Download Lecture 2: Glycogen metabolism (Chapter 15)

Lecture 2: Glycogen metabolism (Chapter 15)
First….
Fig. 15.1
Review:
Animals use glycogen for ENERGY STORAGE.
Glycogen is a highly-branched polymer of glucose units:
Basic structure is similar to that of amylopectin, but with only about
8 to 12 glucose units between branch points (n = 4 to 6).
GLYCOGEN BREAKDOWN INSIDE CELLS:
Glycogen's glucose units are mobilized by their
sequential removal from the glucan chain's
nonreducing ends — that is, the ends that lack a
C1-OH group. This is the reducing end of glucose:
Fig. 15-2a
The ends of some
sugars have a free
anomeric carbon,
which can act as a
mild reducing agent.
(In glycogen, however,
the reducing end is
actually bound by a
protein named
GLYCOGENIN.)
The branched structure of glycogen permits the rapid
release of glucose simultaneously from every
non-reducing end of every branch:
(These red arrows
point to the nonreducing ends.)
Fig. 15-2b
(modified)
Only ONE
reducing
end per
molecule
(Note that the number of glucose units between branch points in this
figure is not accurate. Don’t let this confuse you!)
Reminder: The reducing end is bound by GLYCOGENIN
GLYCOGENIN
Fig. 15-2b
(modified)
Why use glycogen to store energy rather than just
using fat? (Since fat is more abundant than glycogen in
the body and also stores energy)
1. Muscles "mobilize" (i.e., convert to energy)
glycogen faster than fat.
2. Fatty acid residues cannot be metabolized
anaerobically (that is, without oxygen).
(If you want to burn fat while you are exercising,
you must be able to breathe fairly easily.)
3. Animals cannot convert fat to glucose, so fat
metabolism cannot maintain blood glucose levels.
(Glucose is ”brain food"— it is the major energy form
that crosses the blood-brain barrier.)
Glycogenolysis (or glycogen breakdown)
requires 3 major enzymes:
1) GLYCOGEN PHOSPHORYLASE (Fig. 15-4; more later)
2) GLYCOGEN DEBRANCHING ENZYME (Fig. 15-6)
3) PHOSPHOGLUCOMUTASE (Fig. 15-7):
Glycogenolysis requires 3 major enzymes:
1) GLYCOGEN PHOSPHORYLASE
(or simply PHOSPHORYLASE)
See Fig. 15-4 (next slide) for GP’s reaction mechanism.
Note that GP catalyzes bond cleavage by PHOSPHOROLYSIS,
as opposed to HYDROLYSIS.
The overall reaction is:
Glycogen(n residues) + Pi
inorganic
phosphate
Glycogen (n-1) + G-1-P
Glucose-1-phosphate
GLYCOGEN PHOSPHORYLASE MECHANISM:
Fig. 15-4:
Phosphorylase has a “random
sequential”enzyme mechanism
that involves PLP (pyridoxyl-5’phosphate), a vitamin B6
derivative:
NOTE:
Phosphorylase only releases units that are
5 or more from the branch point, leaving a
“LIMIT BRANCH”….
Glycogenolysis requires 3 major enzymes:
2) GLYCOGEN DEBRANCHING ENZYME (Fig. 15-6)
GDE has two enzymatic activities:
A) A “debranching” transglycosylase
activity
B) An hydrolysis activity
A
B
Glycogenolysis requires 3 major enzymes:
3) PHOSPHOGLUCOMUTASE reaction:
G-1-P
G-1,6-P
G-6-P
Glucose-1,6-bisphosphate
Glucose-6-phosphate
Fig. 15-7: Phosphoglucomutase Mechanism
(Note that this reaction is fully reversible.)
Fig. 15-1: G-6-P is a major intermediate in glucose metabolism
Glucose-6-phosphatase
hydrolyzes G-6-P to
Glucose + Pi in LIVER
Important in nucleotide
synthesis
Fig. 15-1: G-6-P is a major intermediate in glucose metabolism
Brief overview next...
Glycogen SYNTHESIS requires 3 major enzymes, and
occurs by a SEPARATE PATHWAY from glycogenolysis:
1) UDP-GLUCOSE PYROPHOSPHORYLASE (Fig. 15-9):
G-1-P + UTP
Uridine triphosphate
UDP-glucose (UDPG) +
Uridine diphosphate glucose
PPi
inorganic
pyrophosphate
2) GLYCOGEN SYNTHASE (Fig. 15-10):
UDPG + Glycogen(n units)
UDP + Glycogen(n+1 units)
This reaction must be “primed” by GLYCOGENIN
3) GLYCOGEN BRANCHING ENZYME (Fig. 15-11) or
AMYLO (1,4
1,6) TRANSGLYCOSYLASE.
GENERAL RULES FROM ABOVE:
BIOSYNTHETIC AND DEGRADATIVE PATHWAYS OF
METABOLISM ARE (ALMOST) ALWAYS COMPLETELY
DIFFERENT. THAT IS, THEY USED DIFFERENT
ENZYMES.
POLYMERIZATION OF MONOMERIC UNITS INTO
MACROMOLECULES USUALLY REQUIRES A
‘PRIMER’ TO INITIATE THE REACTION. THAT IS,
THE FIRST TWO UNITS CANNOT BE LINKED BY
THE ENZYME THAT DOES THE POLYMERIZATION.
1. GLYCOGEN PHOSPHORYLASE
(or simply PHOSPHORYLASE)
• Removes GLUCOSE UNITS from the NONREDUCING
ends of GLYCOGEN.
• Is a FAST enzyme: the outermost branches of glycogen
are degraded in seconds in muscle tissue.
• Is a dimer of identical 842-residue subunits (Fig. 15-3).
1. GLYCOGEN PHOSPHORYLASE
(continued)
• Catalyzes the CONTROLLING STEP in glycogen
breakdown.
• The standard-state free-energy change ( G°') for
phosphorylase reactions is + 3.1 kJ/mol, but the
intracellular [Pi] / [G1P] ratio is about 100, so G
in vivo is actually about - 6 kJ/mol.
1. GLYCOGEN PHOSPHORYLASE
(continued)
• It is a highly and complexly regulated enzyme, both by:
• ALLOSTERIC INTERACTIONS (Fig. 15-13) — ATP,
G6P & glucose inhibit it; AMP activates it
and
• COVALENT MODIFICATION by phosphorylation
and dephosphorylation (Fig. 15-5).
Yields 2 major forms of phosphorylase —
Phosphorylase A: Has a phosphoryl group esterified to
Ser-14 in each subunit (more active)
Phosphorylase B: Is not phosphorylated (less active)
Look at Kinemages Exercise 14 on the CD with VVP textbook!
1. GLYCOGEN PHOSPHORYLASE
(continued)
• Only releases units that are 5 or more from the branch.
WHY?
1. GLYCOGEN PHOSPHORYLASE
(continued)
• Only releases units that are 5 or more from the branch.
WHY?
Robert Fletterick (www.ucsf.edu/pibs/faculty/fletterick.html)
solved the 3D structure of Phosphorylase A: Its crevice
can admit 4 or 5sugar residues, but it is too narrow to
admit a branch.
Fig. 15-1: G-6-P is a major intermediate in glucose metabolism
NEXT...
Glycogen SYNTHESIS requires 3 major enzymes, and
occurs by a SEPARATE PATHWAY from glycogenolysis:
1) UDP-GLUCOSE PYROPHOSPHORYLASE (Fig. 15-9):
G-1-P + UTP
Uridine triphosphate
UDP-glucose (UDPG) +
Uridine diphosphate glucose
PPi
inorganic
pyrophosphate
2) GLYCOGEN SYNTHASE (Fig. 15-10):
UDPG + Glycogen(n units)
UDP + Glycogen(n+1 units)
This reaction must be “primed” by GLYCOGENIN
3) GLYCOGEN BRANCHING ENZYME (Fig. 15-11) or
AMYLO (1,4
1,6) TRANSGLYCOSYLASE.
1) UDP-GLUCOSE PYROPHOSPHORYLASE (Fig. 15-9):
G-1-P + UTP
Uridine triphosphate
UDP-glucose (UDPG) +
Uridine diphosphate glucose
PPi
inorganic
pyrophosphate
The G°’ of this reaction is nearly ZERO, but
the PPi formed is hydrolyzed to 2 Pi
(orthophosphate) in a highly EXERGONIC
reaction the the omnipresent enzyme,
INORGANIC PYROPHOSPHATASE. Therefore,
the overall reaction is also highly exergonic:
G°’ (kJ/mol)
GIP + UTP
UDPG + PPi
~0
H2O + PPi
2 Pi
- 33.5
GIP + UTP
UDPG + 2 Pi
- 33.5
OVERALL
UDPG is a HIGH ENERGY compound that can donate
GLYCOSYL units to the growing glycogen chain. No
further energy is required for glycogen synthesis.
IMPORTANT GENERAL NOTE:
The cleavage of a nucleoside triphosphate
(NTP) to form PPi is a common synthetic
strategy. The free energy of PPi hydrolysis
(by inorganic pyrophosphatase) can be
utilized together with the free energy of NTP
hydrolysis to drive an otherwise endergonic
reaction to completion. (We will see this
over and over and over this semester!)
2) GLYCOGEN SYNTHASE MECHANISM (Fig. 15-10):
UDPG + Glycogen(n units)
UDP + Glycogen(n+1 units)
The glycosyl unit of
UDPG is transferred to
the C(4)-OH on one of
the non-reducing ends
of glycogen, forming
an (1 4) glycosidic
bond. Note that this
step makes -amylose,
not the branched
structure of glycogen.
The ∆G°’ for this reaction is -13.7 kJ/mol, making this reaction spontaneous
(exergonic) under the same conditions that glycogen breakdown is exergonic.
Therefore, the rates of the two reactions must be independently and tightly controlled.
For each molecule of GIP that is converted to glycogen,
one molecule of UTP is hydrolyzed to UDP + Pi.
The UTP is replenished by the enzyme
NUCLEOSIDE DIPHOSPHATE KINASE:
UDP + ATP
UTP
+
ADP
(UTP hydrolysis is energetically equivalent to
ATP hydrolysis.)
GLYGOGENIN and Glycogen “Priming”
Glycogen synthesis can only occur by extending an
already existing (1 4)-linked glucan chain.
Therefore, how can it get started in the first place?
Answer: The first step in glycogen synthesis is the
attachment of a glucose residue to the -OH group on
Tyr-194 of GLYCOGENIN. This attachment step is done
by the enzyme TYROSINE GLUCOSYLTRANSFERASE.
Glycogenin then autocatalytically extends the glucan
chain by up to 7 residues long (also donated by UDPG).
Glycogen synthase can then attach glucose residues to
this glycogen “primer”. Each molecule of glycogen is
associated with ONE molecule each of glycogenin and
glycogen synthase.
3) GLYCOGEN BRANCHING ENZYME (Fig. 15-11) or
AMYLO (1,4
1,6) TRANSGLYCOSYLASE:
Breaks (1 4) glycosidic
bonds and forms (1 6)
linkages. Transfers terminal
chain segments of about 7
residues to the C(6)-OH
groups of glucose residues.
Each transferred segment
must come from a chain of at
least 11 residues, and the
attachment point must be at
least 4 residues away from
another branch point.
Segment can be moved to the
same or a different chain.
Note: Not to be confused with Glycogen Debranching Enzyme!
Control of glycogen metabolism is very complex.
It involves:
• allosteric regulation of both GS & GP
• substrate cycles
• enzyme-catalyzed covalent modification of both GS &GP
• covalent modifications are under hormonal control in the
body, through their own enzymatic cascades
In LIVER:
Glycogen metabolism is ultimately controlled by
GLUCAGON — a 29 amino acid-long polypeptide hormone
that is secreted from the pancreas into the bloodstream
(liver cells have glucagon receptors).
In MUSCLES (and various other tissues):
Is controlled by the adrenal hormones EPINEPHRINE
(adrenalin) and NOREPINEPHRINE (noradrenalin).
These hormones act at cell surfaces to stimulate
ADENYLATE CYCLASE, thus increasing [cAMP].
cAMP acts inside cells as a ‘second messenger’ for
the hormones. Cells have many cAMP-dependent
PROTEIN KINASES whose activities increase upon
cAMP binding. (Reminder: Kinases catalyze the
transfer of phosphoryl groups between ATP and
other molecules, proteins in this case.)
Liver maintains blood [glucose] at ~5 mM; if it drops to
half of this, a coma results. Upon blood [glucose]
decrease, the liver releases glucose to the blood;
glucose triggers pancreas to release glucagon, which
causes increase [cAMP] in liver, which in turn
stimulates glycogen breakdown. Glucose diffuses
freely out of liver cells, causing an increase in blood
[glucose]. High blood [glucose] causes release of
INSULIN from the pancreas to the blood. The rate of
glucose TRANSPORT across many cell membranes
increases in response to insulin.