section 15.2
Glycogen Metabolism
287
N7 N10
1
.
.
20
____
I
I
'CB-N - PLSRTLSVSSLPGLEDWEDEFOLENSVLF
5. Glycogen synthase kinase-4 (GSK-4; site N7;
relatively specific for glycogen synthase).
6
. Casein kinase-2 (site C46; broadly specific;
physiologically important substrates not yet known).
7. A novel protein kinase that may be activated by
cAMP-dependent protein kinase (site N10).
FIGURE 15-9
Regulation of glycogen synthase by multisite phosphorylation. The
location of phosphorylation sites (*) and the protein kinases that
phosphorylate at these sites (boxes) are shown. Phosphorylations occur
only at N- and C-terminal regions of the enzymes, as indicated by CB-N
and CB-C, respectively. The single-letter abbreviations for amino acids are
used (see Chapter 2). cAMP-PK = cyclic AMP-dependent protein kinase;
CAM-MPK = Ca
2
+/calmodulin-dependent multiprotein kinase; PhK =
phosphorylase kinase; GSK = glycogen synthase kinase; CK = casein
kinase; N10-PK = A novel protein kinase. [Reproduced with permission
from P. Cohen, Protein phosphorylation and hormone action.
Proc. R. Soc.
Lond. ( B io l)
234,
115 (1988).]
K.d
for glucose-
6
-phosphate, or decreased
K\
for ADP, in-
organic phosphate, or other inhibitors, or a change in Vmax.
At least seven different protein kinases can phospho-
rylate muscle glycogen synthase
in vitro,
but whether
all are involved
in vivo
is not established. Some are
moderately specific for glycogen synthase, while oth-
ers phosphorylate a variety of proteins. They are listed
below, together with the sites that they phosphorylate
(Figure 15-9).
1. cAMP-dependent protein kinase (sites N7, C87, and
C
1 0 0
, broad specificity; mediates effects of many
hormones).
2. Phosphorylase kinase (site N7; main function
in vivo
probably is activation of glycogen phosphorylase).
3. Ca
2
+/calmodulin-dependent multiprotein kinase (sites
N7 and C100; relatively specific for glycogen
synthase).
4. Glycogen synthase kinase-3 (GSK-3; sites C30, C34,
C38 and C42; also phosphorylates inhibitor-2 (see
below), thereby activating protein phosphatase-
1
).
In muscle, cAMP-dependent protein kinase (Chap-
ter 30) mediates the inhibitory effect of epinephrine
on glycogenesis by phosphorylating protein phosphatase
inhibitor-1 and phosphorylating sites N7, C87, and C l00.
The inhibitory effect of cytosolic Ca2+ on glycogen
synthesis may be mediated by calmodulin-dependent
multiprotein kinase. GSK-3 may be a key control en-
zyme; it phosphorylates sites on glycogen synthase that
have the greatest effect on synthase activity. GSK-3
also activates protein phosphatase
- 1
by phosphorylating
inhibitor-
2
, however, and these seemingly opposing effects
have not been fully explained. Stimulation by insulin of
glycogen synthesis in muscle may be mediated by GSK-3
or phosphatases that remove the phosphate group at the
GSK-3 sites.
Reactivation of glycogen synthase requires removal of
the phosphate groups by a phosphatase. Four enzymes that
dephosphorylate glycogen synthase or one or more kinases
in vitro
have been isolated from skeletal muscle.
1. Protein phosphatase-1 (Mg
2
+/ATP-dependent
phosphatase; multisubstrate protein phosphatase;
M.W. of catalytic subunit is 35,000; major enzyme in
regulation of glycogen metabolism in skeletal muscle;
dephosphorylates glycogen phosphorylase, /3-subunit
of phosphorylase kinase, and at least three sites of
glycogen synthase; regulated by inhibitor-
1
,
inhibitor-2, and GSK-3 + Mg2+- ATP).
2. Protein phosphatase-2A (polycation-dependent
phosphatase, M.W. ~200,000; M.W. of catalytic
subunit ~38,000; function unclear).
3. Protein phosphatase-2B (calcineurin; dimer; catalytic
subunit of M.W. 60,000 has Ca2+ binding site;
Ca2+-binding subunit of M.W. ~ 18,000, partially
homologous with calmodulin and troponin C;
N-terminal glycine blocked by amide linkage with
myristic acid; absolute requirement for Ca2+;
regulated by Ca
2
+/calmodulin; dephosphorylates
a-subunit of phosphorylase kinase and inhibitory but
not glycogen phosphorylase or synthase or /3-subunit
of phosphorylase kinase).
4. Protein phosphatase-2C (monomer; M.W. 43,000;
represents a small fraction of glycogen synthase
phosphatase in skeletal muscle; may primarily
regulate other metabolic pathway
in vivo).
