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Biochem > Enzymes > Flashcards

Enzymes Flashcards

1
Q

enzymes are…

A

catalysts of biological reactions; accelerate reactions by millions fold

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2
Q

common features for enzymes and inorganic catalysts:

A
  1. Catalyze only thermodynamically possible reactions 2. Are not used or changed during the reaction.
  2. Don’t change the position of equilibrium and direction of the reaction
  3. Usually act by forming a transient complex with the reactant, thus stabilizing the transition state
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3
Q

specific features of enzymes:

A
  1. Accelerate reactions in much higher degree than inorganic catalysts
  2. Specificity of action
  3. Sensitivity to temperature
  4. Sensitivity to pH
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4
Q

enzymes have an optimum ____ at which they function most efficiently

A

temperature

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5
Q

enzymes, like all proteins, ____ at high temperature and lose activity

A

denature

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6
Q

enzymes will denature above ____

A

45-50 degrees C

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7
Q

most enzymes have temperature optimum of ____

A

37 degree C

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8
Q

every enzyme has a pH optimum because ionizable amino acids, such as ___, ____, and ____ participate in the catalytic reactions

A

histamine; glutamate; cysteine

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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

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10
Q

each enzyme has max activity at a particular pH (optimum pH); for most enzymes the optimum pH is ___

A

7

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11
Q

contain firmly bound metal ions at the enzyme active sites (examples: iron, zinc, copper, cobalt)

A

metalloenzymes

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12
Q

example of metalloenzyme

A

carbonic anhydrase contains zinc

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13
Q

coenzymes act as _____; ____, _____ or groups of atoms can be transferred

A

group-transfer reagents; hydrogen, electrons

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14
Q

classification of coenzymes

A

metabolite coenzymes (synthesized from common metabolites) and vitamin-derived coenzymes (derivatives of vitamins)

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15
Q

_____ cannot be synthesized by mammals, but must be obtained as nutrients

A

vitamins

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16
Q

TPP is….

A

thiamine pyrophosphate; a derivative of thiamine (vitamin B1)

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17
Q

TPP participates in reactions of:

A
  1. oxidative decarboxylation, 2. transketo-lase enzyme reactions
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18
Q

PLP is…

A

pyridoxal phosphate; derived from vitamin B6 family of vitamins

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19
Q

PLP is a coenzyme for enzymes catalyzing _______

A

reactions involving amino acid metabolism (isomerizations, decarboxylations, transamination)

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20
Q

6 classes of enzymes

A

oxidoreductases, transferases, hydrolases, lyases (synthases), isomerases, ligases (synthetases)

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21
Q

oxidoreductase enzymes

A

dehydrogenases, peroxidases, oxidases

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22
Q

transferase enzymes

A

hexokinase, transaminases

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23
Q

hydrolase enzymes

A

alkaline phosphatase, trypsin, esterases, peptidases, glycosidases

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24
Q

Lyase (synthase) enzymes

A

fumarase, dehydratases, pyruvate decarboxylase

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25
Q

isomerase enzymes

A

Triose phosphate isomerase, phosphogluco-mutase, alanine racemase

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26
Q

ligase (synthetase) enzymes

A

pyruvate carboxylate, DNA ligase

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27
Q

oxidoreductases

A

catalyze oxidation-reduction reactions

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28
Q

transferases

A

catalyze group transfer reactions

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29
Q

hydrolases

A

Catalyze hydrolysis reactions where water is the acceptor of the transferred group

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30
Q

lyases

A

Catalyze lysis of a substrate, generating a double bond in a nonhydrolytic, nonoxidative elimination

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31
Q

isomerases

A

catalyze isomerization reactions

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32
Q

ligases (synthetases)

A

catalyze ligation, or joining of two substrates; require chemical energy (ex: ATP)

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33
Q

specific region in the enzyme to which substrate molecule is bound

A

active site

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34
Q

characteristics of active sites

A
  • 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
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35
Q

active site of lysozyme

A

consists of six amino acid residues which are far apart in sequence

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36
Q

the active site contains _____ and binds substrates through multiple ______

A

functional groups (-OH, -NH, -COO, etc.); weak interactions (noncovalent bonds)

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37
Q

Fischer theory (lock and key model)

A

the enzyme active site (lock) is able to accept only a specific type of substrate (key)

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38
Q

specificity of enzymes

A

absolute, relative, stereospecificity

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39
Q

absolute specificity

A

one enzyme acts only on one substrate (example: urease decomposes only urea; arginase splits only arginine)

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40
Q

relative specificity

A

one enzyme acts on different substrates which have the same bond type (example: pepsin splits different proteins)

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41
Q

stereospecificity

A

some enzymes can catalyze the transformation only substrates which are in certain geometrical configuration, cis- or trans-

42
Q

One international unit (IU) of enzyme catalyzes conversion of 1 µmol of substrate to product per minute

A

definition of enzyme activity

43
Q

the specific activity of an enzyme is a measure of the number of ____ protein

A

IU/mg

44
Q

rate of catalysis

A

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
Q

rate rises linearly as ____ and then levels off at ____

A

[S] increases; high [S] (saturated)

46
Q

first researchers who explained the shape of the rate curve (1913)

A

Leonor Michaelis and Maud Menten

47
Q

During reaction enzyme molecules, E, and substrate molecules, S, combine in a reversible step to form an ______

A

intermediate enzyme-substrate complex (ES complex)

48
Q

indicates the speed or efficiency of a reaction

A

rate constant (k)

49
Q

In fixed, saturated [S], _____

A

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
Q

in a tissue or cell, different chemical agents (______) can inhibit the enzyme activity

A

metabolites, substrate analogs, toxins, drugs, metal complexes, etc.)

51
Q

inhibitor (I) binds to an enzyme and prevents the formation of _____ or break it down

A

ES complex

52
Q

after combining with enzyme (EI complex is formed) can rapidly dissociate

A

reversible inhibitors

53
Q

EI complex is held together by _____ and enzyme is inactive only when bound to inhibitor

A

weak, noncovalent interactions

54
Q

3 basic types of reversible inhibition

A

competitive, uncompetitive, noncompetitive

55
Q

competitive inhibition

A
  • 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
Q

example of competitive inhibition

A

benzamidine competes with arginine for binding to trypsin

57
Q

noncompetitive inhibition

A
  • 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
Q

uncompetitive inhibition

A

Uncompetitive inhibitors bind to ES not to free E

This type of inhibition usually only occurs in multisubstrate reactions

59
Q

irreversible enzyme inhibition

A

very slow dissociation of EI complex

Tightly bound through covalent or noncovalent interactions

60
Q

irreversible inhibitors

A

group-specific reagents
substrate analogs
suicide inhibitors

61
Q

group-specific reagents

A

react with specific R groups of amino acids

62
Q

substrate analogs

A

–structurally similar to the substrate for the enzyme

-covalently modify active site residues

63
Q

suicide inhibitors

A
  • 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
Q

methods of regulation of enzyme activity

A

allosteric control, reversible covalent modification, isozymes (isoenzymes), proteolytic activation

65
Q

allosteric enzymes have a second ______distinct from the active site

A

regulatory site (allosteric site)

66
Q

allosteric enzymes contain more than one _____

A

polypeptide chain (they have quaternary structure)

67
Q

bind noncovalently to allosteric site and regulate enzyme activity via conformational changes

A

allosteric modulators

68
Q

2 types of modulators

A

negative (inhibitor) and positive (activator)

69
Q

negative modulators

A

–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
Q

positive modulators

A

–binds to the allosteric site and stimulates activity

–usually it is the substrate of the reaction

71
Q

____ attachment of a molecule to an amino acid side chain of a protein can modify activity of enzyme

A

covalent

72
Q

common covalent modification of protein activity

A

phosphorylation of ATP

73
Q

multiple forms of an enzyme which differ in amino acid sequence but catalyze the same reaction

A

isoenzymes

74
Q

isoenzymes can differ in:

A

kinetics, regulatory properties, the form of coenzyme they prefer, and distribution in cell and tissues

75
Q

isoezymes are coded by _____

A

different genes

76
Q

some ____ are regulated by enzymes that exist in different molecular forms (isoenzymes)

A

metabolic processes

77
Q

tetramer (four subunits) composed of two types of polypeptide chains, M and H

A

lactate dehydrogenase

78
Q

5 isozymes of LDH

A 
H4 – heart       
 HM3
 H2M2
 H3M                 
 M4 – liver, muscle
79
Q

high affinity, best in aerobic environment

A

H4

80
Q

lowest affinity, best in anaerobic environment

A

M4

81
Q

important for diagnosis of different diseases

A

isoenzymes

82
Q

many enzymes are synthesized as ____ (zymogens) that are activated by ______

A

inactive precursors; proteolytic cleavage

83
Q

_____ only occurs once in the lifetime of an enzyme molecule

A

proteolytic activation

84
Q

examples of specific proteolysis

A

•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
Q

different enzymes that catalyze sequential reactions in the same pathway are bound together

A

multienzyme complexes

86
Q

different activities may be found on a single, multifunctional polypeptide chain

A

multifunctional enzymes

87
Q

“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

A

metabolite channeling

88
Q

metabolite channeling can greatly _____of a reaction

A

increase rate

89
Q

channeling is possible in _____ and _____

A

multienzyme complexes; multifunctional enzymes

90
Q

enzymes can be complex or ____, which contain a ____ and a _____ called a ____

A

holoenzymes; protein part; nonprotein part; cofactor

91
Q

when enzymes only contain protein, they are called ____

A

simple

92
Q

complex or holoenzymes consist of a protein part called the ____ and a nonprotein part called the ____

A

apoenzyme; cofactor

93
Q

cofactors can be either ____ or ____

A

prosthetic groups; coenzymes

94
Q

prosthetic groups are usually small _____ molecules or atoms that are usually tightly bound to the ____

A

inorganic; apoenzyme

95
Q

coenzymes are large ____ molecules that are loosely bound to the _____

A

organic; apoenzyme

96
Q

common covalent modification of protein activity

A

phosphorylation

97
Q

donor molecule in phosphorylation

A

ATP

98
Q

example of modified protein after phosphorylation

A

glycogen phosphorylase

99
Q

protein function after phosphorylation

A

glucose homeostasis, energy transduction

100
Q

you can make phosphorylation more or less active by _____

A

adding or removing a phosphate group

Biochem (14 decks)

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