Enzymes
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- Created by: LBCW0502
- Created on: 09-01-18 12:08
Are enzymes highly efficient catalysts?
Yes
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What is the rate of enzyme catalysed reactions?
~10^6 (10^12 times faster than uncatalysed reactions). Many orders of magnitude faster than chemically catalysed reactions
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Enzymes are effective catalysts under which conditions?
Very mild conditions, 37 degrees Celsius, pH of 7 and atmospheric pressure
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Describe how enzymes exhibit a high degree of reaction specificity
Specific with respect to substrate/product
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Contrast solid phase synthesis with ribosomal synthesis
For solid phase, there is a low yield with incomplete reactions and side products. Ribosomal synthesis is effectively error free (1 amino acid per 1000)
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What are the types of bonds with stabilise the ESC?
Van der Waals interactions, hydrogen bonding, electrostatic interactions and entropic contributions due to the hydrophobic effect
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Substrates and binding site shows which concepts?
Geometric/topographical complementarity. Electronic/chemical complementarity. Lock and key hypothesis. Induced fit hypothesis
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What does chymotrypsin catalyse?
Peptide bond hydrolysis and ester bond hydrolysis
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What does carboxypeptidase A catalyse?
Hydrolysis of C-terminal peptide bonds for all residues except Arg, Lys and Pro
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Most enzymes catalyse a specific reaction on a specific substrate in which conditions?
In vivo
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Enzymes are efficient catalysts of which type of reactions?
Acid base reactions, acyl intermediate reactions, charge-charge interactions
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Enzymes are less efficient for which type of reactions?
Oxidation and reduction reactions
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What are cofactors?
Metal ions e.g. Zn 2+
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Give examples of coenzymes
NADP, NAD
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What is a holoenzyme?
An enzyme with a cofactor
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What is an apoenzyme?
An enzyme without a cofactor
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Describe the components in hexokinase
N-terminal domain, C-terminal domain and active site
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Describe an open conformation
Active site exposed and substrate binds to active site
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Describe a closed conformation
Domains close in behind substrate to exclude water
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Describe features of serine proteases
Endopeptidases characterised by reactive serine residue at active site. E.g. trypsin, chymotrypsin, elastase, thrombin, subtilisin. Share common catalytic mechanism but different specificities
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Outline the serine protease catalytic mechanism
Peptide substrate binds to enzyme. Oxyanion/tetrahedral intermediate formed. Acyl enzyme. Remove amine. Oxyanion/tetrahedral intermediate. Remove carboxyl. Enzyme regenerated
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How does pH affect enzyme activity?
When pH is decreased, anionic groups are neutralised. When pH is increased, cationic groups are neutralised. At pH 7, all charged groups present. Extreme pH values lead to peptide bonds hydrolysed/enzyme denatured. Optimum temperature/maximum rate
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How does temperature affect enzyme activity?
Low temperature causes enzyme to be frozen. High temperatures cause enzyme to be denatured. Optimum temperature for maximum rate
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What is the Michaelis Menten Equation?
v = Vm. [S0] / Km + [S0]
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What is Vm?
The maximum rate (achieved at high [S] and when E is saturated).
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What is Km?
[S] when v is half the maximum (include units)
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Describe the shape of a graph with [P] against time
Curve (where v is the gradient)
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Describe the shape of a graph with v against [S0]
Rectangular hyperbola
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What is k1?
The constant for E + S -> ES
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What is k-1?
The constant for ES -> E + S
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What is k2?
The constant for ES -> E + P
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What is the overall rate of reaction expression?
v = d[P]/dt = k2[ES]
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What is the expression for the rate of appearance of ES?
d[ES]/dt = k1[E].[S]
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What is the expression for the rate of disappearance of ES?
-d[ES]/dt = k-1[ES] + k2[ES]
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What is the expression for a steady state?
-d[ES]/dt = d[ES]dt ([ES] ~ constant)
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Derivation of Michaelis Menten Equation
See enzyme kinetics lecture slides
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What is the Eadie Hofstee Linearization expression?
v = Vm- Kmv / [S0]
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Describe the Eadie Hofstee Linearization plot
v against v/[S0]. Y-intercept is Vm, X-intercept is Vm/Km and the gradient is -Km. Straight line with a negative correlation
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What is the Hanes Linearization expression?
Km/Vm + [S0]/Vm = [S0]/v
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Describe the Hanes Linearisation plot
[S0]/v against [S0]. Y intercept is Km/Vm, X-intercept is -Km and the gradient is 1/Vm. Straight line with a positive correlation
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What is the Lineweaver Burke Linearization expression?
1/v = Km/Vm . 1/[S0] + 1/Vm
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Describe the Lineweaver Burke Linearization plot
1/v against 1/[S0]. Y-intercept is 1/Vm, X-intercept is -1/Km and the gradient Km/Vm. Straight line with a positive correlation
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Experiment
BBT4/5
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Give examples of enzyme inhibitors used as drugs
Saquinavir, Aspirin, Amoxycillin, Digoxin, Carbidopa, 5-Fluorouracil, Trimethoprim, Captopril, Viagra
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What are the two basic forms of enzyme inhibition?
Reversible (able to recover) and irreversible (unable to recover)
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What are the three basic forms of reversible inhibition?
Competitive, uncompetitive and non-competitive (mixed/most common)
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Describe competitive inhibition
Inhibitor and substrate compete for same binding site. (Expressions). Graph of Vm against v/[S0] shows line closer to intercept when inhibitor is used
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Give examples of competitive inhibitors as drugs
Captopril, Saquinavir, Trimethoprim
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Describe uncompetitive inhibition
Substrate must bind before inhibitor (or inhibitor binds to allosteric site). (Expressions). Graph shows line sloping to y-axis with inhibitor present
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Give examples of uncompetitive inhibitors as drugs
Finasteride, Methotrexate
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Describe non-competitive/mixed inhibition
Substrate and inhibitor bind independently. (Expressions). Graph shows line gets closer to origin when inhibitor is present (but is parallel to line without inhibitor)
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Give examples of mixed inhibitors as drugs
Etoposide, Tacrine
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Other cards in this set
Card 2
Front
What is the rate of enzyme catalysed reactions?
Back
~10^6 (10^12 times faster than uncatalysed reactions). Many orders of magnitude faster than chemically catalysed reactions
Card 3
Front
Enzymes are effective catalysts under which conditions?
Back
Card 4
Front
Describe how enzymes exhibit a high degree of reaction specificity
Back
Card 5
Front
Contrast solid phase synthesis with ribosomal synthesis
Back
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