Download Principles of BIOCHEMISTRY

Principles of
BIOCHEMISTRY
Fifth Edition
12.1 Gluconeogenesis
12.2 Precursors for Gluconeogenesis
A. Lactate
B. Amino Acids
C. Glycerol
D. Propionate and Lactate
E. Acetate
12.3 Regulation of Gluconeogenesis
12.4 The Pentose Phosphate Pathway
A. Oxidative Stage
B. Nonoxidative Stage
C. Interconversions Catalyzed by Transketolase
and Transaldolase
12.5 Glycogen Metabolism
A. Glycogen Synthesis
B. Glycogen Degradation
12.6 Regulation of Glycogen Metabolism in Mammals
A. Regulation of Glycogen Phosphorylase
B. Hormones Regulate Glycogen Metabolism
C. Hormones Regulate Gluconeogenesis and Glycolysis
12.7 Maintenance of Glucose Levels in Mammals
12.8 Glycogen Storage Diseases
All species can synthesize glucose from simple two carbon
and three-carbon precursors by gluconeogenesis (糖質新生作
用). Some species, notably photosynthetic organisms (光合作
用生物), can make these precursors by fixing (固定) CO2
leading to the net synthesis of glucose from inorganic
compounds.
Every glucose molecule used in glycolysis had to be
synthesized in some species. The pathway for
gluconeogenesis shares some steps with glycolysis, the
pathway for glucose degradation, but four reactions specific
to the gluconeogenic pathway are not found in the
degradation pathway.
These reactions replace the metabolically irreversible
reactions of glycolysis.
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Carbohydrate metabolism
In addition to fueling the production of ATP (via glycolysis and
the citric acid cycle), glucose is also a precursor of the ribose (
核糖) and deoxyribose (去氧核糖) moieties of nucleotides (核苷
酸) and deoxynucleotides (去氧核苷酸) .
The pentose phosphate pathway (戊糖磷酸途徑) is responsible
for the synthesis of ribose as well as the production of reducing
equivalents in the form of NADPH.
Glucose availability is controlled by regulating the uptake and
synthesis of glucose and related molecules and by regulating
the synthesis and degradation of storage polysaccharides
composed of glucose residues.
Glucose is stored as glycogen (肝醣) in bacteria and animals
and as starch in plants. Glycogen and starch can be degraded
to release glucose monomers that can fuel energy production
via glycolysis or serve as precursors in biosynthesis reactions.
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Carbohydrate metabolism
12.1
Gluconeogenesis
Gluconeogenesis
All organisms have a pathway for glucose biosynthesis, or
gluconeogenesis. This is true even for animals that use
exogenous glucose (外源性葡萄糖) as an important energy
source because glucose may not always be available from
external sources or intracellular stores.
Some mammalian tissues, primarily liver and kidney can
synthesize glucose from noncarbohydrate precursors such as
lactate and alanine.
Under fasting conditions, gluconeogenesis supplies almost all
of the body’s glucose.
2 Pyruvate + 2 NADH + 4 ATP + 2 GTP + 6 H2O + 2 H+
Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi
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12.1 Gluconeogenesis
It is convenient to consider pyruvate as the starting point for
the synthesis of glucose. The pathway for gluconeogenesis
from pyruvate is compared to the glycolytic pathway in Figure
12.1. Note that many of the intermediates and enzymes are
identical.
All seven of the near-equilibrium reactions of glycolysis
proceed in the reverse direction during gluconeogenesis.
Enzymatic reactions unique to gluconeogenesis are required
for the three metabolically irreversible reactions of
glycolysis.
These irreversible glycolytic reactions are catalyzed by PK,
PFK-1, and hexokinase. In the biosynthesis direction these
reactions are catalyzed by different enzymes.
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12.1 Gluconeogenesis
Figure 12.1 Comparison of gluconeogenesis and glycolysis
The synthesis of one molecule of glucose from two molecules
of pyruvate requires four ATP and two GTP molecules as
well as two molecules of NADH. The net equation for
gluconeogenesis is:
Four ATP equivalents are needed to overcome the
thermodynamic barrier to the formation of two molecules of
the energy-rich compound PEP from two molecules of
pyruvate.
In glycolysis the conversion of PEP to pyruvate is a
metabolically irreversible reaction catalyzed by PK.
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12.1 Gluconeogenesis
A. Pyruvate carboxylase (PC) (丙酮酸羧化酶)
In the conversion of pyruvate to glucose with the two
enzymes required for synthesis of PEP. The two steps involve
a carboxylation followed by decarboxylation. In the first
step, pyruvate carboxylase catalyzes the conversion of
pyruvate to OAA.
Catalyzes a metabolically irreversible reaction. The reaction
is coupled to the hydrolysis of one molecule of ATP (Figure
12.2).
Allosterically activated by acetyl CoA.
Accumulation of acetyl CoA from fatty acid oxidation signals
abundant energy, and directs pyruvate to OAA for
gluconeogenesis.
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12.1 Gluconeogenesis
Figure 12.2 Pyruvate carboxylase reaction.
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12.1 Gluconeogenesis
Bicarbonate (HCO3-) is one of the substrates in the reaction
shown in Figure 12.2. Bicarbonate is formed when CO2
dissolves in water so the reaction is sometimes written with
CO2 as a substrate.
The pyruvate carboxylase reaction plays an important role in
fixing CO2 in bacteria and some eukaryotes. This role may
not be so obvious when we examine gluconeogenesis since the
CO2 is released in the very next reaction; however, much of
the OAA that is made is not used for gluconeogenesis.
Instead, it replenishes (回補) the pool of citric acid cycle
intermediates that serve as precursors to the biosynthesis of
amino acids and lipids.
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12.1 Gluconeogenesis
B. Phosphoenolpyruvate Carboxykinase (PEPCK)
(磷酸烯醇式丙酮酸羧激酶)
Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes
the conversion of OAA to PEP (Figure 12.3). This is a wellstudied enzyme with an induced-fit binding mechanism
similar to that described for yeast hexokinase and citrate
synthase (CS；檸檬酸合成酶).
In most species, the enzyme displays no allosteric kinetic
properties and has no known physiological modulators.
The level of PEPCK activity in cells influences the rate of
gluconeogenesis. This is especially true in mammals where
gluconeogenesis is mostly confined to cells in the liver,
kidneys, and small intestine.
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12.1 Gluconeogenesis
Figure 12.3 Phosphoenolpyruvate
carboxykinase reaction.
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12.1 Gluconeogenesis
The two-step synthesis of PEP from pyruvate is common in
most eukaryotes, including humans. This is the main reason
why it’s usually shown when the gluconeogenesis pathway is
described (Figure 12.1).
However, many species of bacteria can convert pyruvate
directly to PEP in an ATP-dependent reaction catalyzed by
phosphoenolpyruvate synthetase (磷酸烯醇式丙酮酸合成酶)
(Figure 12.4).
The products of this reaction include AMP and Pi. The second
phosphoryl from ATP is transferred to pyruvate. Thus, two ATP
equivalents are used in the conversion of pyruvate to PEP.
This is a much more efficient route than the eukaryotic two-step
pathway catalyzed by PC and PEPCK. The presence of
phosphoenolpyruvate synthetase in bacterial cells is due to
the fact that efficient gluconeogenesis is much more important
in bacteria than in eukaryotes.
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12.1 Gluconeogenesis
Figure 12.4 Phosphoenolpyruvate synthetase reaction.
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12.1 Gluconeogenesis
C. Fructose 1,6-bisphosphatase (F1,6BPase) (果糖-1,6-二磷酸酶)
The reactions of gluconeogenesis between PEP and F1,6BP are
simply the reverse of the near-equilibrium reactions of
glycolysis.
The next reaction in the glycolysis pathway, catalyzed by PFK1, is metabolically irreversible.
In the biosynthesis direction, this reaction is catalyzed by the
third enzyme specific to gluconeogenesis, fructose 1,6bisphosphatase (F1,6BPase).
This enzyme catalyzes the conversion of F1,6BP to F6P.
F1,6BPase is allosterically inhibited by AMP and fructose
2,6-bisphosphate (F2,6BP).
PFK-1 and F1,6BPase that catalyze the interconversion of
F6P and F1,6BP are reciprocally controlled by the
concentration of F2,6BP.
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12.1 Gluconeogenesis
The fructose 1,6-bisphosphatase reaction
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12.1 Gluconeogenesis
D. Glucose 6-phosphatase (葡萄糖-6-磷酸酶)
The final step of gluconeogenesis is the hydrolysis of G6P to
form glucose. The enzyme is glucose 6-phosphatase.
In mammals, glucose is an important end product of
gluconeogenesis since it serves as an energy source for
glycolysis in many tissues. Glucose is made in the cells of the
liver, kidneys, and small intestine (小腸) and exported to the
bloodstream.
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12.1 Gluconeogenesis
In these cells, glucose 6-phosphatase is bound to the
endoplasmic reticulum (ER; 內質網) with its active site in
the lumen. The enzyme is part of a complex that includes a
glucose 6-phosphate transporter (G6PT) and a phosphate
transporter.
G6PT moves G6P from the cytosol to the interior of the ER
where it is hydrolyzed to glucose and inorganic phosphate.
Phosphate is returned to the cytosol and glucose is transported
to the cell surface (and the bloodstream) via the secretary
pathway.
Glucose 6-phosphatase is found only in cells from the liver,
kidneys, and small intestine, so only these tissues can
synthesize free glucose.
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12.1 Gluconeogenesis
12.2
Precursors for Gluconeogenesis
Precursors for Gluconeogenesis
The main substrates for G6P synthesis are pyruvate, citric
acid cycle intermediates, three-carbon intermediates in the
pathway (e.g. G3P), and two-carbon compounds such as acetyl
CoA.
Acetyl CoA is converted to OAA in the glyoxylate cycle (乙醛
酸循環), that operates in bacteria, protists, fungi, plants, and
some animals. Some organisms can fix inorganic carbon by
incorporating it into two carbon and three-carbon organic
compounds (e.g., Calvin cycle；卡爾文循環). These
compounds enter the gluconeogenesis pathway resulting in net
synthesis of glucose from CO2.
Major gluconeogenic precursors in mammals:
(1) Lactate
(2) Most amino acids (especially alanine),
(3) Glycerol (from triacylglycerol hydrolysis)
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12.2 Precursors for Gluconeogenesis
A. Lactate
Glycolysis generates large amounts of lactate in active muscle
and red blood cells. Lactate from these and other sources enters
the bloodstream and travels to the liver where it is converted to
pyruvate by the action of lactate dehydrogenase (LDH；乳酸
脫氫酶).
Pyruvate can then be a substrate for gluconeogenesis. Glucose
produced by the liver enters the bloodstream for delivery to
peripheral tissues, including muscle and red blood cells. This
sequence is known as the Cori cycle (Figure 12.5).
The conversion of lactate to glucose requires energy, most of
which is derived from the oxidation of fatty acids in the liver.
Thus, the Cori cycle transfers chemical potential energy in the
form of glucose from the liver to the peripheral tissues (週邊
組織).
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12.2 Precursors for Gluconeogenesis
Figure 12.5 Cori cycle.
Glucose is converted to L-lactate in muscle cells. Some of this lactate is
secreted and passes via the bloodstream to the liver. Lactate is converted to
glucose in the liver and the glucose is secreted into the bloodstream where it
is taken up by muscle cells. Both tissues are capable of synthesizing
glycogen and mobilizing it.
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12.2 Precursors for Gluconeogenesis
B. Amino Acids
Carbon skeletons of most amino acids are catabolized to
pyruvate or citric acid cycle intermediates. The end products
of these catabolic pathways can serve directly as precursors for
synthesis of G6P in cells that are capable of gluconeogenesis.
In peripheral mammalian tissues, pyruvate formed from
glycolysis or amino acid catabolism must be transported to the
liver before it can be used in glucose synthesis.
The Cori cycle is one way of accomplishing this transfer by
converting pyruvate to lactate in muscle and reconverting it to
pyruvate in liver cells.
The glucose-alanine cycle (葡萄糖-丙氨酸循環):
(1) Transamination of pyruvate yields alanine which travels to the liver.
(2) Transamination of alanine in the liver yields pyruvate for
gluconeogenesis.
(3) Glucose is released to the bloodstream.
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12.2 Precursors for Gluconeogenesis
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12.2 Precursors for Gluconeogenesis
Amino acids become a major source of carbon for
gluconeogenesis during fasting when glycogen supplies
are depleted.
The carbon skeleton of aspartate is also a precursor of
glucose. Aspartate is the amino group donor in the urea
cycle, a pathway that eliminates excess nitrogen from the
cell (Section 17.9B).
Aspartate is converted to fumarate in the urea cycle and
then fumarate is hydrated to malate that is oxidized to OAA.
In addition, the transamination of aspartate with aketoglutarate directly generates OAA.
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12.2 Precursors for Gluconeogenesis
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12.2 Precursors for Gluconeogenesis
Figure 12.6 Conversion of pyruvate to alanine.
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12.2 Precursors for Gluconeogenesis
C. Glycerol
The catabolism of triacylglycerols produces glycerol and
acetyl CoA. As mentioned earlier, acetyl CoA contributes to
the net formation of glucose through reactions of the
glyoxylate cycle (Section 13.8).
Glycerol can be converted to glucose by a route that begins
with phosphorylation to glycerol 3-phosphate, catalyzed by
glycerol kinase (Figure 12.7).
Glycerol 3-phosphate enters gluconeogenesis after conversion
to DHAP. This oxidation can be catalyzed by a flavin
containing glycerol 3-phosphate dehydrogenase complex
embedded in the inner mitochondrial membrane.
The outer face of this enzyme binds glycerol 3-phosphate and
electrons are passed to ubiquinone (Q) and subsequently to
the rest of the membrane-associated electron transport chain.
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12.2 Precursors for Gluconeogenesis
Figure 12.7 Gluconeogenesis from glycerol.
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12.2 Precursors for Gluconeogenesis
D. Propionate and Lactate
In ruminants (反芻動物)- cattle, sheep, giraffes, deer, and
camels- the propionate (丙酸鹽) and lactate produced by the
microorganisms in the rumen (瘤胃；反芻動物的第一胃) are
absorbed and enter the gluconeogenesis pathway.
Propionate is converted to propionyl CoA (丙醯基輔酶A) and
then to succinyl CoA (琥珀醯輔酶A). Succinyl CoA is an
intermediate of the citric acid cycle that can be metabolized
to OAA.
Lactate from the rumen is oxidized to pyruvate. Pyruvate
and OAA are substrates for gluconeogenesis
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12.2 Precursors for Gluconeogenesis
Precursors for gluconeogenesis
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12.2 Precursors for Gluconeogenesis
12.3
Regulation for Gluconeogenesis
Regulation of Gluconeogenesis
Gluconeogenesis is carefully regulated in vivo. Glycolysis and
gluconeogenesis are opposing catabolic and anabolic pathways
that share some enzymatic steps but certain reactions are unique
to each pathway.
Short-term regulation of gluconeogenesis is exerted at two
sites—the reactions involving pyruvate and PEP and those
that interconvert F1,6BP and F6P (Figure 12.8). When there
are two enzymes catalyzing the same reaction (in different
directions), modulating the activity of either enzyme can alter
the flux through the two opposing pathways. For example,
inhibiting PFK-1 stimulates gluconeogenesis since more F6P
enters the pathway leading to glucose rather than being
converted to F1,6BP. Simultaneous control of F1,6BPase also
regulates the flux of F1,6BP toward either glycolysis or
gluconeogenesis.
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12.3 Regulation for Gluconeogenesis
Figure12.8 Regulation of liver glycolysis and gluconeogenesis
by metabolites
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12.3 Regulation for Gluconeogenesis
F2,6BP is formed from F6P by the action of the enzyme
phosphofructokinase-2 (PFK-2) (Figure 12.9). In mammalian
liver, a different active site on the same protein catalyzes the
hydrolytic dephosphorylation of F2,6BP, re-forming F6P. This
activity of the enzyme is called F2,6BPase. The dual activities
of this bifunctional enzyme control the steady state
concentration of F2,6BP and, ultimately, the switch between
glycolysis and gluconeogenesis.
The allosteric effector F2,6BP activates PFK-1 and inhibits
F1,6BPase. Note that an increase in F2,6BP has reciprocal
effects: it stimulates glycolysis and inhibits gluconeogenesis.
Similarly, AMP affects the two enzymes in a reciprocal
manner; inhibiting F1,6BPase and activating PFK-1. The
regulation of the bifunctional enzyme PFK-2/F2,6BPase will
be described after we cover glycogen metabolism.
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12.3 Regulation for Gluconeogenesis
Figure12.9 Interconversion of b-D-F6P and b-D-F2,6BP.
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12.3 Regulation for Gluconeogenesis
12.4
The Pentose Phosphate Pathway
The Pentose Phosphate Pathway
五碳糖磷酸途徑意義：
(五碳糖磷酸途徑)
(1) 合成許多反應所需之NADPH + H+，如：脂肪酸及膽固醇之合成。
G6P + 12 NADP+ + 7H2O → 6 CO2 + 12 NADPH + 12 H+ + H3PO4
(2) 生成核酸(嘌呤或嘧啶)合成所需之ribose 5-phosphate
G6P + 2 NADP+ +H2O → R5P + CO2 + 2 NADPH + 2 H+
The pentose phosphate pathway is a pathway for the synthesis
of three pentose phosphates: ribulose 5-phosphate (Ru5P；核
酮糖-5-磷酸), ribose 5-phosphate (R5P；核糖-5-磷酸), and
xylulose 5-phosphate (Xu5P；木酮糖-5-磷酸). R5P is required
for the synthesis of RNA and DNA. The complete pathway has
two stages: an oxidative stage and a nonoxidative stage
(Figure 12.10).
In the oxidative stage, NADPH is produced when glucose 6phosphate is converted to the five-carbon compound Ru5P.
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12.4 The Pentose Phosphate Pathway
The nonoxidative stage of the pentose phosphate pathway
disposes of the pentose phosphate formed in the oxidative
stage by providing a route to gluconeogenesis or glycolysis.
In this stage, Ru5P is converted to the intermediates F6P and
G3P.
If all the pentose phosphate were converted to these
intermediates, the sum of the nonoxidative reactions would be
the conversion of three pentose molecules to two hexose
molecules plus one triose molecule.
(Ru5P)
(F6P)
43
(G3P)
12.4 The Pentose Phosphate Pathway
Figure 12.10 Pentose phosphate
Pathway (2 slides)
Occurrence of the Pentose Phosphate Pathway
Pathway is active in tissues that synthesize fatty acids or
steroids (liver, mammary and adrenal glands, and adipose
tissue).
Red blood cells use this pathway to produce needed NADPH
to maintain reduced iron.
All enzymes in the cycle occur in the cytosol.
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12.4 The Pentose Phosphate Pathway
A. Oxidative Stage
The three reactions of the oxidative stage of the pentose
phosphate pathway are shown in Figure 12.11. The first two
steps are the same as those in the bacterial Entner-Doudoroff
pathway (Section 11.7).
The first reaction, catalyzed by glucose 6-phosphate
dehydrogenase (G6PDH), is the oxidation of G6P to 6phosphogluconolactone. This step is the major regulatory site
for the entire pentose phosphate pathway.
G6PDH is allosterically inhibited by NADPH (feedback
inhibition). This simple regulatory feature ensures that the
production of NADPH by the pentose phosphate pathway is
self-limiting.
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12.4 The Pentose Phosphate Pathway
The next enzyme of the oxidative phase is 6phosphogluconolactonase (6GPL；6-磷酸葡糖酸内酯酶) that
catalyzes the hydrolysis of 6-phosphogluconolactone to the
sugar acid 6-phosphogluconate. Finally, 6-phosphogluconate
dehydrogenase (6-磷酸葡萄糖脫氫酶) catalyzes the oxidative
decarboxylation of 6-phosphogluconate.
This reaction produces a second molecule of NADPH, Ru5P,
and CO2. In the oxidative stage, therefore, a six-carbon sugar is
oxidized to a five-carbon sugar plus CO2 and two molecules of
NADP are reduced to two molecules of NADPH.
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12.4 The Pentose Phosphate Pathway
(6-磷酸葡萄糖脫氫酶)
(6-磷酸葡萄糖酸)
Figure 12.11 Oxidative stage of the
pentose phosphate
pathway
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12.4 The Pentose Phosphate Pathway
B. Nonoxidative Stage
The nonoxidative stage of the pentose phosphate pathway
consists entirely of near equilibrium reactions. This stage of
the pathway provides five-carbon sugars for biosynthesis and
introduces sugar phosphates into glycolysis or
gluconeogenesis. Ru5P has two fates: an epimerase can
catalyze the formation of Xu5P, or an isomerase can catalyze
the formation of R5P (Figure 12.12).
R5P is the precursor of the ribose (or deoxyribose) portion of
nucleotides. The remaining steps of the pathway convert the
five-carbon sugars into glycolytic intermediates. Rapidly
dividing cells that require both R5P and NADPH (for the
reduction of ribonucleotides to deoxyribonucleotides)
generally have high pentose phosphate pathway activity.
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12.4 The Pentose Phosphate Pathway
Figure 12.12 Conversions of ribulose 5-phosphate.
50
12.4 The Pentose Phosphate Pathway
In most cells, the G3P and F6P produced by the pentose
phosphate pathway are used to resynthesize G6P. This G6P
molecule can reenter the pentose phosphate pathway. In that
case, the equivalent of one molecule of glucose is completely
oxidized to CO2 by six passages through the pathway.
After six molecules of G6P are oxidized, the six Xu5P
produced can be rearranged by the reactions of the pentose
phosphate pathway and part of the gluconeogenic pathway to
form five G6P molecules. (Recall that two G3P molecules are
equivalent to one F1,6BP molecule.) If we disregard H2O and
H+, the overall stoichiometry for this process is
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12.4 The Pentose Phosphate Pathway
C. Interconversions Catalyzed by Transketolase
and Transaldolase
The interconversions of the nonoxidative stage of the pentose
phosphate pathway are catalyzed by two enymes called
transketolase (轉酮醇酶) and transaldolase (轉醛醇酶).
Both enzymes have broad substrate specificities (基質專一性).
They catalyze the exchange of two- and three-carbon
fragments between sugar phosphates
For both enzymes, one substrate is an aldose, one substrate is a
ketose.
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12.4 The Pentose Phosphate Pathway
Figure 12.13 Reaction catalyzed by transketolase.
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12.4 The Pentose Phosphate Pathway
Figure 12.14 Reaction catalyzed by Transaldolase.
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12.4 The Pentose Phosphate Pathway
12.5
Glycogen Metabolism
A. Glycogen Synthesis
Glycogen Metabolism
De novo (重新) glycogen synthesis requires a preexisting
primer of four to eight a-1-4 linked glucose residues. This
primer is attached to a specific tyrosine residue of a protein
called glycogenin (肝醣原) (Figure 12.15) via the 1-hydroxyl
group of the reducing end of the short polysaccharide. The
primer is formed in two steps. The first glucose residue is
attached to glycogenin by the action of a glucosyltransferase
(葡萄糖基轉移酶) activity that requires UDP-glucose.
Glycogenin itself catalyzes this reaction as well as the
extension of the primer by up to seven more glucose residues.
Thus, glycogenin is both a protein scaffold for glycogen and
an enzyme.
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12.5 Glycogen Metabolism
Figure 12.15 Glycogenin from rabbit.
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12.5 Glycogen Metabolism
Each glycogen molecule (which can contain thousands of
glucose residues) contains a single glycogenin protein at its
center. Synthesis and degradation of glycogen require separate
enzymatic steps.
Further glycogen addition reactions begin with G6P that can be
converted to G1P by the action of phosphoglucomutase (葡萄
糖磷酸變位酶).
Glycogen synthesis and degradation is mostly a way of storing
G6P until it is needed by the cell.
Cellular glucose converted to G6P by hexokinase.
Three separate enzymatic steps are required to incorporate one
G6P into glycogen.
Glycogen synthase (肝醣合成酶) is the major regulatory step.
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12.5 Glycogen Metabolism
(UDP-葡萄糖焦磷酸化酶)
Figure 12.16 Synthesis of glycogen
in eukaryotes.
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12.5 Glycogen Metabolism
Figure 12.17 The glycogen synthase reaction.
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12.5 Glycogen Metabolism
Another enzyme, amylo-(1,4→1,6)-transglycosylase (轉糖苷
酶), catalyzes branch formation in glycogen. This enzyme,
also known as the branching enzyme, removes an
oligosaccharide of at least six residues from the
nonreducing end of an elongated chain and attaches it by an
linkage to a position at least four glucose residues from the
nearest branch point.
These branches provide many sites for adding or removing
glucose residues, thereby contributing to the speed with which
glycogen can be synthesized or degraded.
The complete glycogen molecule has many layers of
polysaccharide chains extending out from the glycogenin core
(Figure 12.18).
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12.5 Glycogen Metabolism
Figure 12.18 A glycogen molecule.
Two polysaccharides (blue) are attached to each core glycogenin molecule.
Each chain core has 8~14 residues and two branches. Not all branches are
shown. Seven layers are numbered but typical glycogen molecules have
8~12 layers, depending on the species.
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12.5 Glycogen Metabolism
B. Glycogen Degradation
The glucose residues of starch and glycogen are released from
storage polymers through the action of enzymes called
polysaccharide phosphorylases: starch phosphorylase (澱粉
磷解酶；in plants) and glycogen phosphorylase (in other
organisms). These enzymes catalyze the removal of glucose
residues from the nonreducing ends of starch or glycogen,
provided the monomers are attached by a 1→4 linkages.
In contrast to hydrolysis (group transfer to water),
phosphorolysis (磷酸解作用) produces phosphate esters. Thus,
the first product of polysaccharide breakdown is a-D-glucose
1-phosphate (the Cori ester) (柯里酯、葡萄糖-1-磷酸酯，葡萄
釀酒的中間產物), not free glucose.
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12.5 Glycogen Metabolism
The phosphorolysis reaction catalyzed
by glycogen phosphorylase is shown in
Figure 12.19.
Pyridoxal phosphate (PLP；磷酸吡哆
醛) is a prosthetic group (輔基) in the
active site of the enzyme.
The phosphate group of PLP appears to relay a proton to the
substrate phosphate to help cleave the scissile (易切斷的) bond
of glycogen.
Note that glycogen phosphorylase catalyzes a remarkable
reaction since it only uses glycogen and inorganic
phosphates as substrates in a reaction that produces a
relatively “high energy” compound, G1P (Table 10.1).
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12.5 Glycogen Metabolism
Figure 12.19 Cleavage of a glucose residue from the
nonreducing end of a glycogen chain,
catalyzed by glycogen phosphorylase.
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12.5 Glycogen Metabolism
Glycogen phosphorylase is a dimer of identical subunits.
The catalytic sites lie in the middle of each subunit. It binds
phosphate and the end of a glycogen chain (Figure 12.20).
The large glycogen particle binds to a nearby site and the
chain being degraded passes along a groove on the surface
of the enzyme. Four or five glucose residues can be
cleaved sequentially before the enzyme has to release a
glycogen particle and re-bind. Thus, in contrast to glycogen
synthase, glycogen phosphorylase is partially processive
(the enzyme remains bound to the end of the growing chain
and addition reactions are very rapid)(酵素在重複的催化過程
中不分離).
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12.5 Glycogen Metabolism
Figure 12.20 Binding and catalytic sites on
glycogen phosphorylase.
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12.5 Glycogen Metabolism
The enzyme stops four glucose residues from a branch point
(an a (1→6) glucosidic bond) leaving a limit dextrin (限制
糊精). The limit dextrin can be further degraded by the action
of the bifunctional glycogen debranching enzyme (Figure
12.21).
A glucanotransferase (葡聚醣基轉移酶) activity of the
debranching enzyme catalyzes the relocation of a chain of
three glucose residues from a branch to a free 4-hydroxyl
end of the glycogen molecule. Both the original linkage and
the new one are a (1→4).
The other activity of glycogen debranching enzyme, amylo1,6-glucosidase, catalyzes hydrolytic (not phosphorolytic)
removal of the remaining a (1→6) linked glucose residue.
68
12.5 Glycogen Metabolism
Figure 12.21 Degradation of glycogen.
69
12.5 Glycogen Metabolism
12.6
Regulation of Glycogen Metabolism
in Mammals
Regulation of Glycogen Metabolism in Mammals
Muscle glycogen is fuel for muscle contraction.
Liver glycogen is mostly converted to glucose for
bloodstream transport to other tissues.
Both mobilization and synthesis of glycogen are regulated
by hormones.
Insulin (胰島素), glucagon (升糖激素、血糖激素) and
epinephrine (腎上腺素) regulate mammalian glycogen
metabolism.
71
12.6 Regulation of Glycogen Metabolism in Mammals
A. Regulation of Glycogen Phosphorylase
Glycogen phosphorylase (GP) and glycogen synthase (GS)
control glycogen metabolism in liver and muscle cells.
GP is responsible for the breakdown of glycogen to produce
G1P. In muscle cells, G1P is converted to G6P that is used in
glycolysis to produce ATP. In liver cells, G6P is hydrolyzed
to free glucose that is secreted into the bloodstream where it
can be taken up by other tissues.
GP and GS are reciprocally regulated both covalently and
allosterically (when one is active the other is inactive).
Covalent regulation by phosphorylation (-P) and
dephosphorylation (-OH).
72
12.6 Regulation of Glycogen Metabolism in Mammals
The activity of GP is regulated by several allosteric effectors
and by covalent modification (phosphorylation). Let’s
take a few minutes to study the regulation of GP because not
only is it important in glycogen metabolism, it’s also
historically important.
The enzyme exists in four different forms as shown in
Figure 12.22. The unphosphorylated form is called
glycogen phosphorylase b (GPb) and the phosphorylated
form is called glycogen phosphorylase a (GPa).
The enzyme is phosphorylated by a kinase enzyme and
dephosphorylated by a phosphatase.
73
12.6 Regulation of Glycogen Metabolism in Mammals
Like other allosterically regulated enzymes, GP adopts two
conformations; the R conformation is the active
conformation and the T conformation is much less active.
This is depicted in Figure 12.22 as a change in the shape of
the catalytic site: In the R conformation, inorganic phosphate
(a substrate of the reaction) can bind and in the T
conformation binding of inorganic phosphate is inhibited.
Unphosphorylated GPb can exist in both inactive T
conformations and active R conformations. The allosteric site
of the enzyme binds several effectors that cause a shift in
conformation.
74
12.6 Regulation of Glycogen Metabolism in Mammals
Figure 12.22 Regulation of glycogen Phosphorylase.
75
12.6 Regulation of Glycogen Metabolism in Mammals
When ATP is bound, the activity of the enzyme is inhibited
(T state). This is the normal state of activity since
physiological concentrations of ATP are high and relatively
constant.
When the AMP concentration rises, it displaces ATP from the
allosteric site causing a shift to the active R conformation and
activation of glycogen breakdown. In muscle cells, increasing
AMP concentration results from strenuous muscle activity
and signals the need for more G1P to stimulate ATP
production by glycolysis.
The enzyme is inhibited by G6P (feedback inhibition).
There’s no need to continue glycogen breakdown if G6P
concentration is sufficient to fuel glycolysis.
76
12.6 Regulation of Glycogen Metabolism in Mammals
The structures of GPa and GPb are shown in Figure 12.23 in
order to illustrate the structural changes that take place when
the enzyme is phosphorylated and dephosphorylated.
The phosphoryl group is covalently attached to serine residue
14 (Ser-14) near the N-terminal end of the protein.
In the unphosphorylated state (GPb), the N-terminal residues,
including Ser-14, associate with the surface near the catalytic
site. In the phosphorylated state (GPa), phosphoserine-14
interacts with two positively charged arginine residues near
the allosteric site.
The remarkable shift in the location of the N-terminal end of
the chain cause other conformation changes in the enzyme;
notably, a reorientation of two a helices, the tower helices, on
the other side of the dimer interface. This, in turn, affects the
position of the 280s loop controlling the transition between the
active R conformation and the inactive T conformation.
77
12.6 Regulation of Glycogen Metabolism in Mammals
Figure 12.23 Phosphorylated and unphosphoylated
forms of glycogen phosphorylase.
78
12.6 Regulation of Glycogen Metabolism in Mammals
GPa is relatively insensitive to ATP, AMP, and G6P. In muscle
cells, GPa will be formed in response to hormones that signal the
need for glucose and strenuous (激烈的) muscle activity.
This promotes rapid mobilization of glycogen. In liver cells, the
liver version of GP responds to the same hormones but in this
case glycogen breakdown leads to excretion of glucose that can
be taken up by muscle cells.
Liver GPa is inhibited by glucose by shifting GPa to the T
conformation. This makes sense since the presence of a high
concentration of free glucose means that it’s not necessary to
continue producing glucose from glycogen.
The muscle version of GP is not inhibited by glucose since
muscle cells rarely see significant concentrations of free glucose.
Muscle cells don’t convert G6P to glucose and any glucose taken
up from the bloodstream is quickly phosphoryated by
hexokinase to G6P.
79
12.6 Regulation of Glycogen Metabolism in Mammals
B. Hormones Regulate Glycogen Metabolism
Insulin, glucagon, and epinephrine are the principal
hormones that control glycogen metabolism in mammals.
Insulin is produced by b-cells of the pancreas, is secreted
when the concentration of glucose in the blood increases.
High levels are associated with the fed state.
Insulin increases rate of glucose transport into muscle,
adipose tissue via GLUT 4 transporter.
Insulin stimulates glycogen synthesis in the liver via the
second messenger phosphatidylinositol
3,4,5-trisphosphate (PIP3).
80
12.6 Regulation of Glycogen Metabolism in Mammals
Glucagon (29 AAs) is secreted by the a cells of the pancreas
in response to low blood glucose (elevated glucagon is
associated with the fasted state).
Stimulates glycogen degradation to restore blood glucose to
steady-state levels
Glucagon is extremely selective in its target because only
liver cells are rich in glucagon receptors. The effect of
glucagon is opposite that of insulin and an elevated glucagon
concentration is associated with the fasted state.
81
12.6 Regulation of Glycogen Metabolism in Mammals
The adrenal glands release the catecholamine epinephrine
(兒茶酚胺腎上腺素) (aka adrenaline), in response to neural
signals that trigger the fight or flight response (Figure 3.5c).
The epinephrine precursor, norepinephrine (正腎上腺素),
also has hormone activity. Epinephrine stimulates the
breakdown of glycogen. It triggers a response to a sudden
energy requirement whereas glucagon and insulin act over
longer periods to maintain a relatively constant concentration
of glucose in the blood.
Epinephrine binds to b-adrenergic receptors of liver and
muscle cells and to a1-adrenergic receptors of liver cells.
The binding of epinephrine to b-adrenergic receptors or of
glucagon to its receptors activates the adenylyl cyclase
signaling pathway. The second messenger, cyclic AMP
(cAMP), then activates protein kinase A (PKA).
82
12.6 Regulation of Glycogen Metabolism in Mammals
PKA phosphorylates a number of other proteins causing
significant changes in metabolism.
When glucagon binds to its receptor it stimulates adenylate
cyclase causing an increase in cAMP that leads to activation
of PKA. PKA phosphorylates GS converting the “a” form to
the inactive “b” form. This blocks glycogen synthesis.
PKA also phosphorylates another kinase called
phosphorylase kinase. As the name implies, this is the
kinase that phosphorylates GP.
PKA activates phosphorylase kinase leading to conversion of
GPb to the active form, GPa. The result is an increase in the
rate of degradation of glycogen (Figure 12.24).
83
12.6 Regulation of Glycogen Metabolism in Mammals
PP1
PP1
Figure 12.24 Effects of glycogen metabolism.
84
12.6 Regulation of Glycogen Metabolism in Mammals
The net effect of glucagon (or epinephrine) is to block
synthesis of glycogen and stimulate its breakdown.
The reciprocal regulation of these two enzymes is an
important feature of regulation in this pathway.
GS and GP are dephosphorylated by phosphoprotein
phosphatase-1 (PP1；第一型磷蛋白磷酸水解酶), an enzyme
that acts on many other substrates. As shown in Figure 12.25,
dephosphorylation leads to reciprocal inactivation of GP and
activation of GS. This results in synthesis of glycogen from
UDP-glucose and inhibition of glycogen breakdown.
Insulin stimulates the activity of PP1, thus causing the uptake
of glucose into glycogen and its depletion in the bloodstream.
85
12.6 Regulation of Glycogen Metabolism in Mammals
Figure 12.25 Effects of glycogen metabolism.
86
12.6 Regulation of Glycogen Metabolism in Mammals
Reciprocal Regulation of GP and GS
 Covalent Regulation
Active form “a”
Inactive form “b”
Glycogen phosphorylase -P
-OH
Glycogen synthase
-P
-OH
 Allosteric Regulation by G6P
 GP a (active form) - inhibited by G6P
 GS b (inactive form) - activated by G6P
87
12.6 Regulation of Glycogen Metabolism in Mammals
C. Hormones Regulate Gluconeogenesis and Glycolysis
Fructose 1,6-bisphosphatase (F1,6BPase) and PFK-1 are the key
enzymes involved in the decision to either degrade glucose or
synthesize it. These two enzymes are reciprocally regulated by the
effector F2,6BP (Figure 12.8). This effector molecule is synthesized
from F6P by PFK-2 and it is dephosphorylated back to F6P by
fructose 2,6-bisphosphatase (F2,6BPase) (Figure 12.9).
These two enzymatic activities are located on the same bifunctional
protein. The relationship among the four enzymes and their
products is summarized in Figure 12.26.
The F2,6BPase and PFK-2 activities in the bifunctional enzyme (
雙功酵素) are regulated by phosphorylation in a reciprocal manner.
When the protein is phosphorylated, the enzyme acts as a
F2,6BPase and the phosphofructokinase activity is inhibited.
Conversely, when the enzyme is unphosphorylated it acts as a
phosphofructokinase and the F2,6BPase activity is inhibited.
88
12.6 Regulation of Glycogen Metabolism in Mammals
Figure 12.26 The role of fructose 2,6-bisphosphate in
regulating glycolysis and gluconeogenesis.
89
12.6 Regulation of Glycogen Metabolism in Mammals
This is the same mode of reciprocal regulation we encountered
with GP and GS, except this time the two enzyme activities are
on the same molecule.
In the presence of glucagon, PKA is active and it
phosphorylates the bifunctional enzyme (Figure 12.27). Thus,
glucagon stimulates gluconeogenesis and inhibits glycolysis in
liver cells causing glucose levels in the bloodstream to rise.
At the same time, epinephrine (腎上腺素) can stimulate
glycogen degradation and inhibit glycogen synthesis in muscle
cells. The result is more glucose for muscle cells and more ATP
from glycolysis.
90
12.6 Regulation of Glycogen Metabolism in Mammals
Figure 12.27 Effect of glucagon on gluconeogenesis.
91
12.6 Regulation of Glycogen Metabolism in Mammals
12.7
Maintenance of Glucose Levels in
Mammals
Maintenance of Glucose Levels in Mammals
Mammals maintain blood glucose levels within strict limits
by regulating both the synthesis and degradation of glucose.
Glucose is the major metabolic fuel in the body.
Some tissues, such as brain, rely almost entirely on glucose
for their energy needs. The concentration of glucose in the
blood seldom drops below 3 mM or exceeds 10 mM.
When the concentration of glucose in the blood falls below
2.5 mM, glucose uptake into the brain is compromised, with
severe consequences.
Conversely, when blood glucose levels are very high, glucose
is filtered out of the blood by the kidneys accompanied by
osmotic loss of water and electrolytes.
93
12.7 Maintenance of Glucose Levels in Mammals
Role of the Liver in Metabolism
The liver participates in the interconversions of all types of
metabolic fuels: carbohydrates, amino acids and fatty acids.
Anatomically (在解剖學上), the liver is centrally located in
the circulatory system (Figure 12.28). Most tissues are
perfused (佈滿) in parallel with arterial system (動脈系統)
supplying oxygenated blood and the venous circulation (靜
脈循環) returning blood to the lungs for oxygenation.
The liver, however, is perfused in series with the visceral
tissue (內臟組織) (gastrointestinal tract [胃腸道], pancreas
[胰臟], spleen [脾臟], and adipose tissue [脂肪組織]); blood
from these tissues drains into the portal vein (門靜脈) and
then flows to the liver.
94
12.7 Maintenance of Glucose Levels in Mammals
Figure 12.28 Placement of the
liver in circulation
system
95
12.7 Maintenance of Glucose Levels in Mammals
The consumption of glucose by tissues removes dietary
glucose from the blood. When glucose levels fall, liver
glycogen and gluconeogenesis become the sources of
glucose. However, since these sources are limited, hormones
act to restrict the use of glucose to those cells and tissues
that absolutely depend on glycolysis for generating ATP
(kidney medulla [腎髓質], retina [視網膜], red blood cells,
and part of brain).
Other tissues can generate ATP by oxidizing fatty acids
mobilized from adipose tissue.
In the a960’s, George Cahill examined the glucose
utilization of obese patients (肥胖的患者) as they underwent
therapeutic starvation (Figure 12.29).
96
12.7 Maintenance of Glucose Levels in Mammals
Figure 12.29 Five phases of glucose homeostasis.
Graph illustrates glucose utilization after 100 g
glucose consumption then 40 day fast.
97
12.7 Maintenance of Glucose Levels in Mammals
12.8
Glycogen Storage Diseases
Glycogen Storage Diseases
Many metabolic enzymes in humans are encoded by gene
families. Different versions are expressed in different
tissues.
In the case of enzymes involved in glycogen metabolism,
the most common versions are found in liver and muscle. A
deficiency in one of these enzymes will produce severe
symptoms but may not be lethal.
There are nine types of glycogen storage diseases resulting
from defects in glycogen metabolism.
99
12.8 Glycogen Storage Diseases
Type I: The most common glycogen storage disease is called
von Gierke disease (肝醣儲積症、馮吉爾克氏症). It is caused
by a deficiency in glucose 6-phosphatase. Patients are unable
to secrete glucose leading to accumulation of glycogen in the
liver and kidneys.
Type II: Patients suffering from type II disease, known as
Pompe’s disease (龐貝氏症), suffer from reduced activity of
a-1,4-glucosidase, or acid maltase, an enzyme required for
glycogen breakdown in lysozomes. The defect causes
glycogen to accumulate in lysosomes leading to problems
with muscle tissue, especially in the heart. In the most severe
forms, children die within the first few years of life.
100
12.8 Glycogen Storage Diseases
Type III: Cori disease (肝醣儲積症第三型、科里氏症),
characterized by defects in the gene encoding the glycogen
debranching enzyme in liver and muscle. People suffering
from this disease have weakened muscles because they are
unable to mobilize all of the stored glycogen. Some defects have
very mild symptoms.
Type IV: Anderson’s disease (安德森氏症), the mutations occur
in the gene for liver branching enzyme found on chromosome
3. Long-chain polysaccharides accumulate in patients with these
mutations, resulting in death within a few years from heart
failure or liver failure.
Type V: McArdle’s disease (麥克阿德爾氏症) is caused by a
deficiency of muscle glycogen phosphorylase. Individuals
having this genetic disease cannot perform strenuous exercise
and suffer painful muscle cramps (痙攣).
101
12.8 Glycogen Storage Diseases
Type VI: Hers’ disease (赫斯氏症) is a mild form of glycogen
storage disease due to a deficiency in liver glycogen
phosphorylase.
Type VII: Mutations in the gene for muscle PFK-1 cause
Tarui’s disease (肌肉磷酸果糖激酶缺乏症), characterized by
inability to exercise and muscle cramps.
Type VIII: Now recognized as a subtype of type IX.
Type IX: This form of glycogen storage disease manifests as
muscle weakness and/or muscle cramps. The symptoms are
usually mild. All subtypes are due to mutations in the genes
for the various subunits of glycogen phosphorylase kinase.
102
12.8 Glycogen Storage Diseases
Summary
1. Gluconeognesis is the pathway for glucose synthesis from
noncarbohydrate precursors. The seven near-equilibrium
reactions of glycolysis proceed in the reverse direction in
gluconeogenesis. Four enzymes specific to gluconeogenesis
catalyze reactions that bypass the three metabolically
irreversible reactions of glycolysis.
2. Noncarbohydrate precursors of glucose include pyruvate,
lactate, alanine, and glycerol.
3. Gluconeogenesis is regulated by glucagon, allosteric
modulators, and the concentrations of its substrates.
103
Summary
4. The pentose phosphate pathway metabolizes glucose 6phosphate to generate NADPH and ribose 5-phosphate. The
oxidative stage of the pathway generates two molecules of
NADPH per molecule of glucose 6-phosphate converted to
ribulose 5-phosphate and CO2.
5. The nonoxidative stage includes isomerization of ribulose 5phosphate to ribose 5-phosphate. Further metabolism of
pentose phosphate molecules can covert them to glycolytic
intermediates. The combined activities of transketolase and
transaldolase convert pentose phosphates to triose phosphates
and hexose phosphates.
6. Glycogen synthesis is catalyzed by glycogen synthase, using
a glycogen primer and UDP-glucose.
104
Summary
7. Glucose residues are mobilized from glycogen by the action
of glycogen phosphorylase. Glucose 1-phosphate is then
converted to glucose 6-phosphate.
8. Glycogen degradation and glycogen synthesis are
reciprocally regulated by hormones. Kinases and
phosphatases control the activities of the interconvertible
enzymes glycogen phosphorylase and glycogen synthase.
9. Mammals maintain a nearly constant concentration of
glucose blood. The liver regulates the amount of glucose
supplied by the diet, glycogenolysis, and other fuels.
10. Glycogen storage diseases result from defects in genes
required for glycogen metabolism.
105
Summary
Cori Cycle
就是能量於骨骼肌組織、肝臟之間的循環。在新陳代謝過程中，乳酸(lactate)可經由
無氧糖解反應(anaerobic glycolysis)於肌肉組織中產生，而部分的乳酸會進入肝臟中
，並進一步利用gluconeogenesis產生葡萄糖。而葡萄糖又可回到肌肉組織中，再次利
用後產生乳酸，以此循環 。
肌肉組織的活動需要能量，而可藉由分解在肌肉中的肝醣得之，分解肝醣產生的
ATP可為被肌肉組織利用的能量來源。儘管遍佈肌肉組織活動的ATP用於提供能量，
但其必須持續的補充。當氧氣供應充足時，經分解肝糖所產生的G6P可進行有氧氧
化作用，使pyruvate進入citric acid cycle中，完成有氧呼吸作用，並產生大量ATP。然
而當氧氣短缺時，例如肌肉進行劇烈運動時，只單靠一般呼吸作用不能將足夠的氧
氣供給到組織進行citric acid cycle，導致無法完全氧化，所以能量經由無氧呼吸過程
的G6P釋放。分解葡萄糖經G6P至pyruvate，此過程為glycolysis。而糖解作用單經由
無氧呼吸可產生丙酮酸酯，又可被轉換成乳糖。在柯氏循環中，當pyruvate被轉換成
乳酸可使糖解作用繼續進行。在肌肉組織中pyruvate被還原成乳酸的機制為乳酸脫氫
酶(lactate dehydrogenase)。作用產生機制是利用在糖解作用中氧化NADH產生NAD+
，利用NADH氧化後的兩個電子傳遞給pyruvate以產生乳酸。此可維持NAD+的濃度
並可使糖解作用的過程繼續。於是乳酸被釋放至血液中，同時可循環至心肌並氧化
，而乳酸在長時間激烈運動下聚集會限制運動員的表現。部分的乳酸將進入肝臟中
，並再次氧化成pyruvate。
106
Cori Cycle