Lesson 9.2 - Types of proteins Flashcards
What are the two types of proteins?
Globular and fibrous
Features of globular proteins?
- Soluble in water
- Compact
- Spherical shape roughly
When are globular proteins formed?
When the protein folds into its tertiary structure.
- Hydrophobic R-groups in amino acids are kept away from the aqueous environment.
- Hydrophilic R-groups in amino acids are on the surface of proteins.
Why are globular proteins soluble in water?
Hydrophilic amino acids on the surface of the protein.
Why is it important for globular proteins to be soluble in water for their function?
- Chemical reactions
- Immunity
- Contraction
Examples of globular proteins?
- Insulin
- Haemoglobin
- Catalase
What is insulin?
A globular protein. It is the hormone involved in the regulation of blood glucose concentration.
Why is it important that insulin is soluble?
Transported in the bloodstream
Structure of insulin?
Have to fit to specific receptors on cell-surface membranes to have effect. Precise shapes.
Conjugated proteins?
Type of globular protein. Contains non-protein component called a prosthetic group.
Name of proteins without prosthetic groups?
Simple proteins
Types of prosthetic groups:
- Lipids can combine with proteins to form lipoproteins
- Carbohydrates can combine with proteins to form glycoproteins
- Metal ions
- Molecules derived from vitamins
Examples of globular conjugated proteins that contain haem prosthetic group?
- Haemoglobin
- Catalase
What is haemoglobin?
Red, oxygen carrying pigment found in red blood cells.
Structure of haemoglobin?
Quaternary protein made from 4 polypeptides (2 alpha and 2 beta subunits).
Each subunit contains a prosthetic haem group.
Purpose of prosthetic haem group in haemoglobin?
Iron II (Fe2+) ion found in each haem group. Each can combine reversibly with an O2 molecule.
This enables haemoglobin to transport oxygen around the body.
What is catalase?
Enzyme that catalyses (increases the rate of reaction) for the break down of hydrogen peroxide.
Structure of catalase?
Quaternary structure containing 4 haem prosthetic groups.
Purpose of haem prosthetic groups in catalase?
Iron II ions allows catalase to interact with hydrogen peroxide and speed breakdown.
Why should hydrogen peroxide be removed from the body?
Common by-product of metabolism. Damaging to cells and cell components when accumulates.
What are fibrous proteins?
Long, insoluble molecules. Strong and do not tend to be folded into complex three-dimensional shapes.
Why are fibrous proteins insoluble?
High proportion of amino acids with hydrophilic R-groups in their primary structures.
Primary structure of fibrous proteins?
Contain limited range of amino acids, usually with small R-groups. Primary sequence of amino acids is quite repetitive. Very organised structures reflected in the roles fibrous proteins have.
Examples of fibrous proteins?
- Keratin
- Collagen
- Elastin
Where is Keratin present?
- Nails
- Hair
- Skin
Amino acids in Keratin?
Large proportion of cysteine (amino acid containing sulfur).
Result of cysteine in Keratin?
Forms disulfide bridges:
- Strong
- Inflexible
- Insoluble materials
What determines the flexibility of hair in comparison to nails?
Less disulfide bonds between cysteine amino acids in Keratin.
Why does hair / skin smell bad when burnt?
Large quantities of sulfur present.
Where is elastin found?
- Elastic fibres
- Small protein fibres
Where are elastic fibres present?
- Walls of blood vessels
- Walls of alveoli in lungs
What is the purpose of elastic fibres in bodily structures?
Give structures the flexibility to expand when needed. Also return to normal size.
Structure of elastin?
- Quaternary structure
- Made from many stretchy molecules called tropoelastin.
- Contains hydrophobic regions with polypeptide chains that are crosslinked.
- Hydrophobic regions on different molecules associate causing elastin molecules to group together.
- Stretch = move apart but remain attached at cross-links. Elastin molecules reassociate making elastin a very elastic molecule
What is collagen?
Connective tissue found in skin, tendons, ligaments and nervous system.
Features of collagen?
Flexible
Structure of collagen:
- Every 3rd amino acid is glycine
- Glycine has a hydrogen R-group, so small
- Fibres wrap tightly around each other to form a triple helix.
- This means that hydrogen bonds form between the peptide chains in collagen, stabilising the quaternary structure of the protein.
- Strong crosslinks between polypeptide chains. This leads to the formation of microfibrils and fibrils.
- Molecules staggered so that there are no weak spots.
- Cross links between different double helix molecules.
