Enzymes

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Author:
jaguavai
ID:
105310
Filename:
Enzymes
Updated:
2011-09-29 22:29:06
Tags:
Pharm 410
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Description:
Enzyme Structure and Function
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  1. Equilibrium constant (Keq)
    • Tells the ratio of products to reactants after a reaction has reached equilibrium
    • Does NOT tell how long it took to reach equilibrium
  2. DGrxn
    Gproducts – G reactants
  3. ΔGact
    • Determines the reaction rate, refers to activation energy.(v)
    • Slow reaction - large activation energy
  4. Enzyme cofactors
    Small molecule required for enzyme activity
  5. Coenzyme
    • An organic cofactor that is loosely bound to the enzyme
    • Examples: NADP(H), NAD(H), ATP
  6. Prostethic group
    • Organic OR inorganic compound tightly bound to the enzyme
    • Examples: Heme, iron-sulfur clusters, some metal ions
  7. Structural mechanisms for enzyme catalysis (3)
    • Entropy reduction
    • General Acid-Base Catalysis
    • Covalent Catalysis
  8. Entropy Reduction
    Substrates bound in the enzyme's active site are oriented to promote a reaction
  9. General acid-base catalysis
    Protons are accepted or donated by amino acids in the active site to promote a reaction
  10. Covalent catalysis
    A temporary covalent bond forms between the enzyme and the substrate to increase the reactivity of the substrate
  11. pH Optima
    pH at which an enzyme will catalyze a reaction at its optimal rate (ACID-BASE catalysis only)
  12. Classes of enzyme reactions (6)
    • Oxidoreductases
    • Transferases
    • Hydrolases
    • Lyases
    • Isomerases
    • Ligase/synthetases
  13. Flipped LDH
    When [LDH1] >[LHD2] in the blood due to M.I.
  14. Diagnosis of heart attack using CK
    • CK-MB represents ~50% of total CK in heart muscle
    • If [CK-MB] is elevated in blood, supports diagnosis of heart attack due to likelihood that damaged heart muscle is the source of extra CK-MB
  15. Enzymes used to diagnose liver disease
    • AST
    • ALT
  16. Enzymes used to diagnose bile-tract problems
    • ALP
    • GGT
    • If both are elevated, it's a bile tract issue
  17. Streptokinase
    • Alteplase, tPA, PLAT
    • Used to bust clots after MI
  18. Asparaginase
    • Elspar
    • Catalyzes conversion of Asn to Asp. Asn is an essential amino acid for ALL. Elspar starves the ALL cells
  19. Lactase
    Catalyzes digestion of lactose in the gut of lactose intolerant folks
  20. Mechanisms of enzyme regulation (6)
    • Product inhibition
    • Allosteric regulation
    • Covalent modification
    • Protein-protein regulation
    • Zymogen cleavage
    • Enzyme synthesis and degredation
  21. Product inhibition
    • Reversible inhibition of an enzyme when the product competes with the substrate for binding at the active site
    • Example:The first step in the metabolism of glucose, catalyzed by a hexokinase enzyme, can be inhibited by the product, glucose-6-phosphate
  22. Allosteric regulation
    • Reversible inhibition/activation that occurs when a molecule causes a conformational change by binding somewhere other than the active site
    • If increased - positive
    • If inhibited - negative
  23. Covalent modification
    • Inhibition/activation occurs when a molecule is covalently attached to the enzyme, causing a conformational change
    • Example: phosphorylation, protein kinases, protein phosphatases
  24. Protein-protein interactions
    • reversible activation/inhibition of an enzyme that occurs when the enzyme is bound by another protein to form a complex
    • Example: calmodulin is a protein that binds and activates to several cellular enzymes, including protein kinases.
  25. Zymogen cleavage
    • Irreversible activation of a zymogen (inactive enzyme precursor) by proteolytic cleavage
    • Causes conformational changes that reveal the active site
    • Example: Chymotrypsin
  26. Enxyme synthesis
    • If more enzyme is produced, reaction rate increases
    • Takes time, hours to days
    • Example: insulin
  27. Michaelis-Menton Equation
    Describes relationship between v and [S]
  28. First order kinetics
    Small change in [S] means significant change in v
  29. Second order kinetics
    Change in [S] has little effect on v
  30. Vmax
    Maximum rate of a reaction, depending on [S]
  31. Km meanings (2)
    • When 1/2 the substrate is bound to the enzyme
    • When v = 1/2 vMax
    • (always expressed as [S]
  32. Enzyme Inhibition Types (4)
    • Reversible competetive
    • Reversible uncompetetive
    • Reversible mixed (and pure noncompetetive)
    • Irreversible
  33. Competetive inhibition
    • Inhibitor competes wiht the substrate for the active site of the enzyme
    • Increases Km
    • Does not affect Vmax (eventually enough substrate can be added to reach Vmax)
  34. Reversible Competetive Inhibitor Examples (2)
    • Atorvastatin
    • Rivastigmine (Exelon)
  35. Uncompetetive inhibition
    • Will only bind to the enzyme while the substrate is also bound. Will not bind free enzyme. (rare)
    • Decreases Km
    • Lowers Vmax
  36. Uncompetetive Inhibitor Example
    • Mycophenelate (IMP dehydrogenase used in purine synthesis)
    • Immune cells depent in purine synthesis pathway , so mycophenelate is used to supress immune system after transpalnts
  37. Mixed & Noncompetetive Inhibition
    • Can bind with free enzyme or enzyme/substrate complex
    • More [E] affinity - increases Km
    • More [ES] affinity - decreases Km
    • Both lower Vmax
    • If affinities are equal, noncpompetetive, Km unchanged but Vmax lowered
  38. Mixed/Noncompetetive Inhibition examples (2)
    • Caspofungin
    • Foscarnet (Foscavir)
  39. Irreversible Inhibition
    • Enzyme is poisoned by inhibitor, rendering it useless
    • If [I] < [E], Km won't be affected but Vmax will be lowered
    • If [I] > [E], reaction won't go
  40. Irreversable Inhibitors Examples (2)
    • Sarin Gas
    • 5-Fu
  41. Kinetics of allosterically regulated enzymes
    • A substrate vs. rate curve for an allosterically-regulated enzyme is sigmoidal rather than hyperbolic and does not fit the Michaelis-Menton equation.
    • For these enzymes, “K0.5” is used instead of “Km” to indicate the substrate concentration at which half maximal velocity/half-saturation is achieved.
    • An allosteric inhibitor shifts the curve to the right and increases K0.5.
    • An allosteric activator shifts the curve to the left and decreases K0.5.
    • An allosteric activator or inhibitor may increase or decrease Vmax, or leave it unchanged.

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