Card Set Information

2011-08-10 10:36:39

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  1. Roles of enzymes
    Catalysis and regulation
  2. Enzymes actions
    depends largely on 3D molecular configuration
  3. Enzyme turnover number
    Number of substrate being converted to product per unit time by one molecule of enzyme = efficiency
  4. Enzyme Specificity
    • Highly specific in their action
    • Due to specific molecular configuration
    • Definite shape complementary only to substrate
    • ABSOLUTE/GROUP specificity
  5. Enzyme Structure
    • Region in which enzyme binds to substrate = active site
    • Active site = 3-12 amino acids organized into precise 3D arrangement in a pocket of the enzyme
  6. Enzyme catalysed reaction
    • E combines with S to form enzyme substrate complex
    • Rxn complete
    • ES complex breaks up, form Product and Enzyme
  7. Mechanism
    • Activation Energy Barrier
    • can be overcome by increasing temp and pressure
    • addition of a catalyst/enzyme
    • enzyme lower Ea required for reaction to start
  8. How enzyme lower Ea?
    • Enzyme serve as orientation surfaces, align substrates in correct orientation for catalysis
    • Enzyme forces subtrate into constrained position, induce stress in bonds
    • increase probability of bond breakage
  9. Lock and Key hypothesis
    • Enzyme = Lock
    • Substrate = Key
    • Can have multiple keys but only 1 lock!

    1894 exactly complementary

    • 1959 induced fit model
    • substrate combines with enzyme, induces a conformational change in enzyme structure
    • Enzyme undergoes conformational change to improve the fit btw substrate and enzyme
  10. Enzyme co factors
    • Non protein components
    • Promote enzyme activity

    • Enzyme w/o cofactor --- apoenzyme
    • Enzyme cofactor complex --- holoenzyme
  11. Co Factors
    inorganic ions - myould enzyme into shape for easy formation of ES complex

    coenzymes - loosely associated with enzyme

    prosthetic groups - small organic cofactors that bound tightly to enzyme permanently
  12. Factors affecting rate of enzyme catalysed reactions
    • Temperature
    • pH
    • Enzyme concentration
    • Substrate concentration
    • Presence of inhibitors
  13. Effect of enzyme concentration
    • Substrate level maintained at high conc.
    • Other factors kept constant

    Rate of rxn is proportional to enzyme conc.
  14. Effect of substrate concentration
    Rate of reaction increase with increasing substrate concentration up to the point where further increase in substrate will have no effect on rxn rate

    At low substrate conc, enzymes will have their active sites unoccupied, hence substrate conc determines rate of rxn

    • At high conc of substrate, active sites are saturated with substrate
    • Any extra substrate must wait until ES complex dissociate into product and enzyme
    • Hence maximum rxn rate
    • Enzyme conc and dissociation time of ES complex are limiting factors
  15. Michaelis constant Km of enzyme
    is the substrate concentration at which the rxn rate is half of its maximum

    Km is the measure of affinity of an enzyme for its substrate

    • Low Km means high affinity
    • High Km means low affinity
  16. Effect of temperature
    • Initial increase in rxn rate with increasing temp
    • Heat provides kinetic energy to molecules
    • Molecules move faster
    • Increase chances of collision
    • More ES complex formed
    • More products formed per unit time

    • Optimum temperature is reached
    • Corresponds to max rate of rxn

    • Above optimum temp
    • Rxn rate decrease drastically
    • Enzymes protein in nature, owe their shape to weak hydrogen bonds and hydrophobic interactions
    • These bonds break at high temps
    • Extreme heat causes irreversible change to secondary and tertiary structures of protein enzyme
    • Causing the enzyme's active site to lose its 3D congfiguration and no longer fit the substrate
    • Active site denature, enzyme cease to function
  17. Temperature coefficient (effect of temp on rate of rxn)
    Q10 = rate of rxn at (x+10)/rate of rxn at x

    Doubled for every 10 degree rise in temp
  18. Effect of pH
    • Narrow pH range
    • Optimum pH where max rate of rxn

    • Ionisation of A.A residues at active site
    • changes in pH affect ionic charges of acidic and basic grps of the A.A residues in the enzyme
    • pH changes inhibit binding/catalysis by causing ions to reassociate

    • Change in 3D shape of active site
    • pH changes disrupt ionic bonding that maintains structure of enzymes active site
    • 3D shape of active site is altered
    • Substrate no longer fit
    • Enzyme activity decrease
  19. Enzyme Inhibition by enzyme inhibitors
    • Reversible Inhibition
    • Irreversible Inhibition
    • Competitive Inhibition
    • Non-competitive Inhibition
  20. Reversible Inhibition
    • Does not perma damage enzyme
    • Inhibitor remove enzyme can resume function
  21. Irreversible Inhibition
    • Drastic even at low concentration
    • Binds to enzymes permanently, making it hard to restore enzyme activity
    • Cause enzyme proteins to precipitate
  22. Competitive Inhibition
    Inhibitor bears structural resemblance to substrate molecule hence compete for binding at same active site

    • When substrate conc is increase
    • substrate can out compete inhibitor
    • so rate of rxn can increase
  23. Non Competitive Inhibition
    • Inhibitor bears no structural similarity to substrate
    • Forms an enzyme-inhibitor complex
    • by binding to a site on enzyme other than active site
    • Globular structure of enzyme altered
    • Binding of non competitive inhibitor cause enzyme to change its shape
    • Active site unreceptive to substrate

    Inhibitor can also attack catalytic sites on enzyme, catalysis cannot take place though substrate can still bind

    Increase substrate conc has no effect of rate of rxn
  24. Graph of inhibition
  25. Allosteric Inhibition/End Product Inhibition
    • Regulatory enzymes = allosteric enzymes
    • Negative Feedback chain

    • Allosteric enzyme have another site
    • Molecules can bind to allosteric site and act as allosteric effectors

    Either inhibit or activates catalysis

    Product produced can bind to allosteric site, change shape of enzyme, less product produce - -ve feedback