1. Proteins
Topic 7.5

2. 7.5 Proteins
7.5.1 Explain the four levels of protein structure, indicating the significance of each level.
7.5.2 Outline the difference between fibrous and globular proteins, with reference to two examples of each protein type.
7.5.3 Explain the significance of polar and non-polar amino acids.
7.5.4 State four functions of proteins, giving a named example of each.

3. Four levels of protein structure
Primary organization
Chain of amino acids held together by polypeptide bonds
20 amino acids may be arranged in any order and is determined by DNA
Primary structure determines the next three levels of protein organization
(Changing one amino acid here may completely alter the structure and function of a protein, as in sickle cell disease)

5. Secondary organization
Created by the formation of hydrogen bonds between the oxygen from the carboxyl group of one amino acid and the hydrogen from the amino group of another
Most common configurations
α-helix
β-pleated sheet

6. Tertiary organization
Polypeptide chain bends and folds over itself because of interactions among R-groups and the peptide backbone
Important in determining the specificity of the proteins known as enzymes
Interactions:
Disulfide bonds
Hydrogen bonds
Hydrophobic/hydrophillic
Ionic bonds

7. Quaternary organization
Involves multiple polypeptide chains which combine to form a single structure
Some include prosthetic or non-polypeptide groups called conjugated proteins
Ex. Hemoglobin contains four polypeptide chains, each of which contains a non-polypeptide group called a heme (contains an iron atom that binds to oxygen)

8. Animations

9. Fibrous and globular proteins
Fibrous proteins
Composed of many polypeptide chains in a long, narrow shape
Insoluble in water
Examples:
Collagen – structure of connective tissue in humans
Actin – component of human muscle
Globular proteins
More 3-D in their shape
Water soluble
Hemoglobin – delivers oxygen to body tissues
Insulin – regulates blood glucose level in humans

10. Polar and non-polar amino acids Refers to property of R group
Hydrophilic
Found in regions exposed to water
Membrane proteins toward interior and exterior of membrane
Create hydrophilic channels in proteins through which polar substances can move
Non-polar
Hydrophobic
Found in regions of proteins that are linked to the hydrophobic area of the cell membrane

11. Significance of polar and non-polar amino acids
Controls position of proteins in membranes, creating hydrophilic channels through membranes
Determinacy of specificity of an enzyme
Each enzyme has a region called the active site
Only specific substrates can combine with a particular active site
Combination is possible when ‘fitting’ occurs which involves general shape and polar properties of the substrate and the amino acids exposed at the active site

12. Function
Protein
Description
Transport
Hemoglobin
Contains iron that transports oxygen from the lungs to all parts of the body in vertebrates
Movement
Actin and myosin
Interact to bring about muscle contraction in animals
Hormone
Insulin
Secreted by pancreas that aids in maintaining blood glucose level in vertebrates
Defense
Immunoglobulins
Group that act as antibodies to fight bacteria and viruses