Download PROTEINS - University of Akron

PROTEINS
COMPOSITION
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Organic
Amino Acids
Central carbon atom
Carboxyl group (COOH)
Amino group (NH2)
Group or side chain - distinguishes one amino
acid from another
AMINO ACIDS
STRUCTURE
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PEPTIDE BONDS-strong bonds
DIPEPTIDES - 2 amino acids
POLYPEPTIDES - many
Nature of side group determines conformation
CONFORMATION - determined by specific
amino acid and sequence – shape of protein in
space
PEPTIDE BONDS
PRIMARY STRUCTURE
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Specific sequence of amino acids joined by
peptide bonds along the peptide chain
Specific sequence determines form and shape
SECONDARY STRUCTURE
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Alpha-helix - ordered; corkscrew
Beta pleated sheet - ordered; zig-zag
Random coil - disordered; side chains prevent
formation of alpha or beta structure
Three dimensional
TERTIARY STRUCTURE
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Built on secondary structure
May contain alpha, beta, and random coil
Two types:
Fibrous proteins - collagen, myosin & actin
rods or fibers
Globular proteins - whey, myoglobin;
spherical or elliptical
QUATERNARY STRUCTURE
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More than one polypeptide chain
Chains not identical
Actomyosin in muscles, casein in milk
REACTIONS & PROPERTIES
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AMPHOTERIC- amino group and carboxyl group
both ionized  can behave as acid or base =
buffering capacity
ISOELECTRIC POINT - point at which protein is
electrically neutral; positive charges = negative
charges
Food processing – knowledge used to make cottage
cheese
REACTIONS CONT’D
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WATER-BINDING CAPACITY - hydrophilic;
helps maintain stability of protein dispersion
Some proteins readily disperse in salt solutions:
SALTING IN - when salt solution increases
dispersibility of protein (brine in ham)
SALTING OUT - high salt concentration; salts
compete with protein for water; during freezing
process
DENATURATION
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Change in secondary, tertiary, quaternary
structure of protein
Does not break bonds - unfolds them
Caused by: heat, pH change, ionic strength;
freezing, surface changes
Permanent in food
Lose functional properties
HYDROLYSIS
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Breaking of peptide bonds
Acid digestion
Proteolytic enzymes: tenderizers
Papain, bromelain for meat
Rennet for cheese
MAILLARD BROWNING
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Non-enzymatic browning
Free carboxyl group reacts with free amino
group
Color and flavor of baked goods
Favored by: High sugar content, high protein
content, high temperatures, high pH, low water
content
Discoloration of food products like powdered
sugar: must be “desugared” enzymatically