Proteins Flashcards
Describe the structure of an amino acid.
Each amino acid consists of one central carbon atom bonded to a hydrogen atom (up), a carboxyl group (COOH) (right), an amino group (NH4) (left), a specific side chain (designated R) (down).
Describe the characteristics of the side chain (R group)
- Variable
- Vary in sizes
- Determines the property of the amino acid
Describe the properties of amino acids.
- Neutral and non polar
- Neutral and polar
- Acidic and polar
- Basic and polar
How do we catagorise R groups?
R group does not carry a charge - neutral
R group contain S, O, N - polar
R group donate H+ - acidic
R group accept H+ - basic
Describe the physical properties of amino acids.
- Colourless, crystalline solids
- Soluble in water
- Insoluble in organic solvents
Describe the chemical properties of amino acids.
- Dissolve in water to form ions - amino group accepts H+, carboxyl group donates H+
- Amphoteric in nature - amino acid solutions can act as buffers (solutions that resist small changes in pH)
How do amino acids link together?
Via the formation of peptide bonds.
What is a peptide bond?
Covalent bonds formed by condensation reactions where a water molecule is removed between 2 amino acids.
OH of carboxyl group + H of amino group = O=C-NH
State the levels of organisation in proteins.
- Primary
- Secondary
- Teritary
- Quaternary
Explain the primary structure of proteins.
It is the type, number and sequence of the amino acids linked by peptide bonds.
Describe the secondary structure of proteins.
- Localised folds and coils
- Intrachain hydrogen bond formation at regular intervals along the polypeptide backbone between O of carbonxyl group in a peptide bond and H attached to nitrogen of another peptide bond
- α-helix & β-pleated sheet
R groups are not involved in H bond formation.
Describe the structure of an α-helix.
- The O of carbonyl group of residue n forms intrachain H bond with H of amino group of residue n+4 in the polypeptide backbone.
- Right-handed coil with H bonds aligned parallel to axis and side chains (R groups) projected outwards.
- 3.6 amino acid residues per complete turn of helix.
R groups are not involved in H bond formation.
Describe the structure of a β-pleated sheet.
- 2 or more adjacent regions held by multiple intrachain hydrogen bonds at regular intervals
- Intrachain H bond formed via O of carboxyl group of one amino acid and H attached to N of amine group of another amino acid
- Parallel, antiparallel: same direction, opposite direction
- R groups project above and below β-pleated sheet
- β-pleated sheet favours amino acids with hydrophobic R groups.
Describe the tertiary structure of a protein.
Polypeptide folds extensively upon itself to form a precise, compact globular shape involving interactions between R groups of amino acids that are far away from one another.
* Types of interactions: disulfide bond, H bond, ionic interaction, hydrophobic interactions.
Describe the quaternary structure of a protein.
Associations of two or more polypeptide chains held together by hydrophobic interactions, H bonds and ionic interactions to form a precise structure between polypeptide units.
