3.1.4 Proteins Flashcards
How would you test for proteins in a sample?
Biuret test: confirms the presence of peptide bonds
- Add an equal volume of sodium hydroxide to a sample at room temp
- Add drops of dilute copper sulphate solution and swirl to mix
- Positive result: change from blue to purple
- Negative result: remains blue
How many amino acids are there and how do they differ?
20 and by their side ‘R’ group
How does a Peptide bond form?
Condensation reaction between 2 amino acids
How are Dipeptides formed?
Condensation reaction between 2 amino acids
What are Amino acids?
Monomers from which proteins are made
How are Polypeptides formed?
Condensation reaction between many amino acids
How many levels of a Protein structure are there?
4
Define Primary structure of a protein
Sequence , number and type of amino acids in the polypeptide is determined by the sequence of codons on the mRNA
Define Secondary structure of a protein
- It’s the shape that the chain of amino acids chains and is either alpha helix or beta pleated sheets
- Hydrogen in the - NH has a slight positive charge
- Oxygen in the -C=O has a slight negative charge
- As a result weak hydrogen bonds can form leading to alpha helices or beta pleated sheets
Describe the 2 types of Secondary protein structure
Alpha helix
- All N-H bonds are on the same side of protein chain
- Spiral shape
- Hydrogen bonds are parallel to the helical axis
Beta pleated sheets
- N-H and C=O groups alternate from one side to the other
Define Tertiary structure of a protein and name bonds present
3D structure formed by the further folding of the polypeptide
- Disulfide bridges
- Ionic bonds
- Hydrogen bonds
Describe Disulfide bridges
Strong covalent s-s bonds between molecules of the amino acid cystiene
Describe Ionic bonds
Relatively strong bonds between charged R groups (PH changes cause bonds to break)
Describe Hydrogen bonds
Numerous and easily broken
Define Quaternary structure of a protein
- Functional protein may consist of more than 1 polypeptide
- Precise 3D structure is held together by the same types of bonds as the tertiary structure
- May involve addition of prosthetic group e.g. metal ions or phosphate groups
Describe the structure and function of Globular proteins
- Spherical and compact
- Hydrophilic R groups face outwards and hydrophobic R groups face inward so usually water soluble
- Involved in metabolic processes e.g. enzymes and haemoglobin
Describe the structure and function of Fibrous proteins
- Can form long chains or fibres
- Insoluble in water
- Useful for structure/support (collagen in skin)
What are Enzymes?
Biological catalysts for intra and extracellular reactions
- Specific tertiary structure determines shape of active site which is complementary to a specific substrate
- Formation of enzyme-substrate (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 the es complex to form
- This puts strains on substrate bonds , lowering activation energy
How have models of Enzyme action changed?
- Initially lock and key model: rigid shape of active site is complementary to only 1 substrate
- Currently induced fit model: also explains why binding at allosteric sites can change shape of active site
How could a student identify the activation energy of a metabolic reaction from an energy level diagram?
Difference between free energy of substrate and peak of curve
Name 5 factors that affect the rate of Enzyme-controlled reactions
- PH
- Temp
- Concentration of inhibitors
- Substrate concentration
- Enzyme concentration
How does Enzyme concentration affect rate of reaction?
- Given that substrate is in excess rate increases proportionally to enzyme concentration
- Rate levels off when max number of es complexes form at any given time
How does Substrate concentration affect rate of reaction?
- Given that enzyme concentration is fixed , rate increases proportionally to substrate concentration
- Rate levels off when max number of es complexes form at any given time
