PROTEINS
type
Function
Example
Enzymatic
Catalyse chemical reactions
Digestive enzymes
Defensive
Protect against disease.
Antibodies
Storage
Storage of amino acids
Casein, Ovalbumin
Transport
Transport of substances
Haemoglobin
Hormonal
Coordination of an organism’s activities
Receptor
Response to chemical stimuli
Contractile & motor
Structural
Insulin
Receptors in nerve cells
Movement of cilia, flagella, muscles
Actin, myosin
Provide support & framework Keratin, elastin, collagen
Amino acids
Amino group + carboxyl group + hydrogen atom + R group/side chain (differentiate)
Non polar – hydrophobic (methionine, tryptophan, glycine)
Polar – hydrophilic (serine, tyrosine, cysteine)
Negative – acidic (aspartic acid, glutamic acid)
Positive – basic (lysine, arginine, histidine)
Peptide bond – covalent bond from a dehydration reaction between carboxyl group of one
protein and amino group of the other protein
Interactions in proteins –
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Hydrogen bonds – between strong polar groups, weak, folding & coiling of polypeptide chains,
break easily & reform
Ionic bonds – between strongly positive & negative amino acids, strong but uncommon
Weak hydrophobic reactions – weak, occur between nonpolar R groups
Disulphide bonds – strong but rare, hold the folded polypeptide chain together, between two
cysteine molecules
Structures –
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Primary – sequence of amino acids in a polypeptide chain
Secondary – polypeptide chain forms localized 3D structures held together by H bonds (Alpha
helix & Beta pleats)
Tertiary – overall 3D shape of proteins held together by hydrophobic, disulfide, ionic & H bonds
(globular proteins)
Quaternary – two or more polypeptide chains form one macromolecule (haemoglobin, collagen)
Protein folding – functional proteins are folded, twisted & coiled with the aid of chaperonins,
chaperonins keep the polypeptide segregated from disruptive chemicals in the cytoplasm while it
spontaneously folds.
Denaturation – the bonds & interactions in a protein are destroyed, causing it to unravel & lose
its natural shape, due to alterations in its environment.
Denaturation causes – chemicals that disrupt the bonds, excessive heat, transfer from aqueous medium
to nonpolar solvent
Reversible – warming milk, Irreversible – frying egg