Transcript 추가6

-효소; 활성화에너지
6.1 효소의 성질
-효소의 독특한 성질(표6.1)
-그림6.1, 전이상태
-자유에너지활성화(ΔG≠)
-활성자리
-정반응 역반응
-평형상태, 평형상수
-자물쇠-열쇠 모형: 유도적응 모형(그림6.2)
-보조인자, 무기이온, 조효소
-홀로효소, 아포효소
Enzymes
Chapter 6
Overview
Section 6.1: Properties of
Enzymes
 Section 6.2: Classification of
Enzymes
 Section 6.3: Enzyme Kinetics
 Section 6.4: Catalysis
 Section 6.5: Enzyme
Regulation
Biochemistry in Perspective

From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
Enzymes are undoubtedly the most important
molecular machines
To proceed at a viable rate, most reactions
require an initial energy input
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
A chemical reaction occurs when colliding molecules
possess a minimum amount of energy called the
activation energy (Ea)
More commonly called free energy of activation (DG‡)
in biochemistry
Many reactions that are spontaneous (-DG) will
proceed at imperceptibly slow rates, because they do
not have the energy or correct orientation
The likelihood of a reaction improves with increasing
the temperature or using a metal catalyst
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
Living systems cannot increase temperature without
the risk of damaging structures, so they use catalysts
(enzymes)
Enzymes can increase reaction rate up to 107 to 1019
Enzymes are also very specific for substrates
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
Figure 6.1 A Catalyst Reduces the
Activation Energy of a Reaction
Catalysts increase reaction rate by lowering
activation energy
The free energy of activation (DG‡) is the amount of
energy to convert 1 mol of substrate (reactant) from
the ground state to the transition state
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
Each enzyme has a specific active site to bind the
substrate
The active site also has amino acid side chains that
take an active role in the catalytic process
The active site is used to optimally orient the
substrate to achieve the transition state at a lower
energy
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.1: Properties of Enzymes
Figure 6.2 The Induced
Fit Model
Two models that describe enzyme binding of
substrate:
Lock and key and induced fit
Some enzymes require certain non-protein
components to function: cofactors and coenzymes
Intact functional enzymes with cofactors are
holoenzymes
The protein component is the apoenzyme
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
6.2 효소의 분류
- -ase를 붙임
- 국제생화학회명명법: 4개의 숫자사용, E.C.는 위원회약자
- 6가지 주요효소(표6.2)
6.3 효소의 반응속도론
-반응속도(그림6.3): 1차 반응, 초기속도
1) 미카엘리스-멘텐 반응속도론
-정상정류상태에서 중간물질복합체 ES생성률은 분해율과 같다면
-미카엘리스-멘텐상수 Km=k2 + k3/k1 ; 기질에 대한 효소의 친화도
- 미카엘리스-멘텐식 v=Vmax[S]/[S] + Km
-식: Vmax (그림6.4)(그림6.5)
-효소의 회전수 Kcat: 기질이 포화된 상태에서 최소분자가 단위시간 당 기질분자를
생성물로 바꾸는 수
-효소들의 상수 (표6.3)
-Kcat/Km: 생리적인 조건하에서 [S]는 보통 Km보다 훨씬 낮다. 이 조건에서
촉매효율을 나타냄
-효소활성(unit): 분당 1 μmol의 생성물을 만드는데 필요한 효소의 양
-비활성(specific activity): 단백질 1mg당 효소활성의 단위수로 효소의 순수도를
나타냄
Section 6.2: Classification of Enzymes
International Union of Biochemistry (IUB)
instituted a naming convention for enzymes, based
upon the type of chemical reaction catalyzed
Six major enzyme categories:
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.2: Classification of Enzymes
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Thermodynamics can predict whether a reaction is
spontaneous, but cannot predict rate
The rate or velocity of a reaction is the change of a
concentration of reactant or product per unit of time
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Initial velocity (v0) is a velocity at
the beginning of a reaction when
the concentration of substrate
greatly exceeds enzyme
concentration
Information about reaction rates
is the quantitative study of enzyme
catalysis, or enzyme kinetics
Figure 6.3a Enzyme
Kinetic Studies
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Kinetics also measures enzyme
affinity for substrates and inhibitors
Order is useful in describing
reactions; it is determined
experimentally
First order is unimolecular (no
collisions required)
Rate = k[A]1
Figure 6.3b Enzyme
Kinetic Studies
Half-life is the time for one-half of
the reactant molecules to be
consumed
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Second order is bimolecular (A + B
 P)
Rate = k[A]1[B]1
When a reaction is zero order, the
rate is not affected by adding more
substrate
Enzyme substrate sites saturated
Figure 6.3 Enzyme Kinetic Studies
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Michaelis-Menten Kinetics
The concept of enzyme substrate complexes:
E+S
k1
k-1
ES
k2
E+P
Introduce the Michaelis constant Km
k-1 + k2
Km =
k1
When Km is experimentally determined, it is a
constant that is characteristic of the enzyme and the
substrate under specific conditions
The lower the value of Km, the greater the affinity
of the enzyme for ES complex formation
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Vmax is the maximum velocity a
reaction can attain
The number of substrate
molecules converted to product
per unit time is kcat
kcat is Vmax over total enzyme
concentration (Et)
Figure 6.4 Initial Reaction
Velocity v0 and Substrate
Concentration [S] for a
Typical Enzyme-Catalyzed
Reaction
ν=
Vmax[S]
[S] + Km
Michaelis-Menten Equation
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Figure 13.7 Substrate saturation curve for an enzyme-catalyzed reaction. The amount of
enzyme is constant, and the velocity of the reaction is determined at various substrate
concentrations. The reaction rate, v, as a function of [S] is described by a rectangular
hyperbola. At very high [S], v = Vmax. That is, the velocity is limited only by conditions
(temperature, pH, ionic strength) and by the amount of enzyme present; v becomes
independent of [S]. Such a condition is termed zero-order kinetics. Under zero-order
conditions, velocity is directly dependent on [enzyme]. The H2O molecule provides a rough
guide to scale. The substrate is bound at the active site of the enzyme.
The Michaelis-Menten
Equation




You should be able to derive this!
Louis Michaelis and Maud Menten's theory
It assumes the formation of an enzymesubstrate complex
It assumes that the ES complex is in rapid
equilibrium with free enzyme
Breakdown of ES to form products is assumed
to be slower than 1) formation of ES and 2)
breakdown of ES to re-form E and S
Figure 13.8
Time course for the consumption
of substrate, the formation of
product, and the establishment of
a steady-state level of the
enzyme-substrate [ES] complex
for a typical enzyme obeying the
Michaelis-Menten, BriggsHaldane models for enzyme
kinetics. The early stage of the
time course is shown in greater
magnification in the bottom
graph.
Understanding Km




The "kinetic activator constant"
Km is a constant
Km is a constant derived from rate
constants
Km is, under true Michaelis-Menten
conditions, an estimate of the
dissociation constant of E from S
Small Km means tight binding; high Km
means weak binding
Understanding Vmax




The theoretical maximal velocity
Vmax is a constant
Vmax is the theoretical maximal rate of the
reaction - but it is NEVER achieved in
reality
To reach Vmax would require that ALL
enzyme molecules are tightly bound with
substrate
Vmax is asymptotically approached as
substrate is increased
The dual nature of the MichaelisMenten equation
Combination of 0-order and 1st-order kinetics
 When S is low, the equation for rate is 1st
order in S
 When S is high, the equation for rate is 0order in S
 The Michaelis-Menten equation describes a
rectangular hyperbolic dependence of v on S!
The turnover number



A measure of catalytic activity
kcat, the turnover number, is the number of
substrate molecules converted to product per
enzyme molecule per unit of time, when E is
saturated with substrate.
If the M-M model fits, k2 = kcat = Vmax/Et
Values of kcat range from less than 1/sec to
many millions per sec
The catalytic efficiency



Name for kcat/Km
An estimate of "how perfect" the enzyme is
kcat/Km is an apparent second-order rate
constant
It measures how the enzyme performs
when S is low
The upper limit for kcat/Km is the diffusion
limit - the rate at which E and S diffuse
together
Section 6.3: Enzyme Kinetics
Figure 6.5 A Michaelis-Menten Plot
The specificity constant
reflects the relationship
between catalytic rate and
substrate binding affinity
(kcat/Km)
Specific activity is a measure
used to identify enzyme
purification
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
2)라인위버-버크 도시법
-미카엘리스-멘텐식의 역수 (그림6.6)
-기울기 (그림6.6)
3)다기질 반응
-순차적반응
-이중대치반응
4)효소억제
-억제제: 약물, 항생제, 독극물 등
*경쟁억제제(그림6.8): 억제제가 자유효소와 가역적으로 결합
-숙신산탈수소효소가 말론산에 의해 억제(그림6.9)
*무경쟁억제제: 효소-기질 복합체와만 결합
*비경쟁억제제 (그림6.10): 효소와 효소-기질 복합체와 모두 결합
*효소억제의 반응속도분석(그림6.11): 2중 역수 도시법으로 나타냄
*비가역억제: -SH기에 알킬화제인 iodoacetate의 결합
*다른자리 입체성효소
-다중소단위, 효소반응속도가 시그모이드형:
헤모글로빈의 산소-결합곡선(그림6.13)
Section 6.3: Enzyme Kinetics
Lineweaver-Burk Plots
Using the reciprocal of the
Michaelis-Menten equation
obtains a more accurate
determination of the values
Figure 6.6 Lineweaver-Burk or
Double-Reciprocal Plot
Slope of the line Km/Vmax
1/Vmax is the Y intercept
-1/Km is the X intercept
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Multisubstrate Reactions
Most reactions involve two or more substrates in
two classes:
Sequential—reaction cannot proceed until all
substrates are bound to the enzyme active site
Ordered and random
Double-Displacement Reactions—first product is
released before second substrate binds
Enzyme is altered by first phase of the reaction
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Enzyme Inhibition
Inhibitors reduce enzyme activity
In living systems inhibitors are important, because
they regulate metabolic pathways
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Enzyme inhibition can be reversible or irreversible:
Reversible inhibition can be counteracted by
increasing substrate levels or removing the inhibitor
Competitive, noncompetitive, and uncompetitive
Irreversible inhibition occurs when the inhibitor
permanently impairs the enzyme (covalent
interaction)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Competitive Inhibitors bind
reversibly to the enzyme at the
active site, thus competing
with substrate binding
Figure 6.8 Michaelis-Menten
Plot of Uninhibited Enzyme
Activity Versus Competitive
Inhibition
Forms enzyme-inhibitor (EI)
complex
Increasing substrate
concentration overcomes
competitive inhibition
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Noncompetitive Inhibitors can
bind reversibly to the ES
complex at a site other than
the active site
Figure 6.10 MichaelisMenten Plot of Uninhibited
Enzyme Activity Versus
Noncompetitive Inhibition
Forms EI + S and EIS
complex
Changes enzyme
conformation
Increased substrate
concentration partially
reverses inhibition
This is the case for pure
noncompetitive inhibition only
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Uncompetitive Inhibitors: a type of uncompetitive
inhibition that involves binding only after substrate
is bound
Ineffective at low substrate concentrations
Kinetic Analysis of Enzyme Inhibition: doublereciprocal plots may be used to distinguish
competitive, noncompetitive, and uncompetitive
inhibition
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Figure 6.11 Kinetic
Analysis of Enzyme
Inhibition
Competitive inhibition increases Km, not Vmax (6.10a)
Pure noncompetitive Vmax lowered Km unchanged
(6.10b)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Figure 6.11
Kinetic Analysis
of Enzyme
Inhibition
Mixed noncompetitive inhibition—both Vmax and Km
change and intersection occur above or below the
horizontal axis due to differences in k values (6.10c & d)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Figure 6.11
Kinetic Analysis
of Enzyme
Inhibition
Uncompetitive—Km and Vmax are changed although
ratio is the same (6.10e)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Allosteric Enzymes have a
sigmoidal curve rather than a
hyperbolic one
Figure 6.13 The Kinetic Profile
of an Allosteric Enzyme
Resembles the oxygen-binding
curve of hemoglobin
Michaelis-Menten kinetics do
not apply to allosteric enzymes
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.3: Enzyme Kinetics
Enzyme Kinetics, Metabolism, and Macromolecular
Crowding
Ultimate goal is understanding enzyme kinetics in
living organisms
In vitro work does not always reflect in vivo reality
Cell shows macromolecular crowding, which
influences reaction rates and equilibrium constants
Systems biologists are using computer modeling, in
vitro, and in vivo data to overcome issues
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
6.4 촉매
-반응단계의 메커니즘, 활성자리의 구조와 기능
-X-선 결정학, 활성자리 곁사슬의 활성조절, 억제제사용
1)유기반응과 전이상태
-두 단계 반응의 에너지 분석표(그림6.14)
1)촉매매커니즘
*근접과 변형효과
*정전기적효과
-물이 제거됨: 물의 유전상수(이온을 둘러싸는 능력과 이온들 사이의 인력을
감소시키는 물의 능력: 용액내에서 의 이온화는 용매의 유전상수에 의존)낮아짐
*산염기촉매
-활성자리 곁사슬에 양성자공여체 및 수용체
-히스티딘의 이미다졸기의 pKa는 생리적pH
-에스테르 가수분해(그림6.15)
*공유결합촉매
-세린 단백질분해효소: 1단계 세린이 친핵체로, 다음 에스테르 결합의 가수분해
Section 6.4: Catalysis
Scientists use X-ray crystallography, chemical
inactivation, and modeling to understand the
catalytic mechanism of enzymes
Organic Reactions and the Transition State
Essential features are the reaction between
electron-deficient atoms (electrophiles) and electronrich atoms (nucleophiles)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
A reaction mechanism is a step-by-step description
of a reaction
Electrons flow from a nucleophile to an electrophile
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.14 Energy Profile
for a Two-Step Reaction
One or more intermediates may form during the
course of a reaction
Examples of reactive intermediates include free
radicals, carbocations, and carbanions
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.14 Energy Profile
for a Two-Step Reaction
In any reaction, only molecules that reach the
transition state can convert into product molecules
Stabilizing the transition state lowers energy of
activation (Ea) and increases reaction rate
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Catalytic Mechanisms
Mechanisms of only a few enzymes are known in
significant detail
Several factors contribute to enzyme catalysis. The
most important are:
Proximity and Strain Effects—the substrate must
come in close proximity to the active site
Electrostatic Effects—charge distribution in the
largely anhydrous active site may help position the
substrate
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.15 Ester Hydrolysis:
Hydroxide Ion Catalysis
Acid-Base Catalysis—proton transfer is an important
factor in chemical reactions
Hydrolysis of an ester, for example, takes place better
if the pH is raised
Hydroxide ion catalysis
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.15 Ester Hydrolysis:
General Base Catalysis
More physiological is the use of general bases and
acids
Side chains of many amino acids (e.g., histidine, lysine,
and aspartate) can be used as general acids or bases
Depends on state of protonation, based on pKa of
functional groups
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.15 Ester Hydrolysis:
General Acid Catalysis
Covalent Catalysis—the formation of an unstable
covalent bond with a nucleophilic group on the
enzyme and an electrophilic group on the substrate
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
2)효소촉매에서 아미노산의 역할
3)보조인자의 역할
-금속이온과 조효소의 역할
*금속이온
-전이금속
-알칼리와 알칼리토금속
-전이금속은 고농도의 양전하를 제공하여 작은분자와 결합; 루이스산으로 친전자체
-금속이온이 활성자리에 기질의 위치를 도와줌: 촉매를 촉진
-예: 탄산무수화효소(carbonic anhydrase)
-전이금속은 원자가(valence state)상태로 산화환원반응을 중재: Fe2+의 가역적 산화,
시토크롬 P450기능에 중요
*조효소
-대부분 비타민: 수용성 지용성, 세포내 형태(표6.4)
-니코틴산
-NAD+, NADP+: 피로인산기, 아데노신, 니코틴아미드(그림6.15a)
-알코올 탈수소효소(그림6.15b)
-리보플라빈(B2)는 FMN, FAD의 전구물질(그림6.16): 두개 수소원자의 공여, 수용체
Section 6.4: Catalysis
The Roles of Amino Acids in Enzyme Catalysis
The active sites of enzymes are lined with amino
acids that create a microenvironment conducive to
catalysis
Residues can be catalytic or noncatalytic
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
In order to participate in catalysis, the amino acid
has to be charged or polar
For example, chymotrypsin action in Figure 6.16
Noncatalytic side groups function to orient substrate
or stabilize transition state
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
The Role of Cofactors in Enzyme Catalysis
Many proteins require nonprotein cofactors
Metals—important metals in living organisms are
alkali metals (Na+, K+, Mg2+, and Ca2+) and transition
metals (Zn2+, Fe2+, and Cu2+)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Alkali metals are usually loosely bound and play
structural roles
Transition metals usually play a functional role in
catalysis as part of a functional group
Metals are good Lewis acids and effective
electrophiles
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Coenzymes—a group of organic molecules that
provide enzymes’ chemical versatility
Contain functional groups that amino acid side
chains do not
Can be tightly or loosely bound and their structures
are often changed by the catalytic process
Most are derived from vitamins
Three groups: electron transfer (NAD+), group
transfer (coenzyme A), and high-energy transfer
potential (nucleotides)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
4)온도와 pH의 효과
*온도
-반응속도는 온도 높을수록 증가(그림6.16-17)
*pH
-수소이온농도의 변화는 활성자리의 화학군의 이온화에 영향
-최적 pH(그림6.18)
5)효소 촉매의 매카니즘
*키모트립신
-세린 단백질분해효소효소활성자리에 His57, Asp102, Ser195. 세린이 중요
-DFP(억제제)의 작용성
-그림6.19a,b, c, d, e, f
*알코올 탈수소효소
-알코올을 알데히드로 산화; 두 개의 전자와 두 개의 양성자를 제거하는 반응
-활성자리에 두 개의 시스테인잔기와 한 개의 히스티딘잔기, Zn2+가 배위결합
-그림6.20
Section 6.4: Catalysis
Effects of Temperature and pH
on Enzyme-Catalyzed Reactions
Change in an environmental
factor could change enzyme
structure and therefore function
Temperature—the higher the
temperature, the faster the
reaction rate; increased number
of collisions
Figure 6.16 The Effect of
Temperature on Enzyme
Activity
Enzymes are proteins and
become denatured at high
temperatures
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.17 The Effect of pH
on Two Enzymes
pH—hydrogen ion concentration affects enzyme
function; therefore, there is a pH optimum
Catalytic activity is related to ionic state of the active
site
Changes in ionizable groups could change structure of
the enzyme
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Detailed Mechanisms of Enzyme Catalysis
Mechanisms of two well-characterized enzymes:
Chymotrypsin—serine protease of 27,000 D
Serine proteases have a triad of amino acids in their
active site (e.g., Asp 102, His 57, and Ser 195)
Hydrolyzes peptide bonds adjacent to aromatic amino
acids
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.18 The Probable
Mechanism of Action of
Chymotrypsin
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.4: Catalysis
Figure 6.18 The Probable
Mechanism of Action of
Chymotrypsin
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Figure 6.19 Alcohol
Dehydrogenase
Alcohol Dehydrogenase—catalyzes the reversible
oxidation of alcohols to aldehydes or ketones
Uses NAD+ as a hydride (H:-) ion acceptor
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
6.5 효소의 조절작용
-대사회로의 조절이유: 안정된 질서 유지, 에너지보존, 환경변화에 대한 적응
(1)유전적통제
-효소유도;락토스오페론
(2)공유결합변형
-인산화 탈인산화:가인산화효소의 활성화
-메틸화, 아세틸화, 뉴클레오티드화 등
-불활성화 상태의 효소전구체 또는 찌모겐에서 활성화과정(그림6.20)
(3)다른자리 입체성자리
-페이스메이커 조절효소: 대사회로의 분기점
-다른자리입체성, 효소인자의 결합의 영향, 음성적 양성적(그림6.21) 시그모이드형
-리갠드에 의한 다중소단위 단백질의 구조변화(그림6.22)
-음성되먹임(그림6.23)
(4)구획화
Section 6.5: Enzyme Regulation
Enzyme regulation is necessary for:
Maintenance of ordered state
Conservation of energy
Responsiveness to environmental changes
Control is accomplished by genetic control, covalent
modification (e.g. phosphorylation) , allosteric
regulation, and compartmentation
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Genetic Control
Genetic control plays an important role in
controlling the synthesis of enzymes
Happens at the DNA level and can lead to repression
or induction of enzyme synthesis
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Covalent Modification
Figure 6.20 The Activation
of Chymotrypsinogen
Several covalent modifications
in enzyme structure cause
changes in function
Types of covalent modification
include phosphorylation,
methylation, acetylation, and
nucleotidylation
Some enzymes produced and
stored as proenzymes or
zymogens
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Allosteric Regulation
Figure 6.21 The Rate of an
Enzyme-Catalyzed Reaction
as a Function of Substrate
Concentration
Enzymes that are regulated
by the binding of effectors at
allosteric sites
Sigmoidal curve, unlike
Michaelis-Menten kinetics
If the effectors are substrates,
then it is homotropic; if the
ligand is different, then it is
heterotropic
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Figure 6.22a Allosteric
Interaction Models
Most allosteric enzymes are multisubunit enzymes
Two theoretical models: concerted and sequential
In the concerted model, all subunits are changed at
once from taut (T) to relaxed (R) or vice versa
An activator shifts the equilibrium in favor of the R
form; an inhibitor shifts in favor of the T form
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Figure 6.22b Allosteric Interaction Models
Concerted model is supported by positive
cooperativity where binding of one ligand increases
subsequent binding
It is not supported by negative cooperativity
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Figure 6.22 Allosteric
Interaction Models
In the sequential model binding of the ligand to one
subunit, it triggers a conformational change that is
passed to subsequent subunits
A more complex model that allows for intermediate
formations
Accounts for both positive and negative cooperativity
Neither model perfectly accounts for all enzyme
behavior
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press
Section 6.5: Enzyme Regulation
Compartmentation
Compartments created by cellular infrastructure
regulate biochemical reactions
Physical separation makes separate control possible
Solves several problems:
Divide and control
Diffusion barriers
Specialized reaction conditions
Damage control
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 Oxford University Press