Proteins Flashcards
What is the general structure of an amino acid?
- COOH group
- R group
- NH2 amine group
Describe how to test for proteins in a sample.
- Add equal volume of sodium hydroxide to sample at room temperature
- Add drops of dilute copper (II) sulfate solution. Swirl to mix
Positive result: colour changes from blue to purple
How many amino acids are there and how do they differ?
20
Differ only by side ‘R’ group
How do dipeptides and polypeptides form?
- condensation reaction forms peptide bond & eliminates molecule of water
- dipeptide: 2 amino acids
- polypeptide: 3 or more amino acids
How many levels of protein structure are there?
4
Define primary structure of a protein.
- sequence, number and type of amino acids in the polypeptide
- determined by sequence of codons on mRNA
Define secondary structure of a protein.
Hydrogen bonds form between O δ- attached to C=O & Hδ+ attached to -NH
Describe the 2 types of secondary protein structure.
α-helix
β-pleated sheet
Define tertiary structure of a protein.
3D structure formed by further folding of polypeptide
- disulfide bridges
- ionic bonds
- hydrogen bonds
Define quaternary structure of proteins.
- functional proteins may consist of more than one polypeptide
- precise 3D structure held together by the same types of bond as tertiary structure
- may involve addition of prosthetic groups
Describe the structure and function of globular proteins.
- spherical & compact
- hydrophilic R groups face outwards & hydrophobic
R groups face inwards = usually water-soluble - involved in metabolic processes
Describe the structure and function of fibrous proteins.
- can form long chains or fibres
- insoluble in water
- useful for structure and support
Outline how chromatography could be used to identify the amino acids in a mixture.
- Use capillary tube to spot mixture onto pencil origin line & place chromatography paper in solvent.
- Allow solvent to run until it almost touches other end of paper. Amino acids move different distances based on relative attraction to paper & solubility in solvent.
- Use revealing agent or UV light to see spots
- Calculate Rf values & match to database
What are enzymes?
- biological catalyst for intra & extracellular reactions
- specific tertiary structure determines shape of active site, complementary to a specific substrate
- formation of enzyme-substrate 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 & 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?
- initially lock and key model
- currently induced fit model: explains why binding at allosteric sites can change shape of active site
Name 5 factors that affect the rate of enzyme-controlled reactions.
- enzyme conc
- substrate conc
- conc of inhibitors
- pH
- temperature
How does substrate concentration affect rate of reaction?
Rate increases proportionally to substrate concentration.
Rate levels off when maximum number of ES complexes for at any given time
How does enzyme concentration affect rate of reaction?
Rate increases proportionally to enzyme concentration
Rate levels off when maximum number of ES complexes form at any given time
How does temperature affect the rate of reaction?
Rate increases as kinetic energy increases and peaks at optimum temperature.
Above optimum, ionic & H-bonds in 3 degrees structure break = active longer complementary
How does pH affect the rate of reaction?
Enzymes have a narrow optimum pH range
Outside range denatures
Contrast competitive and non-competitive inhibitors.
Competitive:
- similar shape to substrate = bind to active site
- do not stop reaction; ES complex forms when inhibitor is released
- increased substrate concentration decreases their effect
Non-competitive:
Bind at allosteric binding site
- may permanently stop reaction; triggers active site to change shape
- increasing substrate concentration has no impact on their effect
Outline how to calculate rate of reaction from raw data.
Change in concentration of product or reactant/time.
Why is it advantageous to calculate initial rate?
Represents maximum rate of reaction before concentration of reactants decreases & ‘end of product inhibition’.
State the formula for pH.
pH = -log10[H+]
