Biological Molecules Flashcards
what is the general basic structure of an amino acid
- COOH (carboxyl group)
- A R variable side group
- NH₂ amine / amino group
How many naturally occurring amino acids are there and how are they distinguished ?
- 20
- differ only by side ‘R’ group
Describe how a peptide bond is formed between two amino acids to form a dipeptide (2)
- Peptide bonds are formed from a condensation reaction
- between the amine and carboxyl groups (or NH2 and COOH)
What is a polypeptide and how is it formed
- condensation reaction forms a peptide bond between 3 or more amino acids
How many levels of protein structure are there?
4
Describe the ‘primary’ structure of a protein
- linear sequence , number and type of amino acids in the polypeptide
- determined by sequence of codons on mRNA
The secondary structure of a polypeptide is produced by bonds between amino acids. Describe how.
[2 marks]
- Hydrogen bonds form between the slightly positive NH group on one amino acid and the slightly negative C=O group on another.
Describe the 2 types of secondary protein structure
a-helix:
- all N-H bonds on same side of protein chain
- spiral shape
- H-bonds parallel to helical axis.
b-pleated sheet:
N-H & C=O groups alternate from one side to another
Define tertiary structure of a protein. Name the bonds present.
3D structure formed by further folding of polypeptide:
- disulfide bonds
- ionic bonds
- hydrogen bonds
Describe each type of bond in the tertiary structure of proteins.
Disulfide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
ionic bond: relatively strong bonds between charged R groups (pH changes cause these to break
hydrogen bonds: numerous & easily broken
Define quaternary structure of a protein
- functional proteins which consist of more than one polypeptide
- may involve the addition of prosthetic groups (moieties)
Describe how to test for proteins in a sample.
Biuret Test confirms presence of peptide bond
1. Add equal volumes of sodium hydroxide to sample at room temperatures
2. Add drops of dilute copper (||) sulfate solution . Swirl to mix
3. Positive result: colour changes from blue to purple
negative result: solution remains blue
What are enzymes?
- biological catalysts for intra & extracellular reactions
- specific tertiary structure determines shape of active site, complementary to a specific sustrate
- formation of (ES) complexes lowers activation energy of metabolic reactions
Explain the induced fit model of enzyme action.
- shape of active site is not directly complementary to substrate and is flexible
- conformational change enables ES complexes to form
- this puts strain on substrate bonds , lowering activation energy
how have models of enzyme action changed ?
- Lock and key method : rigid shape of active site complementary to only use 1 substrate
- Current induced fit model : explains why binding at allosteric sites can change shape of active site