Protein Structure Flashcards
What are proteins made of?
Large molecules made up of chains of amino acids
What is a peptide bond?
A covalent bond linking two amino acids
Formed by a condensation reaction (eliminates a water molecule)
What is the difference between a polypeptide and a protein?
Polypeptide:
- A single molecule made up of a long chain of amino acids
- Linked by peptide bonds- encoded by a single gene
Protein:
- A fully folded functional molecule
- Contains all subunits, prosthetic groups, and cofactors/coenzymes
Example: Haemoglobin protein contains 2 α and 2 β subunits, each with a haem prosthetic group.
How many genes code for haemoglobin?
- 2 genes
- Haemoglobin A codes for the α polypeptide
- Haemoglobin B codes for the β polypeptide
What are the four levels of protein structure?
Primary structure:
- Order of amino acids in a polypeptide chain
Secondary structure:
- Folding of the polypeptide due to hydrogen bonds
- α-helix- spiral and hydrogen bonds
- β-pleated sheet- held by hydrogen bonds
Tertiary structure:
- 3D folding of the polypeptide due to interactions between R groups
Quaternary structure:
- Final 3D shape of a protein formed from more than one polypeptide chain
What additional bonds hold proteins in their 3D shape?
- Hydrophobic interactions (between non-polar R groups).
- Ionic bonds (between ionised amine & carboxylic acid groups).
- Disulfide bridges (covalent bonds between sulfur atoms in cysteines).
What are conjugated proteins?
- Proteins that contain non-protein prosthetic groups
- Prosthetic groups are tightly bound molecules/ions essential for function
Examples: Organic (vitamin, sugar, lipid) or Inorganic (metal ions like Fe in haemoglobin).
How can protein structure be disrupted?
- Denaturation: Breaking of bonds that hold the protein’s shape
- Heating: Breaks hydrogen & ionic bonds
- Strong acids/alkalis: Disrupt ionic bonds
- Causes loss of function and is irreversible
What are globular proteins?
- Spherical, soluble proteins with hydrophilic R groups on the outside
- Can be conjugated- contain prosthetic groups
- Easily denatured by temperature/pH changes.
Example: Haemoglobin (oxygen transport, contains haem prosthetic group).
What are some examples of globular proteins?
Insulin (hormone):
- Function: Lowers blood glucose by stimulating uptake & conversion to glycogen.
- Structure: Single polypeptide, 3 disulphide bonds, post-translationally modified.
Carbonic Anhydrase (enzyme):
- Function: CO2 transport (converts CO2 + H2O into carbonic acid).
- Structure: Zn2+ prosthetic group, held by histidine residues.
Salivary α-Amylase (enzyme):
- Function: Hydrolyses starch into maltose/dextrins.
- Structure: Requires Cl- (cofactor) & Ca2+ (stabiliser).
What are fibrous proteins?
- Structural proteins with long, unbranched, insoluble polypeptide chains
- Have repeated sequences of hydrophobic amino acids
What are some examples of fibrous proteins?
- Collagen (ligaments, tendons, connective tissue).
Structure: Triple helices cross-linked by hydrogen bonds. - Keratin (hair, skin, nails).
Structure: Multiple keratin monomers cross-linked by disulfide bridges. - Elastin (alveoli & blood vessel walls).
Structure: Tropoelastin molecules cross-linked via lysines.
What are the key differences between starch and haemoglobin?
Starch:
- Polysaccharide, glycosidic bonds, unbranched chains
- Functions as an energy store
Haemoglobin:
- Protein, peptide bonds, amino acids.
- Transport function
What are the key differences between fibrous and globular proteins?
Fibrous Proteins:
- Insoluble, structural and non-helicale
- Repeated sequences and one type of chain
Globular Proteins:
- Soluble and functional
- Helical or non-helical and 2 types of chains possible
