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Biochemistry > allosteric enzymes and inhibitors > Flashcards

allosteric enzymes and inhibitors Flashcards

1
Q

an inhibitor is

A

any molecule that acts to reduce the rate of an enzymatic reaction

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2
Q

competitive inhibitors

A

competitive inhibitors resembles the substrate so that is specifically binds to the active sites but differs from the substrate so that is cannot react as the substrate does

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3
Q

many drugs are..

A

competitive inhibitors since active sites are easy to target as the substrate is known so related molecules can be designed. e.g. sildenafil (viagra) mimics the substrate phosphodiesterase

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4
Q

example of competitive inhibitor

A

succinate hydrogenase , an enzyme that converts succinate to fumigate is competiitley inhibited by malonate –> which structurally resembles succinate

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5
Q

competitive inhibitors reduces the..

A

concentration of the free enzymes available for substrate binding –> smaller reaction

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6
Q

Ki

A

dissociation constant (unitsM)

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7
Q

when inhibitors bind covalently

A

the inhibition is permanent

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8
Q

to overcome competitive inhibition..

A

more substrate must be added e.g. outnumber the inhibitor, more likely enzyme will be filled with substrate than inhibitor

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9
Q

total enzyme conc

A

hf

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10
Q

in competitive inhibition only..

A

the Km is effected and the not the Vmax

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11
Q

[E]t

A

conc of all enzymes in system

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12
Q

[E]t =

A

[E] + [ES] +[EI]

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13
Q

[EI]

A

enzyme and inhibitor

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14
Q

competitive inhibition equation related to michaelis menten

A
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15
Q

line weaver burk and competitive inhibition

A

the varying slopes indicate the effect of the inhibitor on Km

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16
Q

the more competitive inhibitor

A

the higher the Km

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17
Q

High Km means

A

Max is reached at a higher [s]

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18
Q

High Km means

A

Max is reached at a higher [s]

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19
Q

worked example calculating Ki e.g. when the inhibitor conc is 50mM, the Km of the enzyme for its substrate increases 3 fold. What is the value of Ki (mM0

A
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20
Q

enzyme-inhibitor complex is..

A

catalytically inactive

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21
Q

an inactivator is..

A

when an inhibitor binds irreversibly to an enzyme

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22
Q

inactivators reduce… but do not effect..

A

effective levels of [E]t and therefore Vmax and does not change Km

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23
Q

Reagents that chemically modify specific amino acids can act as..

A

inactivators

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24
Q

uncompetitive inhibition

A

binds to the enzyme-substrate complex, but not the free enzyme -effects catalytic property but not substrate binding -only significant in multi substrate enzymes

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25
Q

uncompetitive inhibition does not effect

A

substrate binding but does affect catalytic activity

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26
Q

the inhibitor in uncompetitive inhibition binds to

A

directly to the enzyme-substrate complex but not the free enzymes

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27
Q

Kis

A

-inhibitor-substrate

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28
Q

Km and Vmax in uncompetitive inhibition

A

In uncompetitive inhibition, both Km as well as maximum velocity (Vmax) is influenced- decreased

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29
Q

how will Km and Vmax be affected in uncompetitive inhibition

A

decreased ration of Vmax/Km will stay the same

30
Q

in uncompetitive inhibition increasing substrate conc will.

A

NOT reverse the effects

31
Q

mixed inhibition

A

a mixed inhibitor binds tot he enzyme sites that participate in both substrate binding and catalysis Another mode of inhibition which is similar to that of non competitive inhibition but with an active ESI complex is termed the mixed inhibition. Such inhibition is common in metabolic feedback pathways. Enzymes showing this form of inhibition are generally allosteric in nature

32
Q

Non-competitive - allosteric

A

Non-Competitive inhibition is where; the inhibitor binds to the different site in the enzyme. So, in contrast to competitive inhibition, they can bind along with substrate to the enzyme and here both EI and ESI is inactive Fig 3.

33
Q

effects of inhibitors of Km and Vmax: competitive

A

Km- increases VMAX-nothing

34
Q

effects of inhibitors of Km and Vmax: uncompetitive

A

Km- decreases Vmax-Decreases

35
Q

effects of inhibitors of Km and Vmax: mixed

A

Km- increases VMAX- decreases

36
Q

effects of inhibitors of Km and Vmax: non-competitive

A

-non -decreases

37
Q

multi- subunit enzymes- uncompetitive

A

enzymes made up from more than one protein chain (quaternary)- chains can eb identical or diff -sometimes the binding of substrate to one subunit, influences the binding to other subunits–> co-opervtivity between subunits

38
Q

average size of an enzyme

A

300 amino acids

39
Q

binding of the substrate and catalysis is carried out by around..

A

20-30 amino acids

40
Q

regulation of enzyme activity: 2

A

1) control of enzyme availability 2. control of enzyme activity

41
Q

control of enzyme availability

A

the amount of enzyme in a cell depends on both its rate of synthesis and its rate of degradation- controlled by the cell

42
Q

control of enzyme acuity

A

directly regulated through structural alterations that influence substrate binding affinity –> allosteric mechanisms can cause large changes in enzymatic activity

43
Q

allostery

A

refers to the situation where a chemical binds to an enzyme away from its active site and this influences the kinetic properties of the enzyme

44
Q

allosteric effectors can ..

A

increase rate of reaction - allosteric activator decrease rare of reaction- allosteric inhibitor

45
Q

allosteric activator

A

increase rate of reaction

46
Q

allosteric inhibitor

A

decrease rare of reaction

47
Q

proteins are dynamic and this means that

A

small changes in the structure of the protein can be communicated throughout the entire structure

48
Q

ATCase

A

-300KDA, has R and C subunit R=regualtory C=catalytic arranged as two sets of dimers of the catalytic subunit (c3) in complex with the sets of regulatory dimers (r2). Each R2 joins two C3 trimers -Stabilised by zinc ion bout to four cysteine residues -catalyses the first step in the biosynthesis of pyrimidine

49
Q

allosteric behaviour of ATCase

A

it has been demonstrated that both substrate bind cooperatively to the enzyme -ATCase is allosterically inhabited by cytitidine triphosphate (CTP), a pyrimidine nucleotide -ATCase is allosterically activated by adenosine triphosphate (ATP), a purine nucleotide both ATO and CTP affect Km and Vmax and are comparable to competitive inhibitors

50
Q

ATCase structure

A

-300KDA, has R and C subunit R=regualtory C=catalytic arranged as two sets of dimers of the catalytic subunit (c3) in complex with the sets of regulatory dimers (r2). Each R2 joins two C3 trimers -Stabilised by zinc ion bout to four cysteine residues

51
Q

what: regulates the synthesis of purine and pyrimidine nucleotides –> needed in equal amounts for nucleic acid biosynthesis

A

ATCase

52
Q

allosteric graph

A

sigmoidal

53
Q

ATCase graph

A

sigmoidal rather than hyperbolic -consisten with co-operative bonding due to binding of substrate ti int active site favours the conversion of the entire enzyme to the resting state, increasing the activity at the other active sites

54
Q

CTP

A

decreases the enzymes catalytic rate

55
Q

ATP

A

increases the catalytic rate

56
Q

example of two allosteric effectors

A

CTP and ATP on ATCase

57
Q

active site of ATCase

A

each trimer contributes three active sites. Binding of PALA causes major changes in the quaternary structure

58
Q

T state- tense state

A

one of the 2 distinct tertiary structures of ATCase, which predominated in the absence of substrate -favoured by CTP binding- less active

59
Q

R state- relaxed state

A

another that predominates when substrates are bound -favoured by substrate binding- more active qCTP and ATP bind to..

60
Q

T–> R state

A

-catalytic trimers operate along the molecular three fold axis by 12A

61
Q

CTP binding to the regulatory subunit stabilises the..

A

T state -T state and R state ar win equilibrium- the binding of CTP shifts the equilibrium towards the T-state, decreasing the net enzyme activity and reducing the rate of N-carbamoylasparate generation

62
Q

both CTP and ATP

A

the same site on the outer edge of the regulatory subunit 50 A away from catalytic site

63
Q

how far away is the regulatory unit away from catalytic site

A

50A

64
Q

CTP increases the stability of the

A

T state

65
Q

ATP increases the stability of the

A

R state

66
Q

co-opertivity

A

usually the binding substrate to one subunit increases the likelihood of other subunits substrate binding –> positive co-opertivity

67
Q

rare co-opertivity

A

negative co-opertivity–> binding of a substrate reduced the likelihood of another substate binding to the catalytic unit

68
Q

theory of co-operativity

A

assumes that the enzyme can exist in two states: 1)active R (relaxed) state 2)inactive T (tense) state

69
Q

sequential model

A

binding of substrate to one subunit moves that subunit to the relaxed active state. it also makes it more likely that neighbouring subunits will bind

70
Q

concerted model

A

entire enzyme will switch from he T to R state none go -likelihood of this ids determined by the number of substrate molecules bound -after the switch binding of the further substrate is more likely

71
Q

which enzymes do not show M-M kinetics

A

multi-subunit enzymes–> co-operative enzymes usually show a concerted switch to more active state

Biochemistry (18 decks)

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