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Phay0003 > Enzymes > Flashcards

Enzymes Flashcards

1
Q

What are enzymes

A

Biological catalysts

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2
Q

Rules of catalysis

A

1) catalyst cannot catalyse a thermodynamically unfavourable reaction
2) cannot change course of a reaction
3) cannot change the equilibrium of a reaction, only the rate at which eq is reached
4)directing influence
5) catalyst is recoverable

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3
Q

Catalysis specificity

A

Occurs at the active site
Tertiary protein structure that determines the 3D structure

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4
Q

Denaturation

A

Disruption of non covalent bonds by extremes of heat or ph leads to loss of enzyme catalytic activity

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5
Q

Mechanism of action

A

Enzyme lowers activation energy by binding to substrate
For a reaction to occur, there must be a release of energy (exothermic)
Must first pass through a higher energy state = transition state
Activation energy is needed

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6
Q

Effects of enzyme binding to substrates active site

A

1) enzyme position substrates in its active site and line up molecules precisely for reaction so binds can be broken or made more easily = orbital steering
2) some enzymes combine with substrate to form an unstable covalent intermediate in a “transition state” that more readily undergoes reactions to form products
3) increased effective concentration of reactants
4) enzymes may act as proton donors or proton acceptors
5) induce strain or distortion in the bond= conformational change

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7
Q

Active site of acetylcholinesterase

A

Anionic subsite: binds positive quaternary amine
Esterstic subsite: acetylcholine hydrolysed to choline and acetate

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8
Q

Mechanism of action of acetylcholinesterase

A

Anionic site attracts positively charged amine group
Specially activated serine is positioned next to and in line with the carbonyl of the ester group of the substrate
Covalent transition state intermediate is formed
Choline is released
Bond is hydrolysed by water to release acetate

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9
Q

Two ways of measuring enzyme activity

A

Disappearance of a substrate
Appearance of a product

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10
Q

Factors affecting enzyme activity: time

A

Velocity= appearance of product with time
Slowing down of a reaction is because of conc of substrate decreases or the conc of product increase

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11
Q

Factors affecting enzyme activity: enzyme concentration

A

More enzyme present= faster reaction
Can be product inhibition

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12
Q

Factors affecting enzyme activity: temperature

A

High temp= denature
Initial velocity increase with temp

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13
Q

Factors affecting enzyme activity: ph

A

Ph has an effect in the active site where ionisation may affect shape and any ionic binding of substrate

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14
Q

Factors affecting enzyme activity: presence/ absence of cofactors

A

Organic coenzymes or prosthetic groups
Inorganic ions or activators
Cofactors take part in chemical reaction catalysed by enzyme

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15
Q

Factors affecting enzyme activity: substrate concentration

A

V max= all available enzyme is saturated with substrate

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16
Q

Km

A

Dissociation constant for a given enzyme
Concentration of substrate which allows the catalytic reaction to proceed
1/Km is a measure of the affinity between enzyme and substrate

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17
Q

What binds to the allosteric site

A

Effector= can either stimulate or inhibit enzyme activity

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18
Q

What is a isozyme

A

Enzyme which has multiple molecular forms in the same organism catalysing the same reaction

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19
Q

What is multifunctional enzymes

A

Multiple active sites on a single polypeptide chain
Allows metabolic pathway to operate within a single enzyme= speed and control

20
Q

What are Multi enzyme complexes

A

Association of separate enzymes in separate subunits such as differentnpolypeptides

21
Q

What does allosteric means

A

Site on an enzyme that’s different to the active site

22
Q

Difference between isoenzyme, multifunctional enzyme and multi enzyme complex

A

Isoenzyme= diff forms of same enzyme
Multifunctional enzyme= multiple active site
Multi enzyme complex= different enzymes in a complex

23
Q

What are reversible inhibitors

A

Inhibitor can be removed from the enzyme by dialysis or dilution so enzyme activity is returned to normal
React by weak non covalent bonds to form enzyme inhibitors complex

24
Q

What are irreversible inhibitors

A

Binds very tightly to the enzyme
Formation of covalent bonds to form an enzymes inhibitor COMPOUND

25
Q

What is competitive reversible enzyme inhibition

A

Molecules which closely resemble the substrate fit into the active site of enzyme and blocks active site

26
Q

What is Ki

A

Dissociation constant for the eq between enzyme and inhibition
Also the concentration of inhibitor required to slow the reaction to half the rate in the absence of inhibitor

27
Q

What is a non competitive inhibitor

A

React with enzyme-substrate complex and slows the rate of reaction to form enzyme product complexes

Inhibitor and substrate may bother be bound to enzymes

28
Q

What is potency of an inhibitor

A

The strength

29
Q

What is a conformationally restricted competitive inhibitor

A

Higher affinity for the enzyme than the substrate
Potent enough to work in Vivo

30
Q

What are tight binding inhibitors

A

Competitive inhibitors that are conformationally restricted, has many noncovakent interactions leading to long lasting complexes

31
Q

What are transition state analogues

A

Binds more tighter to the transition state
Low Ki

32
Q

What distinguishes competitive inhibitor from a non competitive

A

Competitive= binds to active site, competing
Non competitive = inhibitor binds to another site instead of active site

33
Q

What parameters are important for an inhibitor to work as a drug?

A

High potency= mg or micrograms
Specificity= only works on one specific enzyme

34
Q

What does lower Ki mean

A

Greater affinity of the inhibitor and greater the potency

35
Q

Irreversible anticholinesterases

A

Active site serine reacts with organophosphate to form a covalent intermediate
Inhibition is irreversible
Used as war gases or pesticides

36
Q

What does acetylcholinesterase do and what effect would an inhibitor have on the body?

A

Breaks down acetylcholine into acetic acid and choline by hydrolysis

37
Q

What are the three levels of regulation of enzyme activity

A

1) allosteric regulation
2) covalent modification
3) regulation of enzyme concentration

38
Q

What is feedback inhibition

A

Allosteric inhibitors are often end products of a pathway that feedback and inhibit an early step in the pathway to prevent overproduction and wastage

39
Q

What is feedforward activation

A

Allosteric activators are often substrates for the reaction or the pathway that feedforward to encourage the pathway to dispose of the substrate

40
Q

Difference between allosteric activators and allosteric inhibitors

A

Allosteric activators stabilise the conformation of the substrate bound form
Allosteric inhibitors stabilise the conformation of the no- substrate bound form

41
Q

What is cooperativity

A

Allows an enzyme to respond to small changes in substrate concentration with large changes in catalytic activity

42
Q

How can a precursor enzyme be activated

A

An inactive precursor enzyme can be activated extra cellularly by proteolytic cleavage

43
Q

Covalent modification of enzymes using protein phosphorylation

A

Transfer of a phosphate group from atp onto enzyme is catalysed by a class of enzyme known as protein kinases
Can be reversed by protein phosphatases

44
Q

Control of enzyme concentration can be achieved how

A
  • control of gene transcription
  • control of stability of mRNA
  • control of protein synthesis (rate of mRNA translation)
  • control of the rate of protein degradation
45
Q

Examples of covalent modification of enzymes

A

Proteolytic cleavage: chymotrypsinogen
Reversible protein phosphorylation
Functional group modification

46
Q

What does an allosteric activator do

A

Stimulates enzyme activity
Binding site is distinct from active catalytic site
Type of effector

47
Q

What is the way that allosteric effectors regulate enzyme activity

A

Changing the affinity of the enzyme for its substrate

Phay0003 (4 decks)

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