1. Each amino acid shares a common structure, with an amine group (NH2),
Food proteins.
Proteins are polymers of amino acids and are referred to as polypeptides.
Each amino acid shares a common structure, with an amine group (NH2),
an acid group (COOH), and a central carbon atom bonded to hydrogen
and to a side chain (R). The distinct side chains or groups allow for
Classification into acidic, basic, neutral, aromatic and sulfur-containing
Amino acids (see figure).
Proteins are composed of amino acids joined by peptide bonds (fig).
A peptide bond connects the acid end of one amino acid with the amino
End of another.
Two amino acids bonded together form a dipeptide.
Polypeptides can be several hundreds amino acids long.
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2. The formation of peptide bond – a condensation reaction
(amino acids link together to form polypeptides)
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3. Classification of Amino acids (based upon Their side chains)
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4. Classification of amino acids (based upon Their side chains)
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5. Structure of proteins
There are four specific levels that provide description of what the polypeptide is composed of and its shape:
a) Primary: linear sequence of amino acids in order within a polypeptide.
Secondary: refers to whether the amino acids together assume either α-helix or β-sheet configuration (see fig)
c) Tertiary: the overall three dimensional shape achieved by the folding of the entire protein molecule. This level of structure is stabilized by bonds such as hydrogen and disulphides between amino acids that happen to be in close proximity to one another due to folded situation of the structure.
d) Quatenary: If a protein contains more than one polypeptide chain in its structure (casein, collagen, myosin), its overall spatial structure is referred to as quatenary.
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6. There are four level of protein structure.
Primary structure …backbone of protein molecule.
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8. Repeating sequence N-C-C-N-C-C.
Sections of secondary protein structure. The amino acids of a protein may
Assume the configuration of the alpha helix or the beta sheet. Notice the
Repeating sequence N-C-C-N-C-C.
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9. …secondary structure folding into a 3D conformation
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10. e.g. globular: egg albumen
e.g. fibrous: collagen
e.g. globular: egg albumen
…fold up to bury…
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11. Other examples: Phosphoprotein; protein with phosphate such as that of
Proteins may exist as
Conjugated form: combine with non-protein such as carbohydrate or lipids into complex molecules. E.g. glycoprotein such as those in egg white (ovomucoid and ovalbumin) contain sugar molecules. The bad cholesterol or low density lipoproteins (LDL) are complexed with lipid.
Other examples: Phosphoprotein; protein with phosphate such as that of
Casein in milk.
(b) Non-conjugated form: not bound to other substances, i.e. contain only
Amino acids.
Proteins also have shape.
Spherical, like a ball. Typically soluble in water, and termed globular. e.g. albumin, myoglobin.
Elongated, like a twisted rope. Generally insoluble and are called
Fibrous. E.g. collagen. See table.
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12. Examples of food protein structures and shapes
Protein shape
Egg albumen
globular
spherical
Meat and legume globulins
collagen
fibrous
elongated
elastin
Lipoproteins
conjugated
Protein bound to lipid LDL
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13. …A complex of 2 or more tertiary structures
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15. Chemical reactions and functional properties
Other than nutritional, proteins play other roles. They take parts in
Chemical reactions that affect products’ properties, and most
Importantly possess functional properties.
Buffering
Preventing pH change by undergoing ionization reaction.
This is a function of amino acids that make up the protein structure.
COOH and NH3 groups can ionize and become charged.
COOH ionizes to COO- + H+ i.e. (COOH behave as acids),
NH2 + H+ to NH3+ i.e. (NH2 behave as bases).
Since proteins have both amino and carboxyl groups, they can behave
As both acids or bases (i.e. an amphoteric substance).
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16. At certain pH, in acid range;
Number of positive charges exceed negative on the surface of protein molecule.
In acid pH range, the net charge on the protein would be positive, thus help it to be polar and soluble in water.
At certain pH, in alkaline range;
Total number of negative charges exceed positive on the surface of protein
Molecule. In the alkaline pH range , the net charge on the protein would be
Negative, and helps to be polar and soluble in water.
HOT:
What happen at isoelectric point? i.e. when the net charge is zero!
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19. Angel cake: pasteurized egg vs fresh egg
HOT 1
Adding sugar to surimi
HOT2
Angel cake: pasteurized egg vs fresh egg
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