CHEM215 Lecture Notes - Lecture 3: Hemoglobin, Isoelectric Point, Glutamine
Wk 2:1 – Proteins continued
Protein absorption:
Absorption of all the essential amino acids can be accomplished by:
• Consuming a complete digestible source of protein several times a week (meat,
poultry, fish, eggs, dairy)
Or
• Consuming a mixture of incomplete protein foods eg. Wholegrain (which are
deficient in lysine) and beans, lentils and legumes (which are deficient in
methionine).
These complementary proteins are important in ensuring vegetarians and vegans have a
balanced diet.
Protein quality:
Just because some protein is present in food, doesn’t mean it is bioavailable.
• Animal source provide roughly 15-20g protein per 100g of food. Egg protein is one of
the most complete and digestible of all food proteins.
• Cereals, grains, nuts and legumes provide only 1-2g of protein per 100g of food.
Nitrogen balance:
The human body does not store protein. Since only dietary protein supplies nitrogen, nitrogen
balance can be equated with protein balance.
Consumption of adequate protein achieves nitrogen balance:
Nitrogen input = nitrogen output.
Nitrogen requirements increase during growth, pregnancy and lactation, prolonged illness or
recovery from trauma.
In conditions of starvation or nitrogen imbalance, protein is used for energy and is excreted in
the urine.
The Urea Cycle:
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Wk 2:1 – Proteins continued
The Kjeldahl procedure for protein determination:
• Assumes that all the nitrogen in our food is coming from protein (not always true, but
nitrogen that comes from non-protein sources is insignificant)
• In order to assay for proteins; food sample is boiled in hot H2SO4 under reflux with
heavy metal present as catalyst. Under these conditions protein is broken down into
amino acids, then further broken down into ammonia ions.
The chemical decomposition of the sample is compete when the initially very dark
coloured medium has become clear and colourless.
• When the acid is completed digested, a known amount of solution is placed in a still;
alkali NaOH is added and the liberated ammonia is steam-distilled into an aliquot of
boric acid. The quantity of ammonia in this solution can then be determined by
titration of the excess acid with a
base.
• The proportion of nitrogen in most proteins is ~16% by weight, so a factor of 6.25 is
used to covert nitrogen content to protein content. (6.25 x 16 = 100)
• However deviations from this can occur - cereal proteins contain high levels of
glutamine and gelatine, which can affect the conversion factor.
• The addition of melamine, an organic base that contains 66% nitrogen by mass,
increases the nitrogen content of milk and therefore its apparent protein content —>
this has been done in China to increase the volume of the milk whilst almost
maintaining the correct protein level. This was determined using the Kjeldahl
method.
Chemical reactions, uses and functional properties of proteins in foods.
1. Buffering: the overall charge on a protein depends on the pH of the medium.
2. Denaturation: dénaturation is a disruption of a proteins tertiary structure - the protein
unfolds.
Denaturation is caused by breaking non-covalent bonds (H-bonds). This changes the
properties of a protein - enzymes will be inactivated and solubility decreases.
Whipping egg whites denatures some of the protein and helps stabilise the foam.
We need to denature proteins in our stomach acids so that they are accessible to our
body —> so we can obtain the amino acids from them for structural and functional
uses.
Coagulation of egg white occurs after denaturation - this is the aggregation of
denatured protein molecules forming a precipitate (fried egg)
3. Emulsification: proteins can stabilise emulsions by acting at the oil-water interface -
as protein molecules often have hydrophobic and hydrophilic characteristics they can
situation between the two cases to stabilise them.
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Document Summary
Absorption of all the essential amino acids can be accomplished by: consuming a complete digestible source of protein several times a week (meat, poultry, fish, eggs, dairy) Or: consuming a mixture of incomplete protein foods eg. wholegrain (which are deficient in lysine) and beans, lentils and legumes (which are deficient in methionine). These complementary proteins are important in ensuring vegetarians and vegans have a balanced diet. Just because some protein is present in food, doesn"t mean it is bioavailable: animal source provide roughly 15-20g protein per 100g of food. Egg protein is one of the most complete and digestible of all food proteins: cereals, grains, nuts and legumes provide only 1-2g of protein per 100g of food. Since only dietary protein supplies nitrogen, nitrogen balance can be equated with protein balance. Nitrogen requirements increase during growth, pregnancy and lactation, prolonged illness or recovery from trauma.
