Enzymes Flashcards
What is the mode of action of enzymes? (brief)
Effective collisions to form ES complex
Enzyme specificity: complementary + Lock and Key + induced fit hypothesis
Lowering of Ea
Define enzymes
Enzymes are biological catalysts that increase the rate of rxn and are chemically unaltered at the end of the rxn, and hence can be resued. They are effective in small amts
Describe the lock & key hypothesis
E (lock) have a specific AS that’s complementary in shape and charge to a specific substrate (key)
- -> substrate binds when effective collisions occurs–> ES complex
- -> pdts formed can no longer fit into AS, leave AS, making it available for another substrate to bind
Describe the induced fit model.
AS of the enzyme is complementary but not a perfect fit for the substrate it catalyses.
When substrate bidns to enzyme, it induces a conformational change in the AS of enzyme–> AS is now a more precise fit for the substrate–> more effective catalysis
What is the role of contact/binding aa?
hold substrate in correct orientation and in close proximity via weak interactions e.g. H/ionic/hydrophobic–> ES complex
What is the role of catalytic aa?
have specific R grps that act on bonds in substrate & catalyse conversion of substrate to product
What is the role of structural aa?
Interact w each other to maintain overall 3D conformation of enzyme for proper functioning
Describe the 5 molecular basis of enzyme action
- Hold substrate in close proximity in AS–> increase chance of rxn
- Hold substrate in correct orientation–> expose bonds in substrate to chemical attack
- Strain effect: slight distortion of substrate as it bidns to enzyme, strains bonds to be broken, increase chance of breakage
- Microenvironment effect: provide favourable microenvironment to facilitate rxns between substratess
- Acid-base catalysis: R grps of acidic and basic aa in AS facilitate rxn between substrates
How does increasing temp and temp beyond optimum affect rate of rxn?
↑ temp: ↑ KE of E & S mlcs, ↑ frequency of effective collisions between S & E AS, ↑ rate of formation of ES complex, ↑ no. of substrate mlcs w sufficient energy to overcome the EA barrier to form products→ ↑ rate
↑ temp beyond optimum: ↑ KE of E and S, ↑ intramolecular vibrations & ↑ thermal agitation, weak intramolecular interactions between R grps (*e.g. H/ionic/hydrophobic), that maintain 3D conformation, are disrupted→ denatured enzyme: specific 3D confirmation of AS lost→ AS not complementary in shape & charge to S & cannot bind to it→ rate of ES complex formation and rate of rxn ↓
How does deviation from optimum pH affect rate of reaction?
Affects ionisation of R grps of charged aa⇒ excess H+: COO– → COOH; excess OH–: NH3+ → NH2
Structural aa affected: ionic and H bonds that maintain conf of enzyme AS disrupted→ denatured: 3D conformation of AS altered, not complementary→ rate of ES complex formation drops
Contact & catalytic aa affected: pH change specific charge of aa in AS→ affect temporary binding between enzyme & substrate, no ES complex / catalytic activity of enzyme lost→ affect catalysis
⇒ lowered rate of reaction
Describe effect of enzyme conc on rate. (graph: low to high and plateau)
Low [enzyme]: [enzyme] is limiting. ↑ [enzyme] = proportional ↑ in rate of reaction
↑ [enzyme]→ ↑ freq of effective E-S collision→ ↑ rate of formation of ES complex→ ↑ rate of rxn
Plateau: [enzyme] not limiting. ↑ [enzyme] = no increase in rate of rxn
Describe effect of substrate conc on rate. (graph: low to high and plateau)
Low [substrate]: [substrate] is limiting. ↑ [substrate] = proportional ↑ in rate of reaction
↑ [substrate]→ ↑ freq of effective E-S collision→ ↑ rate of formation of ES complex→ ↑ rate, as AS of enzymes readily available to catalyse reaction
High [substrate]: [E] is limiting, not [S]. Rate remains constant at Vmax
All enzyme AS are saturated w substrate at any one point in time.
How does competitive inhibitors affect rate of rxn?
Similar conf & charge as S→ complementary to AS→ compete w substrate for AS, block substrate binding→ reduces availability of enzyme AS for substrate binding→ ↓ rate
How does non-competitive inhibitors affect rate of rxn?
No structural similarity to substrate→ complementary to site other than AS→ change in 3D conformation of enzyme AS, no longer complementary in shape & charge to substrate→ substrate cannot bind→ ↓ rate
What is the effect of allosteric inhibitors/activators on rate?
Inhibitor (activator) binds to allosteric site→ conformational change in enzyme→ stabilise enzyme in inactive (active) conformation→ inhibits (activates)→ AS of enzyme has a lower (higher) affinity w substrate
