1.4 Proteins Flashcards
What are proteins made up of?
Amino acids are the monomers from which proteins are made
20 amino acids are common in all organisms - differ only in their R group
What is the structure of amino acid?
- Amine group
- Carboxyl group
- R group (variable side chain)
What does a condensation reaction between 2 amino acids form?
Peptide bond
What is the primary structure?
Sequence of amino acids in a polypeptide chain
What is the secondary structure?
- Hydrogen bonding between amino acids (between carbonyl O of one and amino H of another)
- Causes polypeptide chain to fold into a repeating pattern (eg/ alpha helix or beta pleated sheet)
What is the tertiary structure?
- Overall 3D structure of a polypeptide held together by interactions between amino acid side chains:
- Ionic bonds/disulfide bridges/hydrogen bonds
What is the quaternary structure?
- Some proteins are made of 2+ polypeptide chains
- Held together by more hydrogen, ionic and disulfide bonds
What is the Biuret test?
- Add Biuret solution: sodium hydroxide + copper (I) sulfate
- Protein present: purple colour (negative = stays blue)
- Detects presence of peptide bonds
What are enzymes?
Biological catalysts - lowers the activation energy -> speeds up rate of reaction
Describe the lock and key model?
- Active site is a fixed shape/doesn’t change shape; it is complementary to one substrate
- After a successful collision, an enzyme-substrate complex forms leading to a reaction
Describe the induced fit model?
- Before reaction, enzyme active site not completely complementary to substrate/doesn’t fit substrate
- Active site shape changes as substrate binds and E-S complex forms
- This stresses/distorts bonds in substrate making the reaction more likely to happen
How specific are enzymes?
Have a specific shaped tertiary structure and active site
- Active site is complementary to a specific substrate
- Only this substrate can bind to the active site, inducing fit and forming an enzyme-substrate complex
How does enzyme concentration affect rate of reaction?
Increasing enzyme concentration -> rate of reaction increases
- Enzyme concentration = limiting factor
- More enzymes -> more available active sites
- More successful E-S collisions and E-S complexes
At a certain point, rate of reaction plateus
- Substrate concentration = limiting factor (all substrates in use)
How does substrate concentration affect rate of reaction?
Increasing substrate concentration -> rate of reaction increases
- Substrate concentration = limiting factor
- More successful E-S collisions and E-S complexes
At a certain point, rate of reaction plateus
- Enzyme concentration = limiting factor (all active sites saturated)
How does temperature affect rate of reaction?
Increasing the temperature up to optimum temp. -> rate of reaction increases
- Increase in kinetic energy
- More successful E-S collisions and E-S complexes
Increasing temp. above optimum -> rate of reaction falls
- Enzyme denature; tertiary structure and active site change shape
- Fewer E-S collisions and E-S complexes
Rate of reaction 0 when all enzymes denatured
How does pH affect rate of reaction?
pH above/below optimum pH -> rate of reaction decreases
- Enzymes denature; tertiary structure and active site changes shape/hydrogen/ionic bonds break
- Complementary substrate can no longer bind to active site
- Fewer E-S collisions and E-S complexes
pH = -log10[H+]
How do competitive inhibitors affect rate of reaction?
Competitive inhibitors decrease rate of reaction
- Similar shape to substrate
- Competes for/binds to/blocks active site so substrates can’t bind
- Fewer E-S complexes
- Increasing substrate concentration reduces effect of inhibitor
How do non-competitive inhibitors affect rate of reaction?
Non-cometitive inhibitors decrease rate of reaction
- Binds to site away from the active site (allosteric site)
- Enzyme tertiary structure/active sit change shape so substrate can’t bind to active site
- Fewer E-S complexes
- Increasing substrate concentration has no effect on rate of reaction as it causes permanent change to active site
Describe how proteins are digested in the human gut?
- Hydrolysis of peptide bonds
- Endopeptidases break polypeptides into smaller peptide chains
- Exopeptidases remove terminal amino acids
- Dipeptidases hydrolyse dipeptides into amino acids