第二十二章
糖 酵 解
Glycolysis and the
Catabolism of
Hexoses
An overview on D-glucose metabolism
? 大多数机体的能源物质，彻底氧化,?G'o = –
2840 kJ/mole
? 通过磷酸戊糖途径可产生 NADPH 和 ribose-5-P 。
? 可以以多糖 (glycogen or starch) 或转化为脂肪进
行长期储存。
? 几乎所有的生物分子都可由葡萄糖合成（ amino
acids,nucleotides,fatty acids,coenzymes and
other metabolic intermediates.）
? Metabolism of
glucose under
Aerobic and
Anaerobic
Conditions,
An overview on D-glucose metabolism
The Development of Biochemistry
and the Delineation of Glycolysis
Went Hand by Hand
? 1897,Eduard Buchner (Germany),accidental
observation, sucrose (as a preservative) was
rapidly fermented into alcohol by cell-free
yeast extract,
? The accepted view that fermentation is
inextricably tied to living cells (i.e.,the
vitalistic dogma,活力论 ) was shaken and
Biochemistry was born,Metabolism became
chemistry!
? 1900s,Arthur Harden and William Young Pi
is needed for yeast juice to ferment glucose,
a hexose diphosphate (fructose 1,6-
bisphosphate) was isolated,
? 1900s,Arthur Harden and William Young
(Great Britain) separated the yeast juice into
two fractions,one heat-labile,non－
dialyzable zymase (enzymes) and the other
heat-stable,dialyzable cozymase (metal ions,
ATP,ADP,NAD+),
? 1910s-1930s,Gustav Embden and Otto
Meyerhof (Germany),studied muscle
and its extracts,
– Reconstructed all the transformation steps
from glycogen to lactic acid in vitro;
revealed that many reactions of lactic acid
(muscle) and alcohol (yeast)
fermentations were the same!
– Discovered that lactic acid is reconverted
to carbohydrate in the presence of O2
(gluconeogenesis); observed that some
phosphorylated compounds are energy-rich,
? Glycolysis was also known as
Embden-Meyerhof pathway,
? The whole pathway of glycolysis
(Glucose to pyruvate) was
elucidated by the 1940s,
一、糖酵解途径（ Embden-Meyerhof
pathway ）
葡萄糖经 10步 酶促反应，分解为 2分子丙酮酸,
产生 2分子 ATP和 2分子 NADH。
1,糖酵解的十步反应
? A net gain of two ATP,two NADH,
two pyruvates are resulted when a
glucose molecule is oxidized via the
glycolysis pathway,
Glucose + 2 ADP + 2Pi + 2NAD+?
2 pyruvate + 2ATP + 2H2O + 2NADH + 2H+
2,Ten enzymes catalyze the ten
reactions of glycolysis
（ 1） Hexokinase (also glucokinase in liver) 催
化糖酵解的第一步反应。
★ Mg2+ATP2-,not ATP4- 是该酶的另一个底物 ;
★ 己糖激酶是一个调节酶, 6-磷酸葡萄糖和 ATP
是该酶的变构抑制剂。
★ 该反应热力学有利，在细胞中为不可逆反应。
（ 2） Phosphohexose isomerase (also called
phosphoglucose isomerase) catalyzes the
isomerization from glucose 6-P to fructose 6-
P,converting an aldose to a ketose,
（ 3） Phosphofructokinase-1 (PFK-1,磷酸果糖
激酶 -1)催化第二步磷酸化反应。
★ PFK-1 是一个变构酶，催化效率很低，糖酵解
途径进行的速度依赖于该酶的活性水平。
★ PFK-1 是一个调节酶,ATP和 H+是该酶的变构
抑制剂,AMP可解除 ATP对该酶的抑制作用，
因此细胞中 ATP/AMP值对此酶具有明显的调节
作用。
★ H+ 浓度升高抑制该酶的活性。
★ 在动物细胞中，发现三种同工酶,PFK A,B,C。
★ The plant PFK-1 makes use of PPi,instead
of ATP at this step,
（ 4） Aldolase (醛缩酶 ),named for the reverse
reaction catalyzes the cleavage (“lysis”) of
fructose 1,6-bisphosphate from the middle C-C
bond to form two 3-carbon sugars,
dihydroxyacetone phosphate and glyceraldehyde
3-phosphate; this is a reversal aldol
condensation reaction; thermodynamically very
unfavorable under standard conditions,
（ 5） Triose phosphate isomerase (an extremely
efficient enzyme) converts dihydroacetone
phosphate to glyceraldehyde 3-phosphate; an
intramolecular redox reaction (a hydrogen atom
is transferred from C-1 to C-3),
（ 6） Glyceraldehyde 3-phosphate
dehydrogenase 催化糖酵解的唯一的氧
化还原反应。
★ NAD+ 为该酶的辅酶，从底物接受 2e- 。
★ 脱氢反应的同时，底物发生磷酸化，无机
磷酸直接参与反应，砷酸可取代磷酸参与
该反应，但产物为 3-磷酸甘油酸。
★ 巯基为该酶的催化基团，形成硫酯中间产
物，碘乙酸为是该酶的抑制剂。
（ 7） The phosphoglycerate kinase catalyzes
the direct transfer of the anhydride
phosphate in 1,3-BPG to an ADP to generate
an ATP; this is called the substrate-level
phosphorylation; 1,3-BPG is a high energy
intermediate that leads to ATP formation,
（ 8） The phosphoglycerate mutase catalyzes
the shift of phosphoryl group on 3-
phosphoglycerate from C-3 to C-2; 2,3-
bisphosphoglycerate is both a coenzyme for
the mutase and an intermediate for the
reaction,
（ 9） Enolase (烯醇酶 ) catalyzes the elimination of
a H2O from 2-phosphoglycerate to generate
phosphoenolglycerate (PEP) with the transfer
potential of the phosphoryl group dramatically
increased (? G 0` changed from –17.6 to –61.9
kJ/mol),
（ 10） The pyruvate kinase (named for the reverse
reaction) 催化糖酵解的第二分子 ATP的合成。
★ 该酶为变构调节酶,ATP,长链脂肪酸、乙酰辅
酶 A,和丙氨酸为抑制剂，果糖 -1,6-二磷酸为激
活剂。
★ 该反应在细胞中为不可逆反应。
★ 该酶至少有三种不同类型的同工酶,L型（肝脏）、
M型（肌肉和脑）,A型（其他组织）。
二
其
他
单
糖
进
入
糖
酵
解
途
径
三、糖酵解途径的调节
1,The rate of glycolysis in mammals is mainly
controlled at the step acted by
phosphofructokinase-1 (PFK-1) (Why?)
(1) PFK-1 catalyzes an irreversible exergonic
reaction,which commits glucose to the
glycolysis pathway (away from the pentose
phosphate pathway),
(2)PFK-1 is a complex tetrameric enzyme
regulated by multiple intracellular signals
(allosteric effectors),ATP,citrate being
negative ones; AMP,ADP and fructose 2,6-
bisphosphate as positive ones,
(3) A regulated bifunctional enzyme (PFK-2 and
FBPase-2) synthesizes (from Fru-6-P) and
degrades fructose 2,6-bisphosphate,
A feedforward stimulation,Fru-6-P
stimulate the synthesis and inhibits the
hydrolysis of Fru-2,6-bisphosphate,which in
turn stimulates PFK-1,
100
F r u-6 -P u m / L
R
e
l
a
t
i
v
e
A
c
t
i
v
a
t
y
o
f
P
F
K
1
0, 1 u m / L
F ru - 2,6 - BP
1, 0 u m / L F ru - 2,6 - BP
A T P u m / L
R
e
l
a
t
i
v
e
A
c
t
i
v
a
t
y
o
f
P
F
K
1
0, 1 u m / L
F ru - 2,6 - B P
1, 0 u m / L F ru - 2,6 - B P
2,Hexokinase and pyruvate kinase also set the
pace of glycolysis
(1) These two enzymes also catalyzed
irreversible exergonic reactions,
(2) Muscle hexokinase is allosterically inhibited
by its reaction product Glc-6-P,which
accumulates when PFK-1 is inhibited,
(3) The liver hexokinase (also called hexokinase
D or glucokinase) has about 100 X less
affinity for glucose than that in muscle and is
not inhibited by Glc-6-P,its main role is to
convert excess glucose to Glc-6-P for
glycogen synthesis,
(4) Pyruvate kinase is allosterically
inhibited by ATP,alanine,acetyl-CoA,
and long-chain fatty acids,
(5) The catalytic activity of the liver
pyruvate kinase isozyme (the L type) is
also controlled by reversible
phosphorylation,
四、无氧条件下，丙酮酸的去路
－－发酵
This occurs to regenerate NAD+ for
the glycolysis pathway to continue
when O2 lacks,
1,乳酸发酵,
Pyruvate is reduced
to lactate when O2
lacks in a reaction
catalyzed by
lactate
dehydrogenase
(occurring in
very active
skeleton muscle,
some bacteria
like lactobacilli)
2,乙醇发酵,
pyruvate is first
decarboxylated and
then reduced by NADH,
catalyzed by pyruvate
decarboxylase and
alcohol dehydrogenase
respectively,
Present only in those
alcohol fermentative
organisms
Present in many
organisms including
human
Hexokinase Glucose
Induced
fit
Irreversible
in cells
One subunit
of the tetrameric
phosphofructokinase-1
(PFK-1) Regulatory
ADP
The committing step
Phosphorolysis
（ 磷酸解作用 ）
C-1 no longer carries
a large positive charge,
hydride ion leaves readily
Energy-rich intermediate
(thioester)
Inactive
enzyme
Proposed action mechanism
Of glyceraldehyde 3-P
dehydrogenase
A proposed action
mechanism for
phosphoglycerate
mutase
第二十三章
三羧酸循环
Tricarboxylic Acid Cycle
一、葡萄糖的有氧
分解代谢
在有氧条件下，葡萄糖
的分解代谢可分为三
个阶段进行讨论,
（ 1）葡萄糖到丙酮酸阶
段同糖酵解途径；
（ 2）三羧酸循环；
（ 3）呼吸链电子传递和
ATP的合成。
1,Pyruvate is oxidized to acetyl-
CoA by the catalysis of pyruvate
dehydrogenase complex
Structure of the pyruvate
dehydrogenase complex
? Threedimensionalimage of PDH complex,showing the
subunit structure:E1,pyruvate dehydrogenase; E2,
dihydrolipoyl transacetylase; and E3,dihydrolipoyl
dehydrogenase,
? The core (green) consists of 60 molecules of E2,
arranged in 20 trimers to form a pentagonal
dodecahedron,The lipoyl domain of E2 (blue) reaches
outward to touch the active sites of E1 molecules
(yellow) arranged on the E2 core,A number of E3
subunits (red) are also bound to the core,where the
swinging arm on E2 can reach their active sites,An
asterisk marks the site where a lipoyl group is
attached to the lipoyl domain of E2,
Number of lipoyl domains varies by
species,
2,The complete oxidation of pyruvate in animal
tissues was proposed to undergo via a cyclic
pathway
(1) O2 consumption and pyruvate oxidation in
minced muscle tissues were found to be
stimulated by some four-carbon dicarboxylic
acids (Fumarate,succinate,malate and
oxaloacetate,five-carbon dicarboxylic acid (a-
ketoglutarate ),or six-carbon tricarboxylic
acids (citrate,isocitrate,cis-aconitate),
(2) A small amount of any of these organic
acids stimulates many folds of pyruvate
oxidation!
(3) Malonate inhibits pyruvate oxidation
regardless of which active organic acid is
added!
(4) Hans Krebs proposed the,citric acid
cycle” for the complete oxidation of
pyruvate in animal tissues in 1937 (he
wrongly hypothesized that pyruvate
condenses with oxaloacetate in his
original proposal),
(5) The citric acid cycle was confirmed to
be universal in cells by in vitro studies
with purified enzymes and in vivo
studies with radio isotopes (“radio
isotope tracer experiments”),
(6) Krebs was awarded the Nobel prize in
medicine in 1953 for revealing the citric
acid cycle (thus also called the Krebs
cycle),
3,The acetyl group (carried by CoA) is
completely oxidized to CO2 via the
citric acid cycle
? The 4-carbon oxaloacetate (草酰乙酸 ) acts as
the,carrier” for the oxidation,
? The two carbons released as 2 CO2 in the first
cycle of oxidation are not from the acetyl-CoA
just joined,
? The 8 electrons released are collected by
three NAD+ and one FAD,
? One molecule of ATP (or GTP) is produced
per cycle by substrate-level phosphorylation,
4,The
citric acid
cycle
5,The complete oxidation of one
glucose may yield as many as 32 ATP
? All the NADH and FADH2 will eventually pass
their electrons to O2 after being transferred
through a series of electron carriers,
? The complete oxidation of each NADH
molecule leads to the generation of about 2.5
ATP,and FADH2 of about 1.5 ATP,
? Overall efficiency of energy conservation is
about 34% using the free energy changes
under standard conditions and about 65%
using actual free energy changes in cells,
6,The regulation of TCA
cycle
(1) The pyruvate dehydrogenase complex in
vertebrates is regulated alloseterically and
covalently
? The formation of acetyl-CoA from pyruvate is
a key irreversible step in animals because
they are unable to convert acetyl-CoA into
glucose,
? The complex (in all organisms) is allosterically
inhibited by signaling molecules indicating a
rich source of energy,e.g.,ATP,acetyl-CoA,
NADH,fatty acids; activated by molecules
indicating a lack (or demand) of energy,e.g.,
AMP,CoA,NAD+,Ca2+,
? The activity of the complex (in
vertebrates,probably also in plants,but
not in E,coli) is also regulated by
reversible phosphorylation of one of the
enzymes,E1,in the complex,
phosphorylation of a specific Ser residue
inhibits and dephosphorylation activates
the complex,
? The kinase and phosphatase is also part
of the enzyme complex,The kinase is
activated by a high concentration of ATP,
(2) The rate of the citric acid cycle is controlled
at three exergonic irreversible steps
? Citrate synthase,isocitrate
dehydrogenase and a-ketoglutarate
dehydrogenase;
? Inhibited by product feedback (citrate,
succinyl-CoA) and high energy charge
(ATP,NADH);
? Activated by a low energy charge (ADP)
or a signal for energy requirement
(Ca2+),
二、三羧酸循环的回补反应
The most common anaplerotic reactions covert
either pyruvate or phosphoenolpyruvate to
oxaloacetate or malate,
Some anaerobic bacteria,
lacking the a-ketoglutarate
dehydrogenase enzyme,
make biosynthetic
precursors via the
incomplete citric acid cycle;
could be an early evolution
stage of the citric acid
cycle,
三、三羧酸循环的中间产物是重要的生
物合成的前体分子
? The citric acid cycle is the hub of intermediary
metabolism serving both the catabolic and
anabolic processes (thus an amphibolic
pathway),
? It provides precursors for the biosynthesis of
glucose,amino acids,nucleotides,glucose,
fatty acids,sterols,heme groups,etc,
? Intermediates of the citric acid cycle get
replenished by anaplerotic reactions when
consumed by biosynthesis,
Summary
? Pyruvate is converted to acetyl-CoA by the
action of pyruvate dehydrogenase complex,a
huge enzyme complex,
? Acetyl-CoA is converted to 2 CO2 via the eight-
step citric acid cycle,generating three NADH,
one FADH2,and one ATP (by substrate-level
phophorylation),
? Intermediates of citric acid cycle are drawn off
to synthesize many other biomolecules,
including fatty acids,steroids,amino acids,
heme,pyrimidines,and glucose,
? Oxaloacetate can get supplemented from
pyruvate,via a carboxylation reaction
catalyzed by the biotin-containing pyruvate
carboxylase,
? The activity of pyruvate dehydrogenase
complex is regulated by allosteric effectors
and reversible phosphorylations,
? The rate of the citric acid cycle is
controlled at three exergonic irreversible
steps
Thanks!
▁▂▃▄▅▆▇▇▆▅▄▃▂▁ ▁▂▃▄▅▆▇▇▆▅▄▃▂▁
▁▂▃▄▅▆▇▇▆▅▄▃▂▁ ▁▂▃▄▅▆▇▇▆▅▄▃▂▁
糖 酵 解
Glycolysis and the
Catabolism of
Hexoses
An overview on D-glucose metabolism
? 大多数机体的能源物质，彻底氧化,?G'o = –
2840 kJ/mole
? 通过磷酸戊糖途径可产生 NADPH 和 ribose-5-P 。
? 可以以多糖 (glycogen or starch) 或转化为脂肪进
行长期储存。
? 几乎所有的生物分子都可由葡萄糖合成（ amino
acids,nucleotides,fatty acids,coenzymes and
other metabolic intermediates.）
? Metabolism of
glucose under
Aerobic and
Anaerobic
Conditions,
An overview on D-glucose metabolism
The Development of Biochemistry
and the Delineation of Glycolysis
Went Hand by Hand
? 1897,Eduard Buchner (Germany),accidental
observation, sucrose (as a preservative) was
rapidly fermented into alcohol by cell-free
yeast extract,
? The accepted view that fermentation is
inextricably tied to living cells (i.e.,the
vitalistic dogma,活力论 ) was shaken and
Biochemistry was born,Metabolism became
chemistry!
? 1900s,Arthur Harden and William Young Pi
is needed for yeast juice to ferment glucose,
a hexose diphosphate (fructose 1,6-
bisphosphate) was isolated,
? 1900s,Arthur Harden and William Young
(Great Britain) separated the yeast juice into
two fractions,one heat-labile,non－
dialyzable zymase (enzymes) and the other
heat-stable,dialyzable cozymase (metal ions,
ATP,ADP,NAD+),
? 1910s-1930s,Gustav Embden and Otto
Meyerhof (Germany),studied muscle
and its extracts,
– Reconstructed all the transformation steps
from glycogen to lactic acid in vitro;
revealed that many reactions of lactic acid
(muscle) and alcohol (yeast)
fermentations were the same!
– Discovered that lactic acid is reconverted
to carbohydrate in the presence of O2
(gluconeogenesis); observed that some
phosphorylated compounds are energy-rich,
? Glycolysis was also known as
Embden-Meyerhof pathway,
? The whole pathway of glycolysis
(Glucose to pyruvate) was
elucidated by the 1940s,
一、糖酵解途径（ Embden-Meyerhof
pathway ）
葡萄糖经 10步 酶促反应，分解为 2分子丙酮酸,
产生 2分子 ATP和 2分子 NADH。
1,糖酵解的十步反应
? A net gain of two ATP,two NADH,
two pyruvates are resulted when a
glucose molecule is oxidized via the
glycolysis pathway,
Glucose + 2 ADP + 2Pi + 2NAD+?
2 pyruvate + 2ATP + 2H2O + 2NADH + 2H+
2,Ten enzymes catalyze the ten
reactions of glycolysis
（ 1） Hexokinase (also glucokinase in liver) 催
化糖酵解的第一步反应。
★ Mg2+ATP2-,not ATP4- 是该酶的另一个底物 ;
★ 己糖激酶是一个调节酶, 6-磷酸葡萄糖和 ATP
是该酶的变构抑制剂。
★ 该反应热力学有利，在细胞中为不可逆反应。
（ 2） Phosphohexose isomerase (also called
phosphoglucose isomerase) catalyzes the
isomerization from glucose 6-P to fructose 6-
P,converting an aldose to a ketose,
（ 3） Phosphofructokinase-1 (PFK-1,磷酸果糖
激酶 -1)催化第二步磷酸化反应。
★ PFK-1 是一个变构酶，催化效率很低，糖酵解
途径进行的速度依赖于该酶的活性水平。
★ PFK-1 是一个调节酶,ATP和 H+是该酶的变构
抑制剂,AMP可解除 ATP对该酶的抑制作用，
因此细胞中 ATP/AMP值对此酶具有明显的调节
作用。
★ H+ 浓度升高抑制该酶的活性。
★ 在动物细胞中，发现三种同工酶,PFK A,B,C。
★ The plant PFK-1 makes use of PPi,instead
of ATP at this step,
（ 4） Aldolase (醛缩酶 ),named for the reverse
reaction catalyzes the cleavage (“lysis”) of
fructose 1,6-bisphosphate from the middle C-C
bond to form two 3-carbon sugars,
dihydroxyacetone phosphate and glyceraldehyde
3-phosphate; this is a reversal aldol
condensation reaction; thermodynamically very
unfavorable under standard conditions,
（ 5） Triose phosphate isomerase (an extremely
efficient enzyme) converts dihydroacetone
phosphate to glyceraldehyde 3-phosphate; an
intramolecular redox reaction (a hydrogen atom
is transferred from C-1 to C-3),
（ 6） Glyceraldehyde 3-phosphate
dehydrogenase 催化糖酵解的唯一的氧
化还原反应。
★ NAD+ 为该酶的辅酶，从底物接受 2e- 。
★ 脱氢反应的同时，底物发生磷酸化，无机
磷酸直接参与反应，砷酸可取代磷酸参与
该反应，但产物为 3-磷酸甘油酸。
★ 巯基为该酶的催化基团，形成硫酯中间产
物，碘乙酸为是该酶的抑制剂。
（ 7） The phosphoglycerate kinase catalyzes
the direct transfer of the anhydride
phosphate in 1,3-BPG to an ADP to generate
an ATP; this is called the substrate-level
phosphorylation; 1,3-BPG is a high energy
intermediate that leads to ATP formation,
（ 8） The phosphoglycerate mutase catalyzes
the shift of phosphoryl group on 3-
phosphoglycerate from C-3 to C-2; 2,3-
bisphosphoglycerate is both a coenzyme for
the mutase and an intermediate for the
reaction,
（ 9） Enolase (烯醇酶 ) catalyzes the elimination of
a H2O from 2-phosphoglycerate to generate
phosphoenolglycerate (PEP) with the transfer
potential of the phosphoryl group dramatically
increased (? G 0` changed from –17.6 to –61.9
kJ/mol),
（ 10） The pyruvate kinase (named for the reverse
reaction) 催化糖酵解的第二分子 ATP的合成。
★ 该酶为变构调节酶,ATP,长链脂肪酸、乙酰辅
酶 A,和丙氨酸为抑制剂，果糖 -1,6-二磷酸为激
活剂。
★ 该反应在细胞中为不可逆反应。
★ 该酶至少有三种不同类型的同工酶,L型（肝脏）、
M型（肌肉和脑）,A型（其他组织）。
二
其
他
单
糖
进
入
糖
酵
解
途
径
三、糖酵解途径的调节
1,The rate of glycolysis in mammals is mainly
controlled at the step acted by
phosphofructokinase-1 (PFK-1) (Why?)
(1) PFK-1 catalyzes an irreversible exergonic
reaction,which commits glucose to the
glycolysis pathway (away from the pentose
phosphate pathway),
(2)PFK-1 is a complex tetrameric enzyme
regulated by multiple intracellular signals
(allosteric effectors),ATP,citrate being
negative ones; AMP,ADP and fructose 2,6-
bisphosphate as positive ones,
(3) A regulated bifunctional enzyme (PFK-2 and
FBPase-2) synthesizes (from Fru-6-P) and
degrades fructose 2,6-bisphosphate,
A feedforward stimulation,Fru-6-P
stimulate the synthesis and inhibits the
hydrolysis of Fru-2,6-bisphosphate,which in
turn stimulates PFK-1,
100
F r u-6 -P u m / L
R
e
l
a
t
i
v
e
A
c
t
i
v
a
t
y
o
f
P
F
K
1
0, 1 u m / L
F ru - 2,6 - BP
1, 0 u m / L F ru - 2,6 - BP
A T P u m / L
R
e
l
a
t
i
v
e
A
c
t
i
v
a
t
y
o
f
P
F
K
1
0, 1 u m / L
F ru - 2,6 - B P
1, 0 u m / L F ru - 2,6 - B P
2,Hexokinase and pyruvate kinase also set the
pace of glycolysis
(1) These two enzymes also catalyzed
irreversible exergonic reactions,
(2) Muscle hexokinase is allosterically inhibited
by its reaction product Glc-6-P,which
accumulates when PFK-1 is inhibited,
(3) The liver hexokinase (also called hexokinase
D or glucokinase) has about 100 X less
affinity for glucose than that in muscle and is
not inhibited by Glc-6-P,its main role is to
convert excess glucose to Glc-6-P for
glycogen synthesis,
(4) Pyruvate kinase is allosterically
inhibited by ATP,alanine,acetyl-CoA,
and long-chain fatty acids,
(5) The catalytic activity of the liver
pyruvate kinase isozyme (the L type) is
also controlled by reversible
phosphorylation,
四、无氧条件下，丙酮酸的去路
－－发酵
This occurs to regenerate NAD+ for
the glycolysis pathway to continue
when O2 lacks,
1,乳酸发酵,
Pyruvate is reduced
to lactate when O2
lacks in a reaction
catalyzed by
lactate
dehydrogenase
(occurring in
very active
skeleton muscle,
some bacteria
like lactobacilli)
2,乙醇发酵,
pyruvate is first
decarboxylated and
then reduced by NADH,
catalyzed by pyruvate
decarboxylase and
alcohol dehydrogenase
respectively,
Present only in those
alcohol fermentative
organisms
Present in many
organisms including
human
Hexokinase Glucose
Induced
fit
Irreversible
in cells
One subunit
of the tetrameric
phosphofructokinase-1
(PFK-1) Regulatory
ADP
The committing step
Phosphorolysis
（ 磷酸解作用 ）
C-1 no longer carries
a large positive charge,
hydride ion leaves readily
Energy-rich intermediate
(thioester)
Inactive
enzyme
Proposed action mechanism
Of glyceraldehyde 3-P
dehydrogenase
A proposed action
mechanism for
phosphoglycerate
mutase
第二十三章
三羧酸循环
Tricarboxylic Acid Cycle
一、葡萄糖的有氧
分解代谢
在有氧条件下，葡萄糖
的分解代谢可分为三
个阶段进行讨论,
（ 1）葡萄糖到丙酮酸阶
段同糖酵解途径；
（ 2）三羧酸循环；
（ 3）呼吸链电子传递和
ATP的合成。
1,Pyruvate is oxidized to acetyl-
CoA by the catalysis of pyruvate
dehydrogenase complex
Structure of the pyruvate
dehydrogenase complex
? Threedimensionalimage of PDH complex,showing the
subunit structure:E1,pyruvate dehydrogenase; E2,
dihydrolipoyl transacetylase; and E3,dihydrolipoyl
dehydrogenase,
? The core (green) consists of 60 molecules of E2,
arranged in 20 trimers to form a pentagonal
dodecahedron,The lipoyl domain of E2 (blue) reaches
outward to touch the active sites of E1 molecules
(yellow) arranged on the E2 core,A number of E3
subunits (red) are also bound to the core,where the
swinging arm on E2 can reach their active sites,An
asterisk marks the site where a lipoyl group is
attached to the lipoyl domain of E2,
Number of lipoyl domains varies by
species,
2,The complete oxidation of pyruvate in animal
tissues was proposed to undergo via a cyclic
pathway
(1) O2 consumption and pyruvate oxidation in
minced muscle tissues were found to be
stimulated by some four-carbon dicarboxylic
acids (Fumarate,succinate,malate and
oxaloacetate,five-carbon dicarboxylic acid (a-
ketoglutarate ),or six-carbon tricarboxylic
acids (citrate,isocitrate,cis-aconitate),
(2) A small amount of any of these organic
acids stimulates many folds of pyruvate
oxidation!
(3) Malonate inhibits pyruvate oxidation
regardless of which active organic acid is
added!
(4) Hans Krebs proposed the,citric acid
cycle” for the complete oxidation of
pyruvate in animal tissues in 1937 (he
wrongly hypothesized that pyruvate
condenses with oxaloacetate in his
original proposal),
(5) The citric acid cycle was confirmed to
be universal in cells by in vitro studies
with purified enzymes and in vivo
studies with radio isotopes (“radio
isotope tracer experiments”),
(6) Krebs was awarded the Nobel prize in
medicine in 1953 for revealing the citric
acid cycle (thus also called the Krebs
cycle),
3,The acetyl group (carried by CoA) is
completely oxidized to CO2 via the
citric acid cycle
? The 4-carbon oxaloacetate (草酰乙酸 ) acts as
the,carrier” for the oxidation,
? The two carbons released as 2 CO2 in the first
cycle of oxidation are not from the acetyl-CoA
just joined,
? The 8 electrons released are collected by
three NAD+ and one FAD,
? One molecule of ATP (or GTP) is produced
per cycle by substrate-level phosphorylation,
4,The
citric acid
cycle
5,The complete oxidation of one
glucose may yield as many as 32 ATP
? All the NADH and FADH2 will eventually pass
their electrons to O2 after being transferred
through a series of electron carriers,
? The complete oxidation of each NADH
molecule leads to the generation of about 2.5
ATP,and FADH2 of about 1.5 ATP,
? Overall efficiency of energy conservation is
about 34% using the free energy changes
under standard conditions and about 65%
using actual free energy changes in cells,
6,The regulation of TCA
cycle
(1) The pyruvate dehydrogenase complex in
vertebrates is regulated alloseterically and
covalently
? The formation of acetyl-CoA from pyruvate is
a key irreversible step in animals because
they are unable to convert acetyl-CoA into
glucose,
? The complex (in all organisms) is allosterically
inhibited by signaling molecules indicating a
rich source of energy,e.g.,ATP,acetyl-CoA,
NADH,fatty acids; activated by molecules
indicating a lack (or demand) of energy,e.g.,
AMP,CoA,NAD+,Ca2+,
? The activity of the complex (in
vertebrates,probably also in plants,but
not in E,coli) is also regulated by
reversible phosphorylation of one of the
enzymes,E1,in the complex,
phosphorylation of a specific Ser residue
inhibits and dephosphorylation activates
the complex,
? The kinase and phosphatase is also part
of the enzyme complex,The kinase is
activated by a high concentration of ATP,
(2) The rate of the citric acid cycle is controlled
at three exergonic irreversible steps
? Citrate synthase,isocitrate
dehydrogenase and a-ketoglutarate
dehydrogenase;
? Inhibited by product feedback (citrate,
succinyl-CoA) and high energy charge
(ATP,NADH);
? Activated by a low energy charge (ADP)
or a signal for energy requirement
(Ca2+),
二、三羧酸循环的回补反应
The most common anaplerotic reactions covert
either pyruvate or phosphoenolpyruvate to
oxaloacetate or malate,
Some anaerobic bacteria,
lacking the a-ketoglutarate
dehydrogenase enzyme,
make biosynthetic
precursors via the
incomplete citric acid cycle;
could be an early evolution
stage of the citric acid
cycle,
三、三羧酸循环的中间产物是重要的生
物合成的前体分子
? The citric acid cycle is the hub of intermediary
metabolism serving both the catabolic and
anabolic processes (thus an amphibolic
pathway),
? It provides precursors for the biosynthesis of
glucose,amino acids,nucleotides,glucose,
fatty acids,sterols,heme groups,etc,
? Intermediates of the citric acid cycle get
replenished by anaplerotic reactions when
consumed by biosynthesis,
Summary
? Pyruvate is converted to acetyl-CoA by the
action of pyruvate dehydrogenase complex,a
huge enzyme complex,
? Acetyl-CoA is converted to 2 CO2 via the eight-
step citric acid cycle,generating three NADH,
one FADH2,and one ATP (by substrate-level
phophorylation),
? Intermediates of citric acid cycle are drawn off
to synthesize many other biomolecules,
including fatty acids,steroids,amino acids,
heme,pyrimidines,and glucose,
? Oxaloacetate can get supplemented from
pyruvate,via a carboxylation reaction
catalyzed by the biotin-containing pyruvate
carboxylase,
? The activity of pyruvate dehydrogenase
complex is regulated by allosteric effectors
and reversible phosphorylations,
? The rate of the citric acid cycle is
controlled at three exergonic irreversible
steps
Thanks!
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