Lecture 10 - Proteins part 2 Flashcards
What are the four places that protein digestion occurs in?
mouth, stomach, pancreas, small intestine
How does protein digestion occur in the mouth?
- no enzymatic digestion (unlike TAGS and carbs)
- mechanical breakdown into smaller protein molecules
How does protein digestion occur in the stomach?
- HCl secreted from parietal cells in the gut go into gastric juice denatures the proteins
- pepsin (endopeptidase) enzyme is secreted as pepsinogen, which is an inactive zymogen
How does protein digestion occur in the pancreas?
- pancreatic juice containing zymogens (inactive digestive enzymes) which break down polypeptide chain in a step-wise manner
How does protein digestion occur in the small intestine?
- zymogens are activated
- enzymes break-down peptides
- absorption of AAs
How is HCl secretion into the gut regulated?
by gastrin, acetycholine and histamine - hormones
What are the two functions of HCl in protein digestion?
- denatures proteins - disrupts hydrogen bonds and electrostatic bonds in 2nd 3rd and 4th structures
- activates pepsin
What are some characteristics of pepsin in protein digestion in the stomach?
- active in an acidic pH, inactive at a neutral pH
- HCl causes a conformational change in pepsinogen, allowing it to then autoactivate itself
- pepsin is an endopeptidase, it cleaves peptide bonds within a polypeptide chain
- generates mostly oligopeptide and some free AAs
How does a zymogen become an active enzyme?
zymogen goes through a proteolytic cleavage to become an active enzyme
What is the steps to protein digestion in the small intestine?
- In the pancreas, trypsinogen (zymogen) goes to SI and is converted to trypsin (enzyme) via an enteropeptidase located in the brush border of the SI, trypsin then activates other zymogens (chymotrypsinogen, proelastase, procarboxypeptidases)
- in the pancreas, chymotrypsinogen, proelastase, and procarboxypeptidase A&B (zymogens) go to the SI and are converted to
- chymotrypsinogen -> chymotrypsin
- proelastase -> elastase
- procarboxypeptidases ->
carboxypeptidase
- in the stomach, aminopeptidase (enzyme made in stomach) goes to SI and facilitates cleavage of peptides
- in the stomach, free AAs and small peptides (oligopeptide) go to SI to be broken down
Where are AA absorbed?
in the upper small intestine
What are the two ways that AA are absorbed?
- facilitated diffusion: does not require Na+ or ATP, will not concentrate against gradient, AA goes through intestinal cell membrane
- active transport (>60% of AAs are absorbed this way): Na dependent, requires ATP, concentrate against gradient
- sodium-dependent transporter (indirect ATP requirement)
What are some characteristics of AA absorption?
- essential AAs may be absorbed faster than non-essential AAs
- competition for absorption exists between the AAs
- free AAs have no absorptive advantage over AAs in foods
What is the transporter required for active transport of proteins?
PEPT1 - peptide transporter 1
What is the fate of AA once they have reached the small intestine?
AAs are either transported out of hte intestinal cell or used directly within the enterocyte for:
1. energy: intestinal cells are being sloughed off, replenish dead cells
2. synthesis of new protein
What percentage of essential AAs are used in SI?
30-40%
What is glutamine used in intestinal enterocytes for?
- generate energy for the cell
- stimulate cell proliferation (to replace shed enterocytes)
- increase synthesis of heat shock proteins (chaperones)
- drive mucus production, which helps to prevent bacterial translocation
Where does AA metabolism take place?
The liver takes up AA for circulation
What does the liver use AAs for?
20% of the AA are used to:
- make new proteins/enzymes: albumin and other transport proteins
- make peptide hormones
80% of the other AA are catabolized:
- NH2 sent to the urea cycle
- carbon skeleton sent to kreb’s cycle (for energy) or used for gluconeogenesis or lipogenesis
Are BCAAs taken up by the liver?
no, instead are anabolic signals for tissues like muscle
What is the cycle of AA metabolism in the liver?
In the SI: free AA, di & tri peptides some large protein fragments are absorbed by SI and sent to circulation
Circulation: these AA are transported
Liver: AA etc. are transported through a portal vein into the liver where they are turned into free AAs
What are the four aspects to consider for protein quality?
- AA composition: any protein that provides all essential AA is considered high quality. Animal protein > plant protein
- digestibility: some proteins are more digestible than others. More digestible means higher quality. Animal protein > plant protein.
- Presence of toxic factors: less toxic factors means higher quality. Animal protein > plant protein. plants have phytochemicals
- species consuming the protein: humans, pigs and chickens have similar protein needs. Rumminants have bacteria in the rumen that can make all AAs, so none are considered essential
What are the two ways to assess protein quality?
- Protein efficiency ratio (PER)
- Chemical score (CS)
What is the Protein efficiency ratio
- used to assess protein quality
- official method in Canada
- young rats are fed diets for 4 weeks, diet is good other than protein, which is included at 10% kcal of the diet
- 10% protein is borderline for health. if there is anything wrong with the protein source, the growth of rats will be impaired
- weighed at beginning and end of 4 weeks
What are the pros and cons of the protein efficiency ratio?
pros: simple, cheap, very sensitive to AA balance, digestibility, toxic factors
cons: rats are not humans, growth, not maintenance, you don’t know why a protein is poor quality
What is the chemical score (CS)?
- assess protein quality
- the test protein is chemically digested into free AA
- these are then quantified by chromatography, and mathematically compared to the composition of whole egg protein
What are the pros and cons of chemical score?
pros: simple and cheap, identifies the limiting AA in the food, used to optimize feeds by mixing different sources of protein
cons: doesn’t account for digestibility, assumes whole egg is an ideal protein
What is the formula for PER?
PER = gain in body mass (g)/ total protein intake (g)
What is the formula for CS?
CS = (abundance of first limiting AA in test protein/ abundance of same AA in whole egg) x 100
What is nitrogen balance?
- measure of N intake in the diet and N loss in urine, feces and sweat
What is the formula for nitrogen balance?
Nitrogen balance (NB) = nitrogen intake-nitrogen loss
NB
nitrogen balance
What is the NB values for each group age and nutrition status?
Growth pregnancy and times of tissue repair - NB > 0
When you don’t have enough protein - NB < 0
For most adults - NB = 0 (in balance)
Why is NB < 0 when you don’t have enough protein?
the problem is exacerbated with poor protein quality because body proteins are used as a source of essential AA (in other words, body proteins are broken down to free up essential AA, ultimately leading to a loss of function). NB < 0 is seen in people with serious tissue injuries, wasting diseases and long-term fasting
How may problems with poor protein quality be overcome?
with high protein quantity
When do problems occur in nitrogen balance?
when protein quality and quantity are low
How do protein requirements vary with life stage?
higher protein requirements in:
infancy, childhood, and in teenagers, during pregnancy and lactation
How does too much protein intake occur?
- high protein diets
- protein supplementation
How does protein deficiency occur?
- deficient in both protein quantity and energy
- deficient in protein quantity
What are some characteristics of high protein diets?
- very popular
- low in carbs
What is the atkins diet?
- C:F:P = 3:64:33
- different phases where macronutriet content varies
- C intake very low, F and P intake very high
- criticized because no attention to C or F consumed
What is the south beach diet?
- C:F:P = 30:40:30
- for C intake, there is emphasis on low glycemic index foods
- P is consistent throughout the various phases about 30%
What are the observed clinical results of high protein diets?
- short term weight loss is comparable to other diet approaches
- some studies show improved insulin sensitivity with high protein as compared to high CHO diets (reduced burden on the pancreas to generate insulin to deal with CHO)
- conflicting results with respect to effects on cardiovascular health. A moderate increase in protein appears to be cardioprotective, but high protein may be a concern in the long-term
- people with kidney diseases should avoid high-protein diets
What are protein supplements used for?
- athletes
- help to ensure that the correct balance of AAs are delivered to the muscle
- however this would be the same if a person ate a high quality protein, egg etc.
What do most protein supplements deliver?
high levels of BCAAs, rapidly absorbed and delivered to the muscle
What occurs with protein supplements and aging?
- anabolic response of the muscle to a protein meal gradually diminishes after 40 years of age
- can be improved with protein supplements
What is marasmus?
- protein and energy deficiency
- very low intake of a balanced diet with around 8-10% protein
- because everything is in balance, the body switches to starvation mode: well organized utilization of body fuel stores allows survival, eventually leading to a complete loss of body fat which causes wrinkled appearance to skin
- adults cope better than children
- characterized by complete loss of body fat and muscle, peeling skin, uneven pigmentation
What is kwashiorker?
- protein deficiency
- diet has sufficient calories but is deficient in protein
- only 1-2% protein in diet
- seen in developing countries where agriculture is key to diet
- high carb foods: when child is weaned from mothers breast milk to high carb low protein, problems emerge
- lots of carbs, but no protein to metabolize or transport nutrients
- characterized by enlarged abdomen, burns on the skin and diarrhea
Why does kwashiorker result in an enlarged abdomen
- decreased plasma protein causes an osmotic imbalance in the gut, leading to a swelling of the gut, fluid leaks from blood vessels)
- liver is enlarged due to the inability to export fat from the liver (can’t make VLDL)
