Enzymes

Question 1

What are enzymes

Answer 1

Biological catalysts

Question 2

Rules of catalysis

Answer 2

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

Question 3

Catalysis specificity

Answer 3

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

Question 4

Denaturation

Answer 4

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

Question 5

Mechanism of action of an enzyme

Answer 5

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

Question 6

Effects of enzyme binding to substrates active site

Answer 6

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

Question 7

Active site of acetylcholinesterase

Answer 7

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

Question 8

Mechanism of action of acetylcholinesterase

Answer 8

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

Question 9

Two ways of measuring enzyme activity

Answer 9

Disappearance of a substrate
Appearance of a product

Question 10

Factors affecting enzyme activity: time

Answer 10

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

Question 11

Factors affecting enzyme activity: enzyme concentration

Answer 11

More enzyme present= faster reaction
Can be product inhibition

Question 12

Factors affecting enzyme activity: temperature

Answer 12

High temp= denature
Initial velocity increase with temp

Question 13

Factors affecting enzyme activity: ph

Answer 13

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

Question 14

Factors affecting enzyme activity: presence/ absence of cofactors

Answer 14

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

Question 15

Factors affecting enzyme activity: substrate concentration

Answer 15

V max= all available enzyme is saturated with substrate

Question 16

Km

Answer 16

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

Question 17

What binds to the allosteric site

Answer 17

Effector= can either stimulate or inhibit enzyme activity

Question 18

What is a isozyme

Answer 18

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

Question 19

What is multifunctional enzymes

Answer 19

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

Question 20

What are Multi enzyme complexes

Answer 20

Association of separate enzymes in separate subunits such as differentnpolypeptides

Question 21

What does allosteric means

Answer 21

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

Question 22

Difference between isoenzyme, multifunctional enzyme and multi enzyme complex

Answer 22

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

Question 23

What are reversible inhibitors

Answer 23

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

Question 24

What are irreversible inhibitors

Answer 24

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

Question 25

What is competitive reversible enzyme inhibition

Answer 25

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

Question 26

What is Ki

Answer 26

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

Question 27

What is a non competitive inhibitor

Answer 27

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

Question 28

What is potency of an inhibitor

Answer 28

The strength

Question 29

What is a conformationally restricted competitive inhibitor

Answer 29

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

Question 30

What are tight binding inhibitors

Answer 30

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

Question 31

What are transition state analogues

Answer 31

Binds more tighter to the transition state Low Ki

Question 32

What distinguishes competitive inhibitor from a non competitive

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What does lower Ki mean

Answer 34

Greater affinity of the inhibitor and greater the potency

Question 35

Irreversible anticholinesterases

Answer 35

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

Question 36

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

Answer 36

Breaks down acetylcholine into acetic acid and choline by hydrolysis

Question 37

What are the three levels of regulation of enzyme activity

Answer 37

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

Question 38

What is feedback inhibition

Answer 38

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

Question 39

What is feedforward activation

Answer 39

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

Question 40

Difference between allosteric activators and allosteric inhibitors

Answer 40

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

Question 41

What is cooperativity

Answer 41

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

Question 42

How can a precursor enzyme be activated

Answer 42

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

Question 43

Covalent modification of enzymes using protein phosphorylation

Answer 43

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

Question 44

Control of enzyme concentration can be achieved how

Answer 44

- control of gene transcription - control of stability of mRNA - control of protein synthesis (rate of mRNA translation) - control of the rate of protein degradation

Question 45

Examples of covalent modification of enzymes

Answer 45

Proteolytic cleavage: chymotrypsinogen Reversible protein phosphorylation Functional group modification

Question 46

What does an allosteric activator do

Answer 46

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

Question 47

What is the way that allosteric effectors regulate enzyme activity

Answer 47

Changing the affinity of the enzyme for its substrate