Chapter 5: Properties of Enzymes Flashcards
(72 cards)
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
2
Q
reactants that enzymes act on
A
substrates
3
Q
enzymes exhibit _______, meaning they only act on a single stereoisomer of the substrate
A
stereospecificity
4
Q
enzymes display _____, meaning they do not form wasteful by-products
A
reaction specificity
5
Q
an unstable arrangement of atoms in which chemical bonds are in the process of being broken and made
A
transition state
6
Q
how do enzymes affect the transition state?
A
stabilize transition states by lowering it and decreasing the activation energy
7
Q
what are the six classes of enzymes?
A
- oxidoreductases
- transferases
- hydrolases
- lyases
- isomerases
- ligases
8
Q
- *catalyze oxidation-reduction reactions
- known dehydrogenases
- includes oxidases, peroxidases, oxygenases, or reductases
A
oxidoreductases
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
10
Q
- *catalyze hydrolysis
- a special class of transferases with water serving as the acceptor of the group transferred
A
hydrolases
11
Q
**catalyze lysis of a substrate generating of double bond
A
lyases
12
Q
**catalyze isomerization reactions
A
isomerases
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
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
15
Q
what are the 3 ways to increase the rate of a chemical reaction?
A
- increase the temperature
- increase the concentration of the reactants
- add a catalyst
16
Q
what happens to the velocity when the substrate is used up?
A
velocity becomes 0 and the curve is horizontal
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
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
19
Q
the rate constant for ES formation
A
k1
20
Q
the rate constant for the dissociation of ES back to E + S
A
k-1
21
Q
the rate constant for ES to E + P
A
k2
22
Q
the rate constant E + P forming ES
A
k-2
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
24
Q
what is the Michaelis-Menten equation and what type of curve is it?
A
- ***v0 = Vmax[S]/Km + [S]
* **hyperbolic curve
25
what is the effect of enzyme concentration on reaction velocity?
if the substrate concentration is held constant, the velocity of the reaction is proportional to the enzyme concentration
26
what is the effect of substrate concentration on reaction velocity?
- 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)
27
what are the steady-state conditions?
the rate of appearance of ES = the rate of disappearance of ES
28
what is the Michaelis constant?
Km = (k-1 + k2)/ k1
29
What allows us to determine the turnover number of an enzyme?
Vmax
30
- 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
turnover number (turnover number = k2)
31
what else is k2 called?
kcat
32
- ratio of combined rate constants
- concentration of substrate needed to reach half maximum velocity
- often equivalent to k-1/k1
Km
33
what does a low value of Km mean?
the lower the value of Km, the greater the affinity of the enzyme for the enzyme substrate complex (dissociation is rare)
34
describes E + S --> ES (formation of ES)
catalytic proficiency
35
how do we figure out catalytic proficiency?
kcat/Km
36
what do higher values of catalytic proficiency mean?
indicates more efficient catalysis
37
where is catalytic efficiency important on the graph?
- at the beginning of the graph - near linear portion
| - second order reaction
38
where is the turnover number important on the graph?
- towards the end of the graph - horizontal portion
| - first order reaction in respect to [E]
39
what type of plot is used since it is difficult to determine Km and Vmax from a hyperbolic curve?
Lineweaver-Burk (double reciprocal plot)
40
what is the Lineweaver-Burk equation?
1/v0 = (Km/Vmax) 1/[S] + 1/Vmax
41
what are the different types of reactions with multiple substrates?
- ordered
- random
- ping-pong
42
a product is released before all substrates have bound
ping-pong reaction
43
there is an obligatory order in which substrates bind and products are released
ordered reaction
44
- a compound that binds to an enzyme an interferes with its activity
- most bind reversibly
- some bind covalently and are irreversible
enzyme inhibitor
45
what is the inhibition (dissociation) constant?
Ki = [E][I]/[EI]
46
what does a high Ki indicate? a low Ki?
* *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
what are the types of reversible enzyme inhibition?
- classical competitive inhibition
- non-classical competitive inhibition
- uncompetitive inhibition
- noncompetitive inhibition
48
- 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
competitive enzyme inhibitor
49
- 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)
noncompetitive enzyme inhibitor
50
- binds only to the ES complex and ESI does not go on to give the product
- lowers Vmax AND Km
uncompetitive enzyme inhibitor
51
- 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)
mixed enzyme inhibition
52
inhibitors designed to mimic the shape of the transition state of ES --> P
transition state analogs
53
uses known information about the shape and reactivity of a biological target to design inhibitors
rational drug design
54
- 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
irreversible enzyme inhibitor
55
- enzyme activated reagents
| - disguised inhibitors
suicide inhibitors
56
enzymes whose activity can be modified in a manner that affects the rate of an enzyme-catalyzed reaction
regulated enzymes
57
how are regulated enzymes activity modified?
- 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
- 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
allosteric enzymes
59
the active site becomes available to the substrates when a regulatory molecule binds to a different site on the enzyme
allosteric activation
60
the active site becomes unavailable to the substrates when a regulatory molecule binds to a different site on the enzyme
allosteric deactivation
61
what is another name for the allosteric site?
regulatory site
62
what is another name for the allosteric effector?
regulatory molecule
63
what happens during allosteric inhibition?
- 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
what happens during allosteric activation?
- 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
how does an allosteric activator affect the apparent Km?
lower the apparent Km and raise the activity at a given [S]
66
how does an allosteric inhibitor affect the apparent Km?
raises the apparent Km and lower the activity at a given [S]
67
addition of S leads to an increase in the concentration of enzyme in the ________.
R conformation
68
addition of inhibitor increases the proportion in the _____.
T state
69
activator molecules bind preferentially to the ____, so they increase the ______.
R state; R/T ratio
70
- 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
the concerted model
71
- 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
sequential model
72
how does the regulation of covalent modification work?
- activity of an enzyme can be modulated by the covalent attachment and removal of groups on the polypeptide chain
- phosphorylation is most common
