Enzymes Flashcards
1
Q
enzymes are…
A
catalysts of biological reactions; accelerate reactions by millions fold
2
Q
common features for enzymes and inorganic catalysts:
A
- Catalyze only thermodynamically possible reactions 2. Are not used or changed during the reaction.
- Don’t change the position of equilibrium and direction of the reaction
- Usually act by forming a transient complex with the reactant, thus stabilizing the transition state
3
Q
specific features of enzymes:
A
- Accelerate reactions in much higher degree than inorganic catalysts
- Specificity of action
- Sensitivity to temperature
- Sensitivity to pH
4
Q
enzymes have an optimum ____ at which they function most efficiently
A
temperature
5
Q
enzymes, like all proteins, ____ at high temperature and lose activity
A
denature
6
Q
enzymes will denature above ____
A
45-50 degrees C
7
Q
most enzymes have temperature optimum of ____
A
37 degree C
8
Q
every enzyme has a pH optimum because ionizable amino acids, such as ___, ____, and ____ participate in the catalytic reactions
A
histamine; glutamate; cysteine
9
Q
the pH sensitivity of enzymes results from the effect of _______
A
pH on the ionic charge of amino acid side chains of enzymes
10
Q
each enzyme has max activity at a particular pH (optimum pH); for most enzymes the optimum pH is ___
A
7
11
Q
contain firmly bound metal ions at the enzyme active sites (examples: iron, zinc, copper, cobalt)
A
metalloenzymes
12
Q
example of metalloenzyme
A
carbonic anhydrase contains zinc
13
Q
coenzymes act as _____; ____, _____ or groups of atoms can be transferred
A
group-transfer reagents; hydrogen, electrons
14
Q
classification of coenzymes
A
metabolite coenzymes (synthesized from common metabolites) and vitamin-derived coenzymes (derivatives of vitamins)
15
Q
_____ cannot be synthesized by mammals, but must be obtained as nutrients
A
vitamins
16
Q
TPP is….
A
thiamine pyrophosphate; a derivative of thiamine (vitamin B1)
17
Q
TPP participates in reactions of:
A
- oxidative decarboxylation, 2. transketo-lase enzyme reactions
18
Q
PLP is…
A
pyridoxal phosphate; derived from vitamin B6 family of vitamins
19
Q
PLP is a coenzyme for enzymes catalyzing _______
A
reactions involving amino acid metabolism (isomerizations, decarboxylations, transamination)
20
Q
6 classes of enzymes
A
oxidoreductases, transferases, hydrolases, lyases (synthases), isomerases, ligases (synthetases)
21
Q
oxidoreductase enzymes
A
dehydrogenases, peroxidases, oxidases
22
Q
transferase enzymes
A
hexokinase, transaminases
23
Q
hydrolase enzymes
A
alkaline phosphatase, trypsin, esterases, peptidases, glycosidases
24
Q
Lyase (synthase) enzymes
A
fumarase, dehydratases, pyruvate decarboxylase
25
isomerase enzymes
Triose phosphate isomerase, phosphogluco-mutase, alanine racemase
26
ligase (synthetase) enzymes
pyruvate carboxylate, DNA ligase
27
oxidoreductases
catalyze oxidation-reduction reactions
28
transferases
catalyze group transfer reactions
29
hydrolases
Catalyze hydrolysis reactions where water is the acceptor of the transferred group
30
lyases
Catalyze lysis of a substrate, generating a double bond in a nonhydrolytic, nonoxidative elimination
31
isomerases
catalyze isomerization reactions
32
ligases (synthetases)
catalyze ligation, or joining of two substrates; require chemical energy (ex: ATP)
33
specific region in the enzyme to which substrate molecule is bound
active site
34
characteristics of active sites
- specificity
- small three dimensional region of the protein
- binds substrates through multiple weak interactions (noncovalent bonds)
- there are contact and catalytic regions in the active site
35
active site of lysozyme
consists of six amino acid residues which are far apart in sequence
36
the active site contains _____ and binds substrates through multiple ______
functional groups (-OH, -NH, -COO, etc.); weak interactions (noncovalent bonds)
37
Fischer theory (lock and key model)
the enzyme active site (lock) is able to accept only a specific type of substrate (key)
38
specificity of enzymes
absolute, relative, stereospecificity
39
absolute specificity
one enzyme acts only on one substrate (example: urease decomposes only urea; arginase splits only arginine)
40
relative specificity
one enzyme acts on different substrates which have the same bond type (example: pepsin splits different proteins)
41
stereospecificity
some enzymes can catalyze the transformation only substrates which are in certain geometrical configuration, cis- or trans-
42
One international unit (IU) of enzyme catalyzes conversion of 1 µmol of substrate to product per minute
definition of enzyme activity
43
the specific activity of an enzyme is a measure of the number of ____ protein
IU/mg
44
rate of catalysis
At a fixed enzyme concentration [E], the initial velocity Vo is almost linearly proportional to substrate concentration [S] when [S] is small but is nearly independent of [S] when [S] is large
45
rate rises linearly as ____ and then levels off at ____
[S] increases; high [S] (saturated)
46
first researchers who explained the shape of the rate curve (1913)
Leonor Michaelis and Maud Menten
47
During reaction enzyme molecules, E, and substrate molecules, S, combine in a reversible step to form an ______
intermediate enzyme-substrate complex (ES complex)
48
indicates the speed or efficiency of a reaction
rate constant (k)
49
In fixed, saturated [S], _____
the higher the concentration of enzyme, the greater the initial reaction rate (this relationship will hold as long as there is enough substrate present)
50
in a tissue or cell, different chemical agents (______) can inhibit the enzyme activity
metabolites, substrate analogs, toxins, drugs, metal complexes, etc.)
51
inhibitor (I) binds to an enzyme and prevents the formation of _____ or break it down
ES complex
52
after combining with enzyme (EI complex is formed) can rapidly dissociate
reversible inhibitors
53
EI complex is held together by _____ and enzyme is inactive only when bound to inhibitor
weak, noncovalent interactions
54
3 basic types of reversible inhibition
competitive, uncompetitive, noncompetitive
55
competitive inhibition
* Inhibitor has a structure similar to the substrate thus can bind to the same active site
* The enzyme cannot differentiate between the two compounds
* When inhibitor binds, prevents the substrate from binding
* Inhibitor can be released by increasing substrate concentration
56
example of competitive inhibition
benzamidine competes with arginine for binding to trypsin
57
noncompetitive inhibition
* Binds to an enzyme site different from the active site
* Inhibitor and substrate can bind enzyme at the same time
* Cannot be overcome by increasing the substrate concentration
58
uncompetitive inhibition
Uncompetitive inhibitors bind to ES not to free E
| This type of inhibition usually only occurs in multisubstrate reactions
59
irreversible enzyme inhibition
very slow dissociation of EI complex
| Tightly bound through covalent or noncovalent interactions
60
irreversible inhibitors
group-specific reagents
substrate analogs
suicide inhibitors
61
group-specific reagents
react with specific R groups of amino acids
62
substrate analogs
–structurally similar to the substrate for the enzyme
| -covalently modify active site residues
63
suicide inhibitors
* Inhibitor binds as a substrate and is initially processed by the normal catalytic mechanism then generates a chemically reactive intermediate that inactivates the enzyme through covalent modification
* Suicide because enzyme participates in its own irreversible inhibition
64
methods of regulation of enzyme activity
allosteric control, reversible covalent modification, isozymes (isoenzymes), proteolytic activation
65
allosteric enzymes have a second ______distinct from the active site
regulatory site (allosteric site)
66
allosteric enzymes contain more than one _____
polypeptide chain (they have quaternary structure)
67
bind noncovalently to allosteric site and regulate enzyme activity via conformational changes
allosteric modulators
68
2 types of modulators
negative (inhibitor) and positive (activator)
69
negative modulators
–binds to the allosteric site and inhibits the action of the enzyme
–usually it is the end product of a biosynthetic pathway - end-product (feedback) inhibition
70
positive modulators
–binds to the allosteric site and stimulates activity
| –usually it is the substrate of the reaction
71
____ attachment of a molecule to an amino acid side chain of a protein can modify activity of enzyme
covalent
72
common covalent modification of protein activity
phosphorylation of ATP
73
multiple forms of an enzyme which differ in amino acid sequence but catalyze the same reaction
isoenzymes
74
isoenzymes can differ in:
kinetics, regulatory properties, the form of coenzyme they prefer, and distribution in cell and tissues
75
isoezymes are coded by _____
different genes
76
some ____ are regulated by enzymes that exist in different molecular forms (isoenzymes)
metabolic processes
77
tetramer (four subunits) composed of two types of polypeptide chains, M and H
lactate dehydrogenase
78
5 isozymes of LDH
```
H4 – heart
HM3
H2M2
H3M
M4 – liver, muscle
```
79
high affinity, best in aerobic environment
H4
80
lowest affinity, best in anaerobic environment
M4
81
important for diagnosis of different diseases
isoenzymes
82
many enzymes are synthesized as ____ (zymogens) that are activated by ______
inactive precursors; proteolytic cleavage
83
_____ only occurs once in the lifetime of an enzyme molecule
proteolytic activation
84
examples of specific proteolysis
•Digestive enzymes
–Synthesized as zymogens in stomach and pancreas
•Blood clotting enzymes
–Cascade of proteolytic activations
•Protein hormones
–Proinsulin to insulin by removal of a peptide
85
different enzymes that catalyze sequential reactions in the same pathway are bound together
multienzyme complexes
86
different activities may be found on a single, multifunctional polypeptide chain
multifunctional enzymes
87
“channeling” of reactants between active sites; Occurs when the product of one reaction is transferred directly to the next active site without entering the bulk solvent
metabolite channeling
88
metabolite channeling can greatly _____of a reaction
increase rate
89
channeling is possible in _____ and _____
multienzyme complexes; multifunctional enzymes
90
enzymes can be complex or ____, which contain a ____ and a _____ called a ____
holoenzymes; protein part; nonprotein part; cofactor
91
when enzymes only contain protein, they are called ____
simple
92
complex or holoenzymes consist of a protein part called the ____ and a nonprotein part called the ____
apoenzyme; cofactor
93
cofactors can be either ____ or ____
prosthetic groups; coenzymes
94
prosthetic groups are usually small _____ molecules or atoms that are usually tightly bound to the ____
inorganic; apoenzyme
95
coenzymes are large ____ molecules that are loosely bound to the _____
organic; apoenzyme
96
common covalent modification of protein activity
phosphorylation
97
donor molecule in phosphorylation
ATP
98
example of modified protein after phosphorylation
glycogen phosphorylase
99
protein function after phosphorylation
glucose homeostasis, energy transduction
100
you can make phosphorylation more or less active by _____
adding or removing a phosphate group
