarbonate and proteases /peptidases from the
pancreas
• Proteases/peptidases are secreted as zymogens (inactive enzymes) which are activated in
the small intestine
• Polypeptides are digested by peptidases into tripeptides, dipeptides & AAs
Enterocyte brush border
• Membrane-bound
peptidases digest
peptides at the brush
border
Within enterocyte
• Cytosolic peptidases
digest tripeptides and
dipeptides into amino
acids within the
enterocyte
LIVER- amino acids absorbed into portal vein and transported to liver enter bloodstream
LARGE INTESTINE
Protien absorption
• Amino acids are absorbed into enterocytes via different transporters depending on the
amino acid. These transporters may be sodium-dependent or independent
• 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)
FUnction amino acids
Form rigid tructures and movement structures DIgestio and absorption, metabolism and transport Hormones and neurotransmitters Acid base balanc Immune function fluid balance forming glucose providing energy
ROLE OF PROTEIN IN FLUID
BALANCE
• Blood pressure pushes fluid out of 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 remains in interstitial space resulting in edema
measures of protei quality
Biological Value (BV) of protein
- Protein Efficiency Ratio (PER)
- Chemical Score (CS)
- Protein Digestibility Corrected Amino Acid Score (PDCAAS)
BV?
Biological Value (BV) of protein
BV = how efficiently the dietary protein is converted to body tissue protein. If all 8 (9) EAA are
present, the body can use the dietary protein more efficiently. The measure takes into
account % of urinary and faecal nitrogen excretion:
Nitrogen retained (g) / Nitrogen absorbed (g) x 100
E.g.: Egg white: BV of 100 = 100% of the nitrogen absorbed from the egg protein is retained.
The amino acid composition of egg white (protein part of the egg) matches the body’s
proteins amino acid composition the closest from all dietary protein.
BV is of clinical importance when protein intake must be limited (e.g. in some kidney
diseases, or if dietary intake is reduced)
PER
- Protein Efficiency Ratio (PER)
PER= Weight gain (g) / protein consumed
Compares weight gain of a laboratory animal consuming the test protein with the weight gain of an animal
consuming a reference protein. PER reflects BV because growth and weight gain depends on uptake of
amino acids to form body tissue. Relevant in conditions where protein intake is under the requirements.
CS
- Chemical Score (CS)
Chemical score = milligram (mg) of each EAA per gram of the test food OVER the amount in mg of each
EAA per gram of the reference food. The lowest ratio measured for an EAA (e.g. the “limiting” amino acid)
of the test protein is the chemical score for that tested protein (0 - 1.0).
PDCAAS
- Protein Digestibility Corrected Amino Acid Score (PDCAAS)
PDCAAS = Chemical score X digestibility*
Protein digestibility = amount of amino acids that are absorbed (i.e. that have been freed through
digestion. This is the amino acid profile able to support growth and maintenance. Highest score is 1: milk
proteins, egg and soy.
*Digestibility of animal proteins= ~90-100%; plant proteins = ~70-99%. Digestibility is influenced by other
components of the food.
COMPLETE VERSUS INCOMPLETE PROTEINS
Complete proteins
• Contain all 8 (9) essential amino acids in the right ratio
• Classified as “complete” or high-quality protein
• Include animal proteins (except gelatin), soy and quinoa (but bioavailability may be reduced)
Incomplete proteins
• Do not contain all 8 (9) essential amino acids, or are low in some of the EAAs
• Classified as “incomplete”, or lower quality based on BV
• Plant proteins are generally incomplete. They are limited in sulfur-containing amino acids
(methionine, cysteine, and tryptophan) compared to animal sources
• May contain proteases inhibitors, lectins, and other naturally occurring components in plants
that may affect the structural components of plant proteins & prevent adequate digestion
Complementary proteins
• 2 or more incomplete proteins can be combined to form complementary proteins that contain
all the 8 EAAs (for adults)
• Complementary proteins do not need to be consumed at the same meal
Endogenous nitrogen recycling
• The absorption of AA from cellular debris, mucous,
enzymes etc. for recycling of AAs in the protein turnover constitutes a sizeable amount (estimated at
~90 g /day, but not all appears absorbed)
• The need for combining plant protein sources in
vegetarian and vegan diets has thus been
challenged. Is it really necessary?
Myths
Myths or facts about high protein diets?
• A high protein diet will “overburden” kidneys’ capacity to excrete excess urea and lead to
dehydration and kidney damage: true or false? Reasoning?
• A high protein diet will increase urinary bone calcium excretion and thus increase the risk of
osteoporosis: true or false? Reasoning?
• A high protein diet will “acidify” the body: true or false? Reasoning?
• Gluten is toxic to the body: true or false? Reasoning?
• When excess protein intake is primarily from animal protein, the rest of the diet is low in fiber,
vitamins, minerals and phytochemicals intake, high in saturated /trans fats and cholesterol
intake, affecting health outcomes in the long term: true or false? Reasoning?
