Download Lehninger Principles of Biochemistry 5/e

David L. Nelson and Michael M. Cox
LEHNINGER
PRINCIPLES OF BIOCHEMISTRY
Fifth Edition
CHAPTER 15
Principles of Metabolic Regulation
© 2008 W. H. Freeman and Company
Both the amount and the catalytic activity of an enzyme can be
regulated
1. Extracellular signal: hormonal, neuronal, growth factors etc.
2. Transcription: activate or repress the transcription
3. The stability of mRNA
4. The rate of translation
5. The rate of protein degradation
6. Sequester the enzyme and its substrate in different compartments
7. By the concentration of substrate
8. The presence of allosteric effector
9. Covalent modification
10. Binding of regulatory protein
Adenine Nucleotides play special Roles in Metabolic regulation
1.
2.
3.
4.
5.
It is important to maintain a constant supply and concentration of ATP: [ATP]
drop  reaction rate is decreased.
AMP concentration is more sensitive indicator of cell’s energetic state than is
[ATP]
AMP-activated protein kinase
- regulated by [AMP]
- A reduced nutrient supply or by increase exercise cause the rise in [AMP]
- increase glucose uptake, activates glycolysis and fatty acid oxidation
- suppress energy requiring processes such as fatty acid, cholesterol, and
protein synthesis
NADH and NADPH : change in their mass action ratios have global effects
on metabolism
Glucose
Coordinated Regulation of Glycolysis and Gluconeogensis
Hexokinase
1. Human have four isozyme, encoded by different genes
Isozyme: Different proteins that catalyze the same reaction
2. In myocytes, hexokinase II: high affinity for glucose, inhibited
by G-6-P
3. In liver, hexokinase IV (glucokinase)
- low affinity for glucose: direct regulation by the level of blood
glucose
- not inhibited by G-6-P
- is subjected to inhibition by reversible binding of a regulatory
protein specific to liver
- are transcriptionally regulated: [ATP] low or [glucose] high
Phosphofluctokinase-1
1. Higher enzyme activity: [ADP] or [AMP] high
Low enzyme activity: [ATP] high
2. Citrate: High [citrate]  increases the inhibitory effect of ATP
3. F-2,6-BP: activates PFK-1
Fructose -1,6-bisphosphatase
1. Inhibited by AMP
2. Inhibited by F-2,6-BP
3. PFK-1 and FBPase-1 are gulated in a coordinated and reciprocal
manner
- energy high: gluconeogenesis
- energy low: glycolysis
F-2,6-BP is potent allosteric regulator of PFK-1 and
FBPase-1
1. The special role of liver in maintaining a constant blood glucose
level requires additional regulatory mechanism to coordinate
glucose consumption and production
2. Glucagon: the liver to produce glucose and to stop consuming it
for own need
3. Insulin: the liver to use glucose as fuel and as a precursor for the
synthesis and glycogen and triacylglycerol.
4. Hormonal regulation is mediated by F-2,6-BP
5. F-2,6-BP increase the PFK-1’s affinity for its substrate
- in absence of F-2,6-BP, at physiological condition PFK-1 is
inactive
6. F-2,6-BP reduce the FBPase-1’s affinity for its substrate.
F-2,6-BP is set by the relative rates of its formation and
breakdown
1. Xylulose-5-P, a product of
PPP, increase glycolysis
2. Activates PP2A
 dephosphorylation of
PFK-2/FBPase-2
 activate PFK-2
 increase F-2,6-BP
 activate PFK-1
 Inhibits FBPase-1
 Stimulate glycolysis
Pyruvate kinase
1. Pyruvate carboxylase is
activated by acetyl-coA
2. PEP carboxykinase is
regulation by its synthesis
and breakdown
 Glucagon
 cAMP
 increase transcription of
PEP carboxykinase
 Insulin: opposite effect
Transcriptional regulation of glycolysis and gluconeogenesis
1. Carbohydrate response
element binding protein
(ChREBP)
2. Activates pyruvate kinase,
fatty acid synthase
3. Cyclic AMP response
element binding protein
(CREB)
Glucogon
cAMP
PEP carboxykinase
1. Forkhead box other
(FOXO1) stimulate the
synthesis of gluconeogenic
enzme
 insulin
 PKB activation
 phosphorylation of
FOXO1
 Degradation
 inhibition of
gluconeogenesis
The PEP carboxykinase promoter region showing the complexity of
regulatory input to this gene.
The metabolism of glycogen in animal
1. In vertebrates, glycogen is found primarily in the liver and
skeletal muscle
2. The glycogen in muscle provide a quick source of energy for
aerobic or anaerobic metabolism
3. Liver glycogen serves as a reservoir of glucose for other tissue
when dietary glucose is not available.
Glycogen breakdown is catalyzed by glycogen phosphorylase
1. phosphorylase,
debranching enzyme,
phosphoglucomutase
2. a 1->4 glycosidic bond
between nonreducing
end by inorganic
phosphoate
3. Formation of glucose-1
–phosphoate
4. Called phosphorolysis
1. Debranching enzyme:
oligo a 1->6 to a 1->4
glucan-transferase
2. Phosphorylase reaches a
point 4 glucose residues
assay from an a 1->6
branch point
3. branches are transferred
4. The glucosyl residue at
C-6 is hydrolyzed.
1. G-1-P to G-6-P
2. In muscle, G-6-P can enter
glycolysis
3. In liver, to release glucose
to the blood,G-6phosphatase present in
liver and kidney but not in
other tissue
1. G-1-P to G-6-P
2. In muscle, G-6-P can enter
glycolysis
3. In liver, release glucose to
the blood
4. G-6-P enter ER lumen by
T1 and then convert into G
and Pi
5. G and Pi are transported by
T2 and T3 , repectively.
6. G is transported by
GLUT2 into blood.
Glycogen synthesis
1.
Sugar nucleotide (UDP-glucose) are
the substrates for the polymerization
of monosaccharides
2. Properies of sugar nucleotide for
biosynthesis
- Their formation is metaboically
irreversible
- nucleotide moiety has many groups
that can undergo noncovalent
interaction with enzyme
- nucleotidyl group are excellent
leaving group
- cell can set sugar nucleotide aside
in a pool for one purpose.
1.
2.
3.
Glycogen branching enzyme: amylo (1->4) to (1->6) transglycosylase
Transfer a terminal fragments of 6 or 7 glucose residues to C-6 hydroxyl group
The biological effect of branching is to make the glycogen molecule more soluble and to
increase the number of non-reducing end for more accessible phosphorylase and
synthase.
1.
2.
Glycogen synthase can not
initiate a new glycogen chanin
de novo (at least 4 glucose
residues).
Glycogenin
- the primer on which new
chains are assembled
- catalyzed their assembly
Glycogen phosphorylase is regulated allosterically and
hormonally
1.
Ca2+ binds to and activates
phosphorylase b kinase 
activate phosphorylase b
2.
AMP bind to and activate
phosphorylase, speeding up the
release of G-1-P from glycogen.
Glycogen phosphorylase of liver as a glucose sensor.
1.
2.
3.
In liver, when blood glucose levels return to normal, glucose enters hepatocytes and
binds to an inhibitory allosteric site on phophorylase a.
This binding produces a conformational change that exposes the phosphorylated Ser
residues to PP1
And then dephosphorylated and inactivated.
Glycogen synthase is regulated by phosphorylation
and dephosphorylation
The path from insulin to GSK3 and glycogen synthase
Protein phosphatase I is central to glycogen metabolism
1.
2.
3.
PP1 can remove phosphoryl
groups from all three enzymes in
response to glucagon (liver) and
epinephrin (liver and muscle);
phosphorylase kinase, glycogen
phosphorylase, and glycogen
synthase
Glycogen-targeting protein bind
PPI, glycogen, and each of three
enzyme
PPI is inactivated when
phosphorylated by PKA and is
activated by G-6-P
Regulation of carbohydrate metabolism in the liver
Difference in the regulation of carbohydrate metabolism in liver
and muscle
1.
2.
3.
4.
Myocyte lacks receptors for
glucagon and the enzymatic
machinery for gluconeogenesis
Pyruvate kinase is not
phosphorylated by PKA, thus
glycolysis is not turned off when
[cAMP] is increased.
In fact activated PKA
phosphorylates phophorylase
kinase and increase glycogen
breakdown
Insulin triggers increase
glycogen synthesis in myocyte:
activating PPI and inactivating
GSK3, GLUT4 targeting to
membrane