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Biochemistry > Chapter 5: Properties of Enzymes > Flashcards

Chapter 5: Properties of Enzymes Flashcards

1
Q
  • efficient, selective, biological catalysts
  • speed up the rate of forward and reverse reactions but don’t change the equilibrium concentrations
  • highly specific for reactants
A

enzymes

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2
Q

reactants that enzymes act on

A

substrates

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3
Q

enzymes exhibit _______, meaning they only act on a single stereoisomer of the substrate

A

stereospecificity

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4
Q

enzymes display _____, meaning they do not form wasteful by-products

A

reaction specificity

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5
Q

an unstable arrangement of atoms in which chemical bonds are in the process of being broken and made

A

transition state

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6
Q

how do enzymes affect the transition state?

A

stabilize transition states by lowering it and decreasing the activation energy

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7
Q

what are the six classes of enzymes?

A
  1. oxidoreductases
  2. transferases
  3. hydrolases
  4. lyases
  5. isomerases
  6. ligases
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8
Q
  • *catalyze oxidation-reduction reactions
  • known dehydrogenases
  • includes oxidases, peroxidases, oxygenases, or reductases
A

oxidoreductases

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9
Q
  • *catalyze group transfer reactions
  • many require the help of a coenzyme
  • usually a portion of the substrate is covalently bound to the enzyme or its coenzyme
  • includes kinases
A

transferases

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10
Q
  • *catalyze hydrolysis

- a special class of transferases with water serving as the acceptor of the group transferred

A

hydrolases

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11
Q

**catalyze lysis of a substrate generating of double bond

A

lyases

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12
Q

**catalyze isomerization reactions

A

isomerases

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13
Q
  • *catalyze ligation, or joining, of two substrates
  • these reactions require energy, usually in the form of ATP
  • often referred to as synthetases
A

ligases

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14
Q
  • Model that states it is not until the substrate and enzyme come into contact that they fit together
  • enzyme undergoes conformational change
A

induced fit model

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15
Q

what are the 3 ways to increase the rate of a chemical reaction?

A
  1. increase the temperature
  2. increase the concentration of the reactants
  3. add a catalyst
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16
Q

what happens to the velocity when the substrate is used up?

A

velocity becomes 0 and the curve is horizontal

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17
Q

what does an enzyme catalyzed reaction involve?

A

**the reversible formation of an ezyme-substrate complex (ES), which breaks down to form free enzyme (E) and product (P)

**E + S –> ES –> E + P

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18
Q

Is an enzyme catalyzed reaction first or second order?

A

Pseudo-first order reaction — the rate is affected by the concentration of E but not S, meaning at saturating concentrations of S, there is sufficient S for ever E to react

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19
Q

the rate constant for ES formation

A

k1

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20
Q

the rate constant for the dissociation of ES back to E + S

A

k-1

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21
Q

the rate constant for ES to E + P

A

k2

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22
Q

the rate constant E + P forming ES

A

k-2

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23
Q

why is k-2 neglected?

A
  • the formation of ES is not always product-forming (reversible)
  • there is no product present at the early stages (very low [P]) so k-2 can be ignored
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24
Q

what is the Michaelis-Menten equation and what type of curve is it?

A
  • ***v0 = Vmax[S]/Km + [S]

* **hyperbolic curve

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25
Q

what is the effect of enzyme concentration on reaction velocity?

A

if the substrate concentration is held constant, the velocity of the reaction is proportional to the enzyme concentration

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26
Q

what is the effect of substrate concentration on reaction velocity?

A
  • at low [S], reaction velocity is first-order with respect to substrate (linear)
  • at high [S], reaction is zero-order with respect to substrate (independent of [S])
  • at mid [S], the reaction is mixed-order (proportionality is changing)
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27
Q

what are the steady-state conditions?

A

the rate of appearance of ES = the rate of disappearance of ES

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28
Q

what is the Michaelis constant?

A

Km = (k-1 + k2)/ k1

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29
Q

What allows us to determine the turnover number of an enzyme?

A

Vmax

30
Q
  • the number of substrate molecules converted to product by an enzyme in a unit time when the enzyme is fully saturated with substrate
  • the efficiency of ES –> E + P
A

turnover number (turnover number = k2)

31
Q

what else is k2 called?

A

kcat

32
Q
  • ratio of combined rate constants
  • concentration of substrate needed to reach half maximum velocity
  • often equivalent to k-1/k1
A

Km

33
Q

what does a low value of Km mean?

A

the lower the value of Km, the greater the affinity of the enzyme for the enzyme substrate complex (dissociation is rare)

34
Q

describes E + S –> ES (formation of ES)

A

catalytic proficiency

35
Q

how do we figure out catalytic proficiency?

A

kcat/Km

36
Q

what do higher values of catalytic proficiency mean?

A

indicates more efficient catalysis

37
Q

where is catalytic efficiency important on the graph?

A
  • at the beginning of the graph - near linear portion

- second order reaction

38
Q

where is the turnover number important on the graph?

A
  • towards the end of the graph - horizontal portion

- first order reaction in respect to [E]

39
Q

what type of plot is used since it is difficult to determine Km and Vmax from a hyperbolic curve?

A

Lineweaver-Burk (double reciprocal plot)

40
Q

what is the Lineweaver-Burk equation?

A

1/v0 = (Km/Vmax) 1/[S] + 1/Vmax

41
Q

what are the different types of reactions with multiple substrates?

A
  • ordered
  • random
  • ping-pong
42
Q

a product is released before all substrates have bound

A

ping-pong reaction

43
Q

there is an obligatory order in which substrates bind and products are released

A

ordered reaction

44
Q
  • a compound that binds to an enzyme an interferes with its activity
  • most bind reversibly
  • some bind covalently and are irreversible
A

enzyme inhibitor

45
Q

what is the inhibition (dissociation) constant?

A

Ki = [E][I]/[EI]

46
Q

what does a high Ki indicate? a low Ki?

A
  • *high Ki = inhibitory effect is weak

* *low Ki = inhibitory effect is strong - inhibitor is bound tightly, so the amount of active enzyme present is low

47
Q

what are the types of reversible enzyme inhibition?

A
  • classical competitive inhibition
  • non-classical competitive inhibition
  • uncompetitive inhibition
  • noncompetitive inhibition
48
Q
  • competes with substrate for the same site on the enzyme
  • raises apparent Km
  • Vmax is unchanged (enough [S] will fill the active site with S
A

competitive enzyme inhibitor

49
Q
  • does not compete with substrate for binding, but alters catalytic effectiveness
  • inhibitor can bind to either E or ES
  • lowers Vmax
  • Km is unchanged (affinity is not changing)
A

noncompetitive enzyme inhibitor

50
Q
  • binds only to the ES complex and ESI does not go on to give the product
  • lowers Vmax AND Km
A

uncompetitive enzyme inhibitor

51
Q
  • describes cases that do not conform to classic noncompetitive inhibition
  • both Vmax and Km are affected (usually because the affinity of the inhibitor for E is different than its affinity for ES)
A

mixed enzyme inhibition

52
Q

inhibitors designed to mimic the shape of the transition state of ES –> P

A

transition state analogs

53
Q

uses known information about the shape and reactivity of a biological target to design inhibitors

A

rational drug design

54
Q
  • forms a stable, covalent bond with an enzyme, thus removing active enzyme molecules from the population
  • usually occurs through alkylation or acetylation of a side chain
A

irreversible enzyme inhibitor

55
Q
  • enzyme activated reagents

- disguised inhibitors

A

suicide inhibitors

56
Q

enzymes whose activity can be modified in a manner that affects the rate of an enzyme-catalyzed reaction

A

regulated enzymes

57
Q

how are regulated enzymes activity modified?

A
  • amount of enzyme controlled by regulating rate of its synthesis or degradation
  • enzymes are more active catalysts when its concentration is high
  • enzymes become less active when concentration of substrates decrease or when product accumulates
58
Q
  • those whose properties are affected by a change in structure
  • change shape between active and inactive shapes as a result of the binding of substrates at the active site and of regulatory molecules at other sites
A

allosteric enzymes

59
Q

the active site becomes available to the substrates when a regulatory molecule binds to a different site on the enzyme

A

allosteric activation

60
Q

the active site becomes unavailable to the substrates when a regulatory molecule binds to a different site on the enzyme

A

allosteric deactivation

61
Q

what is another name for the allosteric site?

A

regulatory site

62
Q

what is another name for the allosteric effector?

A

regulatory molecule

63
Q

what happens during allosteric inhibition?

A
  • an enzyme is active in the uncomplexed form, which has a high affinity for its substrate
  • binding of an allosteric inhibitor stabilizes the enzyme in its low-affinity form, resulting in little or no activity
64
Q

what happens during allosteric activation?

A
  • an enzyme is inactive in its uncomplexed form, which has a low affinity for its substrate
  • binding of an allosteric activator stabilizes the enzyme in its high-affinity form, resulting in enzyme activity
65
Q

how does an allosteric activator affect the apparent Km?

A

lower the apparent Km and raise the activity at a given [S]

66
Q

how does an allosteric inhibitor affect the apparent Km?

A

raises the apparent Km and lower the activity at a given [S]

67
Q

addition of S leads to an increase in the concentration of enzyme in the ________.

A

R conformation

68
Q

addition of inhibitor increases the proportion in the _____.

A

T state

69
Q

activator molecules bind preferentially to the ____, so they increase the ______.

A

R state; R/T ratio

70
Q
  • all subunits are either T or R state
  • one substrate binds each subunit
  • substrate can bind either confirmation but binding in T weaker than binding in R
A

the concerted model

71
Q
  • one subunit can be R while the other is T state
  • one substrate binds each subunit
  • substrate can bind either conformation but cooperativity leads to a conformational change from T to R upon substrate binding to any subunit
A

sequential model

72
Q

how does the regulation of covalent modification work?

A
  • activity of an enzyme can be modulated by the covalent attachment and removal of groups on the polypeptide chain
  • phosphorylation is most common

Biochemistry (9 decks)

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