Download Section 9.2: Citric Acid Cycle

-완전혐기성생물, 내기성혐기성, 조건혐기성, 완전호기성생물
구연산회로, 전자수송회로, 산화적인산화반응: 미토콘드리아에서(그림 9.1-2)
9.1 산화환원반응
-산화환원반응: Cu+ + Fe3+ ↔ Cu+2 + Fe2+
-NADH에서 2개의 양성자 와 2개의 전자를 전이 (그림9.3)
-산화환원반응의 반쪽반응: Cu+ ↔ Cu+2 + e- (짝 산화환원쌍)
Fe3+ + e- ↔ Fe2+
-전지화학전지 (그림9.4) : 산화반응으로 전자이동이 일어나 전압발생
-산화환원(redox) 또는 환원전위(reduction potential): 특정물질이 전자를 잃거나
얻는 경향
-표준수소전극을 기준표준으로 하여 전지화학전지에서 측정; 1기압에서 0 볼트
pH 7, 250C, 1기압
2H+ + 2e- ↔ H2
수소농도가 1M인 표준수소전극에 대해 -0.42V이다
-0.42V보다 낮은 환원전위를 갖는 물질은 H+보다 전자에 대해 낮은 친화력갖음
(표9.1:표준환원전위, ΔE0’)
-전자는 ΔE0’가 더 음성인 물질에서 더 양성인 물질로 자발적으로 흐름, ΔE0’가
양성이 됨: ΔG0’ = -nFΔE0’ (전자친화력이 높은 쪽으로 자발적으로 반응이 진행)
Aerobic Metabolism I: The Citric Acid Cycle
Chapter 9
Overview
Section 9.1: Oxidation-Reduction Reactions
 Section 9.2: Citric Acid Cycle
Biochemistry in Perspective

From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Chapter 9: Overview
Aerobic metabolism consists of
three processes: citric acid cycle,
electron transport chain, and
oxidative phosphorylation
Important intermediates:
NADH and FADH2
Figure 9.1 Overview of Aerobic Metabolism
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Chapter 9: Overview
Figure 9.2 Aerobic Metabolism in the
Mitochondrion
Aerobic processes occur in the mitochondrion
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Figure 9.3 Reduction of Pyruvate by NADH
Oxidation-Reduction Reactions
In living organisms, energy-capturing and energyreleasing processes involve redox reactions
Many redox reactions have both an electron (e-) and
a proton (H+) transferred
Conversion of pyruvate and NADH to lactate and
NAD+ (shown above) is under anaerobic conditions
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Figure 9.4 An Electrochemical Cell
Oxidation-Reduction Reactions
Half-reactions make redox reactions more easily
understood
Biochemical reference half-reaction is 2H+ + 2e- 
H2 (reversible)
Gives a reduction potential of -0.42 V
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Oxidation-Reduction Reactions Continued
The relationship between standard reduction
potentials (DEº′)and standard free energy (DGº′) is:
DGº′ = -nF DEº′
Electrons flow spontaneously from a species with a
more negative Eº′ to a species with a more positive Eº′
Living organisms utilize redox coenzymes as highenergy electron carriers (e.g., NADH and FADH2)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
1) 산화환원 조효소
대부분 비타민: 수용성 지용성, 세포내 형태(표6.3)
*니코틴산
-NAD+, NADP+: 피로인산기, 아데노신, 니코틴아미드(그림9.5)
-알코올 탈수소효소
*리보플라빈(B2)는 FMN, FAD의 전구물질(그림9.6): 두개
수소원자의 공여, 수용체
2) 호기성대사
-전자흐름과 에너지(그림9.7)
Section 9.1: Oxidation-Reduction Reactions
Redox Coenzymes: Nicotinic Acid
Two coenzyme forms: nicotinamide adenine
dinucleotide (NAD) and nicotinamide adenine
dinucleotide phosphate (NADP)
Have oxidized (NAD+ and NADP+) and reduced forms
(NADH and NADPH)
NADP+ involved in biosynthetic reactions and NAD+
involved in catabolic reactions
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Figure 9.5 Nicotinamide Adenine
Dinucleotide (NAD)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Redox Coenzymes: Riboflavin
Riboflavin (vitamin B2) is a component of two
coenzymes: flavin mononucleotide (FMN) and flavin
adenine dinucleotide (FAD)
Function in a diverse class of redox enzymes known
as flavoproteins
Function as dehydrogenases, oxidases, and
hydroxylases
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Figure 9.6 Flavin Coenzymes
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.1: Oxidation-Reduction Reactions
Figure 9.7 Electron Flow and Energy
Aerobic Metabolism
Electron transport chain captures most of aerobic
cell’s free energy
Energy transferred from NADH to O2
½O2 + NADH + H+  H20 + NAD+ (-220 kJ/mol)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
9.2 시트르산회로 (그림9.8)
-아세틸CoA에서 조효소 A에 연결됨(그림 9.9)
-아세틸CoA가 산화되며, NADH, FADH2가 생성
-기질수준인산화로 고에너지 GTP생성
-구연산회로에서 조효소의 역할(표9.2)
(1)피루브산의 아세틸CoA로의 전환
-미토콘드리아 기질로 수송된 피루브산은 피루브산 탈수소효소복합체에 의한
일련의 촉매반응으로 아세틸CoA로 전환(산화적 탈카복실화)
-대장균의 피루브산 탈수소효소복합체(표9.3)
-반응(그림9.10), 리포산의 역할(그림9.11)
Section 9.2: Citric Acid Cycle
Figure 9.9 Coenzyme A
Citric acid cycle is used to harvest energy from
acetyl group of acetyl-CoA
Acetyl is derived from catabolism of carbohydrates
(e.g., pyruvate), lipids, and some amino acids
Coenzyme A is an acyl carrier molecule
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.8 The Citric Acid Cycle
In the citric acid cycle, the acetyl group’s carbon
atoms are ultimately oxidized to form CO2
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.8 The Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Transfer of electrons to
carrier molecules from the
citric acid cycle intermediate
molecules forms the reduced
coenzymes NADH and FADH2
Figure 9.8 The Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Citric acid cycle intermediates also play an
important role in a variety of biosynthetic reactions
A variety of coenzymes play important roles in the
citric acid cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Conversion of Pyruvate to Acetyl-CoA
Pyruvate dehydrogenase complex converts pyruvate
to acetyl-CoA
Large complex multienzyme structure
Highly exergonic (DGº′ = -33.5 kJ/mol)
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Thiamine pyrophosphate (TPP)
coenzyme helps decarboxylate
pyruvate
Lipoic acid helps convert an
intermediate (HETPP) into
acetyl-CoA
Figure 9.11 Lipoamide
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Decarboxylation
Action of lipoic acid
Figure 9.10 Reactions Catalyzed by the
Pyruvate Dehydrogenase Complex
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Action of TPP
Formation of AcetylCoA
Figure 9.10 Reactions Catalyzed by the
Pyruvate Dehydrogenase Complex
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
(2)시트르산회로의 반응
-8개의 반응
1. 아세틸 CoA로서 두 개의 탄소도입
-구연산으로 축합반응
2. 구연산은 이성질화하여 쉽게 산화할 수 있는 2차 알코올을 형성한다
-이소구연산
3. 이소구연산은 산화하여 NADH와 CO2를 형성한다
-α-케토글루타르산형성
4. α-케토글루타르산은 산화하여 두 번째 분자 NADH와 CO2를 형성한다.
5. 숙시닐 CoA의 티오에스테르 결합의 절단은 기질수준인산화와 연계되어 있다
-숙신산 티오키나아제
-이인산키나아제
Section 9.2: Citric Acid Cycle
Figure 9.12 Citrate Synthesis
Reactions of the Citric Acid Cycle
Eight reactions in two stages:
1. Liberation of two CO2 from acetyl-CoA
2. Regeneration of oxaloacetate
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Reactions of Citric Acid Cycle Continued
1. Introduction of two carbons as acetyl-CoA-forming
citrate
2. Citrate isomerization
3. Isocitrate is oxidized to form NADH and CO2
4. a-Ketoglutarate is oxidized; forms NADH and CO2
Reactions 3 and 4 are oxidative decarboxylation
reactions
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
6. 4개의 탄소분자인 숙신산은 산화하여 푸마르산과 FADH2를 형성한다
-말론산에 의해 억제
7. 푸마르산은 수화하여 말산이 된다
8. 말산은 산화하여 옥살로아세트산과 세번째 NADH를 형성한다.
(3) 시트르산회로에서의 탄소원자의 운명
-두 분자의 CO2분자로 방출
(4) 양반응 구연산회로
-양반응 구연산회로는 동화와 이화반응에 모두 작용한다
-보충반응(동화작용으로 나간 분자보충): 예로 옥살로아세트산 보충(그림9.13)
(5) 구연산회로의 조절
-구연산 합성효소, 이소구연산 탈수소효소와 α-케토글루타르산 탈수소효소가
매우 긴밀히 조절됨(그림9.14)
-이소구연산 탈수소효소: ADP, NAD+에 의해 촉진(ATP, NADH로 억제)
-구연산만이 미토콘드리아내막을 통과할 수 있다: 에너지요구량이 적어질때,
아세틸-CoA의 세포질이동, 지방산합성, NADPH제공 (그림9.15)
Section 9.2: Citric Acid Cycle
Reactions of Citric Acid Cycle Continued
5. Cleavage of Succinyl-CoA leads to substrate-level
phosphorylation
6. Succinate is oxidized to form fumarate and FADH2
7. Fumarate is hydrated and forms L-malate
8. Malate is oxidized to form oxaloacetate and a third
NADH
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
The Amphibolic Citric Acid
Cycle
Citric acid cycle involved in
anabolic as well as catabolic
processes
Anabolic reactions lead to
the formation of many
important biomolecules
Figure 9.13 Amphibolic Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
The Amphibolic Citric Acid
Cycle Continued
Anaplerotic reactions also
contribute intermediates into
the cycle
Oxaloacetate from
pyruvate or aspartate
Succinyl-CoA from fatty
acids
Figure 9.13 Amphibolic Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Citric Acid Cycle
Regulation
Regulation controlled by
three irreversible
enzymes
Citrate synthase
regulated by substrate
levels, ATP/ADP ratio,
and NADH/NAD+ ratio
Figure 9.14 Control of Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Isocitrate dehydrogenase regulated
by substrate levels, ATP/ADP ratio,
and NADH/NAD+ ratio
a-Ketoglutarate dehydrogenase
regulated by substrate levels, AMP,
and NADH levels
Figure 9.14 Control of Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Calcium regulation cytoplasmic [Ca2+] increase is
followed rapidly by [Ca2+] increase in the matrix
Increases ATP production by stimulating enzymes
that regulate the pace of the citric acid cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.15 Citrate Metabolism
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.15 Citrate
Metabolism
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Citrate Metabolism:
Citrate plays a role in oxaloacetate, malate, and
pyruvate formation
Can also lead to NADPH production used for fatty
acid biosynthesis
Citrate in the cytoplasm can also inhibit glycolysis
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
(6)시트르산회로와 인간의질병: 암과 에너지대사
-글루코스를 젖산으로 빠르게 전환; 낮은 외부 pH, 높은 세포내 pH
-에너지대사?: 만성저산소증(hypoxia)에 의해 촉진
-HIF-1(전사인자 저산소증-유도인자-1) 작용:글루코오스운반단백질, 해당효소,
LDH등 발현
-암발생: 젖산과잉생산, VEGF의 유도와 혈관신생을 유도
-호기성해당과 글루코오스소모의 증가가 생장하는 종양의 특징
(7)글리옥실산회로
-식물, 진균, 원생동물, 박테리아: 2 개의 탄소화합물(에탄올, 아세트산,
아세틸CoA)를 이용하여 성장가능
-구연산회로의 변형: 식물은 글리옥시좀에서 일어남
-회로(그림9.16); 5개 반응
-알돌절단반응 후 생성된 숙신산 말산으로 전환(그림9.17)
-탈카르복실화반응을 우회하여 더 큰 분자를 합성하며, 숙신산은
클르코오스합성에 옥살로아세트산은 이 회로의 유지에 사용
*시트르산회로의 진화역사: Hans Krebs와 구연산회로
-Krebs, Warburg
-Lipman과 Kaplan:아세틸 CoA발견과 구연산합성
-1953년 노벨상수상; Krebs, Lipmann
-1932년, 글리옥실산회로발견
Section 9.2: Citric Acid Cycle
The Glyoxylate Cycle
Occurs in plants and some fungi, algae, protozoans,
and bacteria
Modified citric acid cycle
Five reactions use two-carbon compounds
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.16 The Glyoxylate Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
Figure 9.17 Role of Glyoxylate
in Gluconeogenesis
Glyoxylate cycle produces two molecules: succinate
and oxaloacetate
Succinate can be used to make metabolically
important molecules like glucose
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Section 9.2: Citric Acid Cycle
The Citric Acid Cycle and Human Disease
Most common diseases are severe forms of
encephalopathy
Encephalopathies have been linked to mutations
in a-ketoglutarate dehydrogenase, succinate
dehydrogenase, fumarase, and succinyl-CoA
synthetase
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press
Biochemistry in Perspective
Evolutionary History of
the Citric Acid Cycle
First originated as two
pathways:
1. The reductive
pathway provided
free electron
acceptors
2. The oxidative
pathway generated
a-ketoglutarate, a
biosynthetic
precursor molecule
Figure 9A The Incomplete
Citric Acid Cycle
From McKee and McKee, Biochemistry, International Fifth Edition, © 2012 by Oxford University Press