Enzymes Part 2 (Kinetics) Flashcards
(47 cards)
1
Q
Endergonic Reaction
A
- ΔG > 0
- Non-spontaneous; requires input of energy
- Products have higher free energy than the reactants
2
Q
Exergonic Reaction
A
- ΔG < 0
- Spontaneous
3
Q
How can an endergonic reaction be driven to move forward?
A
By coupling it with an exergonic (favorable) reaction
4
Q
Main molecule utilized in enzyme coupled reactions:
A
ATP
- adenine (nitrogenous base)
- ribose (5 carbon sugar)
- three phosphate groups
- -3.5 total charge
5
Q
ATP has a high energy phosphate bond. Removal of 1 phosphate group leads to ADP and gives off a considerable amount of energy.
What type of reaction occurs and how much energy is given off?
A
- hydrolysis (a phosphate bond is cleaved)
- ΔG = -30.5 kJ/mol
6
Q
ATP + H2O → ADP + Pi
ΔG =
A
-30.5 kJ/mol
7
Q
ADP + Pi → ATP
ΔG =
A
+30.5 kJ/mol
8
Q
What is the molecular basis for the large amount of energy given of by the conversion of
ATP + H2O → ADP + Pi
A
- Electrostatic repulsion between the three phosphates destabilizes ATP molecule.
- ATP has -3.5 charge
- ADP has -2.5 charge
- After hydrolysis, an individual phosphate can be stabilized by resonance stabilization, which makes it more stable than its form in ATP.
9
Q
Equilibrium is a state of:
A
maximum stability
10
Q
A process is spontaneous and can perform work only when it is:
A
moving toward equilibrium
11
Q
Reaction velocity versus [E]:
A
- more enzyme = faster rate.
- linear correlation
- substrate has more enzyme to bind to, increases [ES], increases product formation.
12
Q
Reaction velocity versus [S]:
A
- rate increases asymptomatically with increasing [S]
- Velocity initially increases linearly, but then stabilizes and becomes constant when all enzyme active sites are saturated.
13
Q
If you want to increase Vmax, you need to increase:
A
[E]
14
Q
E + S ⇔ ES ⇒ E + P
What is Km?
A
- Michaelis Constant
- Conversion of E + S ⇔ ES
- reflects affinity for a substrate to an enzyme
- Km = [S] at ½Vmax
15
Q
E + S ⇔ ES ⇒ E + P
What is Kcat?
A
- Rate of ES ⇒ E + P
- (Kcat)([E]) = Vmax
16
Q
(Kcat)([E]) =
A
Vmax
17
Q
Michaelis Menten Equation
A
- v = initial velocity at a given [S]
18
Q
½Vmax =
A
Km
19
Q
Lineweaver-Burk Plot Equation:
A
- x-intercept = -1/Km
- y-intercept = 1/Vmax
20
Q
Km only changes with:
A
pH and temperature
Does not change with [E] or [S]
21
Q
Numerically, Km =
A
[S] at ½Vmax
22
Q
Smaller the Km:
A
- higher affinity of enzyme for substrate
- “tighter ES binding”
23
Q
Larger the Km:
A
- lower affinity of enzyme for substrate
- “less tight ES binding”
24
Q
Vmax is linearly dependent on:
A
[E]
25
Kcat only changes with:
* pH and temperature
* does not change with [E] or [S]
* a constant
26
Conceptually, Vmax is:
maximum velocity with which the enzyme can catalyze the reaction
27
Conceptually, Kcat is:
* kcat = (Vmax)([E])
* turnover number of an enzyme reflecting the number of moles of substrates converted to products per sec per mol of enzyme
* ES converted to E + P
28
Higher the Kcat:
* more product produced per second per mole of enzyme
* "high turnover:
29
Fastest way to regulate enzyme activity:
* phosphorylation/dephosphorylation of the enzyme
* most phosphorylation occurs on the serine, threonine, and tyrosine amino acids of an enzyme (all have -OH groups)
30
Slowest way to regulate enzyme activity:
extracellular signalling
31
Enzyme regulation by specific proteolysis:
* irreversible
32
The two types of enzyme inhibitors:
1. irreversible
2. reversible
* competitive
* uncompetitive
* noncompetitive
33
Irreversible inhibitors are:
* molecules that covalently bind to an active site amino acid of the enzyme to inhibit the activity.
* substrate analogs
* Examples:
1. penicillin
2. sarin (nerve gas)
3. aspirin
34
Reversible inhibitors are:
* molecules that bind reversibly to inhibit enzyme activity
* Three kinds:
1. competitive
2. uncompetitive
3. noncompetitive
35
Competitive inhibitors:
* reversible
* bind to active site of enzyme, compete with the substrate for active site slots.
* Km INCREASED, Vmax UNCHANGED
* can be overcome by increasing [S]
36
Noncompetitive inhibitors:
* reversible
* bind to a separate site of the enzyme (not the active site)
* Km UNCHANGE, Vmax DECREASED
* increasing [S] does not help since their is no competition for the active site
37
Example of a competitive inhibitor:
* statins
* inhibit enzyme involved in cholesterol synthesis
38
Transition State Analogs:
* potent inhibitors
* stable molecules that resemble geometric and/or electronic features of the highly unstable transition state
* binding is often much tighter than the substrate because they fit all elements of the active site
39
Allosteric Enzymes:
* have two binding sites:
1. active site (for substrate)
2. allosteric site (for a noncompetitive inhibitor or effector)
* all are oligomeric (\>1 peptide sequence)
* allosteric molecules do not resemble substrates
40
V versus [S] curve of allosteric enzymes:
* sigmoidal (S-shaped)
* due to cooperative substrate binding
41
The two equilibrium states of allosteric enzymes:
1. R-state (active; strong substrate binding)
2. T-state (inactive; weak substrate binding)
42
Allosteric activators stabilize:
* the R-state (active) of an allosteric enzyme
* increases substrate binding
* increases activity
43
Allosteric inhibitors stabilize:
* the T-state (inactive) of an allosteric enzyme
* decreases substrate binding
* decreases activity
44
K-type Allosteric Inhibitor:
* increase K0.5 (DECREASES AFFINITY)
* no effect on Vmax
45
K-type Allosteric Activator:
* decreases K0.5 (INCREASES AFFINITY)
* no effect on Vmax
46
V-type Allosteric Inhibitor:
* decreases Vmax
* no effect on K0.5
* no effect on affinity
47
V-type Allosteric Activator:
* increases Vmax
* no effect on K0.5
* no effect on affinity
