466 Chapter 15 Enzyme RegulationCov

466 Chapter 15 Enzyme Regulation
Covalent Modification of Glycogen Phosphorylase Trumps
Allosteric Regulation
As early as 1938, it was known that glycogen phosphorylase existed in two forms: the
less active phosphorylase b and the more active phosphorylase a. In 1956, Edwin
Krebs and Edmond Fischer reported that a “converting enzyme” could convert
phosphorylase b to phosphorylase a. Three years later, Krebs and Fischer demon-
strated that the conversion of phosphorylase b to phosphorylase a involved covalent
phosphorylation, as shown in Figure 15.15.
Phosphorylation of Ser14 causes a dramatic conformation change in phosphorylase.
Upon phosphorylation, the amino-terminal end of the protein (including residues 10
through 22) swings through an arc of 120°, moving into the subunit interface (Figure
15.16). This conformation change moves Ser14 by more than 3.6 nm. The phosphory-
lated or a form of glycogen phosphorylase is much less sensitive to allosteric regulation
than the b form. Thus, covalent modification of glycogen phosphorylase converts this
enzyme from an allosterically regulated form into a persistently active form. Covalent
modification overrides the allosteric regulation.
Dephosphorylation of glycogen phosphorylase is carried out by phosphoprotein
phosphatase 1. The action of phosphoprotein phosphatase 1 inactivates glycogen
phosphorylase. The 1992 Nobel Prize in Physiology or Medicine was awarded to Krebs
and Fischer for their pioneering studies of reversible protein phosphorylation as an
important means of cellular regulation.
Enzyme Cascades Regulate Glycogen Phosphorylase Covalent
Modification
The phosphorylation reaction that activates glycogen phosphorylase is mediated by
an enzyme cascade (Figure 15.17). The first part of the cascade leads to hormonal
stimulation (described in the next section) of adenylyl cyclase, a membrane-bound
enzyme that converts ATP to adenosine-3,5-cyclic monophosphate, denoted as cyclic
AMP or simply cAMP (Figure 15.18). This regulatory molecule is found in all eu-
karyotic cells and acts as an intracellular messenger molecule, controlling a wide va-
riety of processes. Cyclic AMP is known as a second messenger because it is the in-
tracellular agent of a hormone (the “first messenger”). (The myriad cellular roles
of cyclic AMP are described in detail in Chapter 32.)
The hormonal stimulation of adenylyl cyclase is effected by a transmembrane sig-
naling pathway consisting of three components, all membrane associated. Binding
of hormone to the external surface of a hormone receptor causes a conformational
change in this transmembrane protein, which in turn stimulates a GTP-binding
protein (abbreviated G protein). G proteins are heterotrimeric proteins consisting of
- (45–47 kD), - (35 kD), and - (7–9 kD) subunits. The -subunit binds GDP or
GTP and has an intrinsic, slow GTPase activity. In the inactive state, the G complex
has GDP at the nucleotide site. When a G protein is stimulated by a hormone–
receptor complex, GDP dissociates and GTP binds to G, causing it to dissociate
from G and to associate with adenylyl cyclase (Figure 15.19). Binding of G (GTP)
activates adenylyl cyclase to form cAMP from ATP. However, the intrinsic GTPase activity
of G eventually hydrolyzes GTP to GDP, leading to dissociation of G (GDP) from
adenylyl cyclase and reassociation with G to form the inactive G complex. This
cascade amplifies the hormonal signal because a single hormone–receptor complex
can activate many G proteins before the hormone dissociates from the receptor, and
because the G-activated adenylyl cyclase can synthesize many cAMP molecules be-
fore bound GTP is hydrolyzed by G. More than 100 different G-protein–coupled re-
ceptors and at least 21 distinct G proteins are known (see Chapter 32).
cAMP
Hormone
Adenylyl
cyclase
G protein
G(GTP) dissociates from
G and binds to adenylyl
cyclase, activating synthesis
of cAMP
Receptor

 
Inactive
adenylyl
cyclase
G protein
Receptor


 
Slow GTPase activity of G
hydrolyzes GTP to GDP
G(GDP) dissociates from
adenylyl cyclase and
returns to G

 


Pi
ATP
GTP GDP
GTP
GDP
GDP
FIGURE 15.19 Hormone binding to its receptor leads via G-protein activation to cAMP synthesis. Adenylyl
cyclase and the hormone receptor are integral plasma membrane proteins; G and G are membrane-
anchored proteins.