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bee pee s > 13- enzymes II > Flashcards

13- enzymes II Flashcards

1
Q

state the michaelis-menten kinetics equilibrium

A

E + S (kf, kr ) ES
ES (-> kcat) -> E + P
- assuming forwards direction

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2
Q

what is Vmax

A
  • the fastest velocity that the enzyme can produce product

- done at very large [s] → shifts the equilibrium completely towards the ES complex (le chateliers principle)

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3
Q

michaelis-menten kinetics equatition

A

V = dP/dt = Vmax[S]/Km + [S]

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4
Q

explain what Kcat is

A
  • turnover rate
  • Measure of how efficiently ES complex produces product
  • Larger Kcat = easier reaction (more efficient the enzyme)
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5
Q

explain what Km is

A
  • [S] at ½ Vmax

- describes how tightly the substrate binds to the enzyme

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6
Q

what is the ratio of Kcat/Km used for

A
  • index enzyme efficiency
  • High ratio = more efficient enzyme (very selective based on what it reacts on)
  • Also used as a specificity constant (tells you if this enzyme operates on one molecule or similar types)
  • Compare ratios for different substrates (tells you which substrate the enzyme prefers)
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7
Q

explain what Kd is

A
  • dissociation constant = kr/kf
  • Measure of binding between E and S
  • Small Kd = tight binding (substrate and enzyme stick together)
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8
Q

what is lineweaver-burk plot

A
  • when you rearrange the equation to give a linear relationship
  • here you measure the velocity at various substrate concentrations
  • 1/V = Km/Vmax[S] + 1/Vmax
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9
Q

what are the three different types of inhibitition?

A
  • competitive inhibition
  • non-competitive inhibition
  • uncompetitive inhibition
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10
Q

explain what competitive inhibition is

A
  • Inhibitor molecule competes with substrate molecule for active site (in equilibrium with the enzyme)
  • Binding is in the active site of the enzyme
  • Substrate cannot bind (inhibitor occupies the space where it would’ve attached)
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11
Q

explain what non-competitive inhibition is

A
  • Inhibitor binds to enzyme but not in active site
  • Substrate binds to active site
  • Inhibitor binding preventing ES complex from forming (prevents or alters conformational change)
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12
Q

explain what uncompetitive inhibition is

A
  • very rare
  • Inhibitor binds to ES complex
  • Binding destroys catalytic ability of ES complex
  • changes the shape of the ES complex or just abolishes the ability of the enzyme
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13
Q

explains what happens to the michaelis-menten kinetics equilibrium in terms of each inhibitor

A
  • competitive inhibitor: I binds to the free E at the beginning of the equation
  • non-competitive inhibitor: I binds to either the E at the beginning of the equation OR the ES
  • uncompetitive inhibition: I binds to the ES
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14
Q

explain what happens to the Lineweaver-Burk plot in terms of each inhibitor

A
  • competitive inhibitor: Vmax does not change, Km changes. y-intercept does not change, x-intercept changes, slope of the line changes
  • non-competitive inhibitor: Vmax changes, Km does not change. y-intercept changes, x-intercept does not change, slope of the line changes
  • uncompetitive inhibitor: Vm changes, Km changes. y-intercept changes, x-intercept changes, slope of the line changes
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15
Q

give an example of competitive drug

A
  • disulfiram (antabuse)
  • Blocks aldehyde dehydrogenase (present in livers)
  • ethanol converted into acetic acid by two enzymes
  • disulfiram blocks the second reaction to create hangovers (used to treat alcoholics)
  • Generates rapid and severe hangover when drinking
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16
Q

give an example of non-competitive drug

A
  • fluconazole
  • Blocks Cytochrome P4502C9 (CYP2C9)
  • Does not bind to active site (binds somewhere else on the enzyme)
  • causes a conformational change that blocks the catalysis of the reaction
17
Q

give an example of uncompetitive drug

A
  • lithium
  • Blocks inositol monophosphatase
  • Used to treat manic depression
    • Exact mechanism of action unknown
    • inhibits a number of different enzymes
18
Q

what are irreversible inhibitors

A
  • Bond covalently to enzyme → screws up the activity of the enzyme
  • either put the drug covalently bonded permanently bonded into the active site (does not allow the substrate to get in) or make a bond elsewhere which generates a conformational change that prevents the reaction from occurring
  • Specific inhibitors usually bond in the active site
19
Q

give an example of an irreversible inhibitor

A
  • penicillin
  • Blocks transpeptidase, enzyme used to finish cell wall construction
  • when the cell wall machinery is prevented from operating, it basically bursts under pressure
  • Penicillin reacts covalently with transpeptidase
  • the OH group was replaced by an ester and cannot act as a catalyst anymore

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