Lesson 12: Enzyme Regulation & Inhibitors

stimulatory enzyme regulation

positive regulation; increases activity

inhibitory enzyme regulation

negative regulation; decreases activity

2 primary mechanisms of enzyme regulation

bioavailability and control of catalytic efficiency

bioavailability

control of enzyme synthesis, localization, and degradation

control of catalytic efficiency

modify the enzyme itself

proteolytic processing

cleave part of protein off

covalent modification

form a covalent bond with a functional group

phosphorylation is a type of

covalent modification

effectors

aka activators; bind to enzyme --> conformational change stimulates enzyme activity

inhbitors

bind to enzyme --> block or alter active site to decrease efficiency

reversible inhibitors

bind non-covalently; associate and disassociate rapidly

irreversible inhibitors

bind covalently, preventing catalysis; dissociate very slowly if at all

naturally occurring inhibitors are usually

reversible

irreversible inhibitors are usually

man-made / drug

suicide inhibitors

act like substrate and bind irreversibly, fail to complete reaction and remain bound until enzyme is degraded

3 ways inhibitors reduce reaction rate

interfere with substrate binding, substrate turnover, or product release

competitive inhibitor

competes with substrate for active site

competitive inhibitor can be overcome by…

flooding with substrate --> subsrate outcompetes inhibitor

structure of competitive inhibitor

resembles substrate, but does not actually react

uncompetitive inhibitor

only binds to ES complex, causes conformational change that prevents turnover or release

uncompetitive inhibitors bind to _ at _

ES complex; site other than active site

uncompetitive inhibitor effect on substrate binding

no effect on substrate binding

KI

dissociation constant for competitive inhibitor

equation for KI

KI = [E][I]/[EI]

KI'

dissociation constant for uncompetitive inhibitor

equation for KI'

KI' = [ES][I]/[ESI]

competitive inhibitor effect on slope of Lineweaver-Burk

increase slope

competitive inhibitor effect on Vmax

no effect on Vmax

competitive inhibitor effect on Km apparent

Km apparent increases by inhibition factor

inhibition factor for competitive inhibitor (name and equation)

𝛼; 𝛼 = 1 + [I]/KI

Michaelis-Menten in presence of competitive inhibitor

Vo = (Vmax[S])/(Km𝛼 + [S])

uncompetitive inhibitor effect on slope of Lineweaver-Burk

no effect on slope

uncompetitive inhibitor effect on Vmax

Vmax decreases by inhibition factor

uncompetitive inhibitor effect on Km apparent

Km apparent decreases by inhibition factor

inhibition factor for uncompetitive inhibitor (name and equation)

𝛼'; 𝛼' = 1 + [I]/KI'

Michaelis-Menten in presence of uncompetitive inhibitor

Vo = (Vmax*[S])/(Km + 𝛼'[S])

mixed inhibitor effect on Vmax

Vmax always decreases

inhibition constant for mixed inhibitors

since mixed inhibitor can bind to E or ES, both KI and K' have an effect

Michaelis-Menten in presence of mixed inhibitor

Vo = (Vmax[S])/(Km𝛼 + 𝛼'[S])

mixed inhibitor where KI = KI'

noncompetitive inhibitor

noncompetitive inhibitor effect on Km

no effect

noncompetitive inhibitor; KI = KI'

inhibitor that binds equally to E and ES

mixed inhibitor where KI > KI' (effect on Km-app)

Km-app decreases

mixed inhibitor where KI' > KI (effect on Km-app)

Km-app increases

mixed inhibitor favoring E > ES

KI > KI'; Km-app decreases

mixed inhibitor favoring ES > E

KI' > KI; Km-app increases

mixed inhibitor effect on Km-app

depends on relationship between KI and KI'; see 41, 43 & 44

noncompetitive KI = KI’ : no effect

KI > KI’: Km-app decreases

KI’ > KI: Km-app increases

inhibitor chart: type of inhibitor, binds to, binding site, relevant constant(s), relevant 𝛼, effect on Vmax, effect on Km-app, draw Lw-B plot

draw it!!