Proteins. Flashcards
What are proteins made out of?
Polymers made of monomers called amino acids.
Chains of amino acids=polypeptides.
Roughly 20 diff amino acids.
Amino acid structure.
NH2- amino group on the left (N-terminal).
Carboxyl group (COOH)- at other end of molecule (C-terminal).
A hydrogen atom.
An R group/ variable group- makes amino acids have different properties (opposite hydrogen atom).
DIAGRAM.
Formation of a peptide bond.
- Amino group of one amino acid reacts with the carboxyl group of another amino acid.
- Results in the elimination of water.
- Called a condensation reaction.
- Forms a peptide bond.
- Results in a dipeptide.
- C-N is a peptide bond.
- H2O as a by product.
- OH lost from one amino acid and H lost from the other= one water molecule and a bond between C and N.
DIAGRAM.
Primary protein structure.
- Order of amino acids in a polypeptide chain.
- Can be joined in any order/combination/number.
- Determined by base sequence on one strand of the DNA molecule.
Secondary protein structure.
- Shape formed by polypeptide chains as a result of Hydrogen bonding between =O on the -CO groups and the -H on the -NH groups in the peptide bonds along the chain.
- Causes to be twisted into 3-D shape.
- Spiral shape=alpha helix (e.g. keratin).
- Beta pleated sheet (fibroin in silk).
DIAGRAMS.
Alpha helix: - Folding chain.
- Forming hydrogen bonds between NH3+ and COO-.
Tertiary protein structure.
- Alpha helix of secondary folds and twists to make a complex, compact 3D structure.
Shape maintained by: - Hydrogen bonds (very weak link).
- Ionic bonds (weaker than covalent bond).
- Disulphide bonds (covalent bonds-strong link).
- Hydrophobic bonds/interactions(between r groups).
- Important to give globular proteins their shape (e.g. enzymes).
DIAGRAM.
Quaternary protein structure.
- Some polypeptide chains are not functional unless in combination.
- Can combine with another chain.
- Can be associated with non-protein groups and form large, complex molecules(e.g. haemoglobin).
- 2 or more polypeptide chains bonded together.
- Not always present.
- Primary, secondary and tertiary are always present.
Fibrous proteins.
Shape- Long, narrow fibres, often contain repeated alpha or beta.
Purpose- Structural for muscle, hair, bones and skin.
Amino acid sequence- Repeated, gets a repetitive shape.
Durability- High means less affected by changes in temperature and pH.
Examples- Collagen(tendons, 3 identical poly chains twisted and linked by hydrogen bonds=very stable molecule), Silk, Keratin.
Insoluble in water
Parallel chains with cross-linkages.
Globular proteins.
Shape- Spherical, no particular pattern, compact.
Purpose- metabolic functions, used in chemical reactions.
Amino acid sequence- Irregular.
Durability- Low=more affected by change.
Examples- Enzymes, hormones, haemoglobin(4 folded polypeptide chains with iron-containing group, haem, at centre of each), antibodies.
Soluble in water.
Protein transport/trafficking process.
1) Nucleus:
- Nucleolus produces ribosomes.
- DNA is the store of instructions used to make proteins.
- mRNA is a copy of the instructions and leaves via the nuclear pore.
2) RER(ribosomes):
- mRNA is converted into the protein’s primary structure.
- Protein leave RER in a vesicle.
3) Golgi body:
- The protein is ‘modified’ into 2 degree, 3 degree and maybe 4 degree structure.
- Protein exits Golgi body in a vesicle.
4) Cell membrane:
- Some proteins are released from the cell.
5) Mitochondria:
- Protein production, packaging and transport requires lots of ATP.
Test for proteins.
Biuret test (qualitative):
1) Add a few drops of biuret reagent (sodium hydroxide and copper(II) sulphate) to sample.
2) More concentrated protein=darker purple colour.
3) Blue=no protein in sample.
Explanation:
NaOH and CuSO4 react to make blue copper hydroxide.
This reacts with the peptide bonds in the protein to make a purple colour (biuret).
Loss and gaining of atoms in an amino acid.
Carboxylic acid terminal (OH) can lose 1 H^+ to become negative O (O^-)= COO^-.
Amino group can gain 1 H^+ to become N^+ or NH3. Happens with an extra hydrogen from an acidic environment
Zwitterion definition.
A molecule with a positive and a negative charge.
E.g. When an amino acid loses an H^+ from the carboxyl group and the amino group gains an H^+.
Buffer definition.
A chemical that prevents small changes in pH (e.g. amino acids).
Because they can accept and donate H^+.
Zwitterions can act as buffers because they can act as a base and an acid.
Donate or accept protons (H+) depending on the pH of the solution in order to neutralise it.
Condensation and hydrolysis definition.
Condensation- Removes water.
Hydrolysis- Adds water.
