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Notes knowledge clips – ProteinsIntroduction to proteins
Animal proteins milk, egg, blood, gelatin
Plant proteins soy, lupine, sunflower
New proteins potato, algae, leaf proteins
Nutrition: essential amino acids
Structure: for instance collagen contains gelatin
Metabolism: for instance anti-bodies
Taste: for instance by Maillard reaction
Build-up of amino acids (amino group and carboxylic acid group). Linked to
central carbon atom. The differences between amino acids are determined by
the side group R.
Peptides: linking carboxylic group of one amino acid with the amino group of
another amino acid. This is done by a peptide bond.
Oligo-peptides: 2-100 amino acids
Protein: 100-300 amino acids adopts a structure
Properties of proteins depends on amino acids, total folding state of protein
Peptides can be formed by hydrolysing proteins
Meat (muscle) proteins: not soluble so they can give structure to the body
Milk proteins: should be soluble so the calf can drink the milk and the proteins
are dissolved in the milk.
Amino acids
Amino group (NH2) and carboxylic group (COOH) are linked to central carbon
atom (alpha carbon atom).
Also a R group (side chain) is connected to the alpha carbon atom.
4th place is taken by an H.
So all places of the carbon are full you can get stereoisomers .
Proteins: only 20 different amino acids (only alpha-L)
You can have L or D stereoisomer. L means that the amino group is on the
left. D means the amino group is on the right. D-amino acids are not
digestible.
Alpha and beta amino acids. Alpha: amino group is attached to the alpha
carbon atom (the central one). Beta: amino group is attached to one carbon
atom lower (the beta carbon)
Aromatic amino acids: for instance tryptophan: contains a benzene ring
(aromatic group). Does not dissolve very well in water because no polar
group.
Aliphatic amino acids: only carbon-hydrocarbon chains (leucine). Does not
dissolve very well in water because no polar group.
, Polar amino acids: for instance asparagine, has an amide function: C double
bonded O and NH2.
Classification of amino acids
o Non-polar & non-charged: hydrophobic, low solubility in water, are
positioned in inside of protein.
Side chains are aliphatic group, aromatic group, imino acid (thio-
ether)
o Polar & non-charged: hydrophilic, high solubility, on outside of
proteins, reactive
Side chains are hydroxyl groups, sulfhydryl groups, amide
groups (can be reactive under certain conditions)
o Polar & charged: very well soluble, outside of protein, can be
reactive, charge depends on pH!!
Side chains are carboxyl group and amino groups
Low pH: carboxylic group is protonated and neutral, amino group
is protonated and thus positively charged.
High pH: carboxylic group is deprotonated and thus negative
charge. Becomes deprotonated and is neutrally charged.
Classification on nutritional value
o Essential not produced by body (should be taken up by diet)
o Conditionally essential certain people should eat more of them
o Non-essential can be produced by the body
o The reactive amino acids are distributed over all these classes. If you
have a lot of reactions (high reactivity), you can loose nutritional value.
Leucine: non polar side groups so non-polar, non-charged
Glutamic acid: contains an additional COOH group = polar, charged
Tyrosine: contains aromatic ring and hydroxylic group (polar) polar, non-
charged amino acid.
Peptides
More reactive than intact proteins sometimes not convenient
Have taste not always desired because sometimes bitter
Taken up more easily by the body
Can have anti-oxidant, blood-pressure properties
Combination of amino-acids (carboxyl group with amino group) peptide
bond. One water molecule is released! Weight of dipeptide is smaller than two
amino acids together.
Tripeptides, or even longer can be formed from this.
Nomenclature: N-terminal side should always be on the left (the amino acid
where the amino group is still free and not bound). Right side: C-terminal
side with free carboxylic group. Cysteine and glutamic acid are bound at alpha
carbon.
However, if you would have glutathione, the amino acids are bound at the
gamma carbon. You then call it: gamma-glutamyl-cysteinyl-glycine. YL
because these amino acids are bound to next amino acid. Between Cys and
Gly – and between glu and cys = corned line. Because glutamic acid is not
linked by alpha but gamma carboxylic group. Difference in digestibility and
functionality
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