24.2 Phosphorylase Is Regulated by Allosteric Interactions and Reversible Phosphorylation

✓ 2 Explain the regulation of glycogen breakdown.

DID YOU KNOW?

Isozymes, or isoenzymes, are enzymes that are encoded by different genes yet catalyze the same reaction. Usually, isozymes display different kinetic parameters or regulatory properties.

Glycogen degradation is precisely controlled by multiple interlocking mechanisms. The focus of this control is the enzyme glycogen phosphorylase. Phosphorylase is regulated by several allosteric effectors that signal the energy state of the cell, as well as by reversible phosphorylation, which is responsive to hormones such as epinephrine, glucagon, and insulin. We will examine the differences in the control of two isozymic forms of glycogen phosphorylase: one specific to liver and one specific to skeletal muscle. These differences are due to the fact that the liver maintains glucose homeostasis of the organism as a whole, whereas the muscle uses glucose to produce energy for itself.

Liver Phosphorylase Produces Glucose for Use by Other Tissues

The dimeric phosphorylase exists in two interconvertible forms: a usually active phosphorylase a and a usually inactive phosphorylase b (Figure 24.5). Each of these two forms exists in equilibrium between an active relaxed (R) state and a much less active tense (T) state, but the equilibrium for phosphorylase a favors the active R state, whereas the equilibrium for phosphorylase b favors the less active T state (Figure 24.6). The role of glycogen degradation in the liver is to form glucose for export to other tissues when the blood-glucose concentration is low. Consequently, we can think of the default state of liver phosphorylase as being the a form: glucose is to be generated unless the enzyme is signaled otherwise. The liver phosphorylase a form thus exhibits the most responsive R ↔ T transition (Figure 24.7). The binding of glucose to the active site shifts the a form from the active R state to the less active T state. In essence, the enzyme reverts to the low-activity T state only when it detects the presence of sufficient glucose. If glucose is present in the diet, there is no need to degrade glycogen.

Figure 24.5: Structures of phosphorylase a and phosphorylase b. Phosphorylase a is phosphorylated on serine 14 of each subunit. This modification favors the structure of the more active R state. One subunit is shown in white, with helices and loops important for regulation shown in blue and red. The other subunit is shown in yellow, with the regulatory structures shown in orange and green. phosphorylase b is not phosphorylated and exists predominantly in the T state. Notice that the catalytic sites are partly occluded in the T state.
Figure 24.6: Phosphorylase regulation. Both phosphorylase b and phosphorylase a exist in equilibrium between an active R state and a less active T state. phosphorylase b is usually inactive because the equilibrium favors the T state. phosphorylase a is usually active because the equilibrium favors the R state. In the T state, the active site is partly blocked by a regulatory structure. The active site is unobstructed in the R state. regulatory structures are shown in blue and green.
Figure 24.7: The allosteric regulation of liver phosphorylase. The binding of glucose to phosphorylase a shifts the equilibrium to the T state and inactivates the enzyme. Thus, glycogen is not mobilized when glucose is already abundant. regulatory structures are shown in blue and green.

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Muscle Phosphorylase Is Regulated by the Intracellular Energy Charge

In contrast to the liver isozyme, the default state of muscle phosphorylase is the b form, owing to the fact that, for muscle, phosphorylase needs to be active primarily during muscle contraction. Muscle phosphorylase b is activated by the presence of high concentrations of AMP, which binds to a nucleotide-binding site and stabilizes the conformation of phosphorylase b in the active R state (Figure 24.8). Recall that phosphofructokinase, the allosteric enzyme that controls glycolysis, is also activated by AMP. ATP acts as a negative allosteric effector by competing with AMP. Thus, the transition of phosphorylase b between the active R state and the less active T state is controlled by the energy charge of the muscle cell. Glucose 6-phosphate also binds at the same site as ATP and stabilizes the less active state of phosphorylase b, an example of feedback inhibition. Under most physiological conditions, phosphorylase b is inactive because of the inhibitory effects of ATP and glucose 6-phosphate. In contrast, phosphorylase a is fully active, regardless of the amount of AMP, ATP, and glucose 6-phosphate present. In resting muscle, nearly all the enzyme is in the inactive b form.

Figure 24.8: The allosteric regulation of muscle phosphorylase. A low energy charge, represented by high concentrations of AMP, favors the transition to the R state. Glucose 6-phosphate and ATP stabilize the T state.

Unlike the enzyme in muscle, the liver phosphorylase is insensitive to regulation by AMP because the liver does not undergo the dramatic changes in energy charge seen in a contracting muscle. We see here a clear example of the use of isozymes to establish the tissue-specific biochemical properties of the liver and muscle. In human beings, liver phosphorylase and muscle phosphorylase are approximately 90% identical in amino acid sequence, yet the 10% difference results in subtle but important shifts in the stability of various forms of the enzyme.

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Biochemical Characteristics of Muscle Fiber Types Differ

Not only do the biochemical needs of liver and muscle differ with respect to glycogen metabolism, but the biochemical needs of different muscle fiber types also vary. Skeletal muscle consists of three different fiber types. Type I or slow-twitch muscle, type IIb (also called type IIx) or fast-twitch fibers, and type IIa fibers, which have properties intermediate between type I and type IIb. Type I fibers, which power endurance activities, rely predominantly on cellular respiration to derive energy. These fibers are powered by fatty acid degradation and are rich in mitochondria, the site of fatty acid degradation and the citric acid cycle. As we will see in Chapter 27, fatty acids are an excellent energy source, but generating ATP from fatty acids is slower than from glycogen. Glycogen is not an important fuel for type I fibers, and consequently the amount of glycogen phosphorylase is low. Type IIb fibers use glycogen as their main fuel. Consequently, the glycogen and glycogen phosphorylase are abundant. These fibers are also rich in glycolytic enzymes needed to process glucose quickly in the absence of oxygen and poor in mitochondria. Type IIb fibers power burst activities such as sprinting and weight lifting. No amount of training can interconvert type I fibers and type IIb fibers. However, there is some evidence that type IIa fibers are “trainable”; that is, endurance training enhances the oxidative capacity of type IIa fibers while burst activity training enhances the glycolytic capacity. Table 24.1 shows the biochemical profile of the fiber types.

Table 24.1 Biochemical characteristics of muscle fiber types

Phosphorylation Promotes the Conversion of Phosphorylase b to Phosphorylase a

In both liver and muscle, phosphorylase b is converted into phosphorylase a by the phosphorylation of a single serine residue (serine 14) in each subunit. This conversion is initiated by hormones. Low blood-glucose concentration leads to the secretion of the hormone glucagon. Fear or the excitement of exercise will cause release of the hormone epinephrine. The rise in glucagon and epinephrine concentration results in phosphorylation of the enzyme to the phosphorylase a form in liver and muscle, respectively. The regulatory enzyme phosphorylase kinase catalyzes this covalent modification.

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!quickquiz! QUICK QUIZ 2

Compare the allosteric regulation of phosphorylase in the liver and in muscle, and explain the significance of the difference.

Comparison of the structures of phosphorylase a in the R state and phosphorylase b in the T state reveals that subtle structural changes at the subunit interfaces are transmitted to the active sites (Figure 24.5). The transition from the T state (the prevalent state of phosphorylase b) to the R state (the prevalent state of phosphorylase a) is associated with structural changes in α helices that move a loop out of the active site of each subunit. Thus, the T state is less active because the catalytic site is partly blocked. In the R state, the catalytic site is more accessible and a binding site for orthophosphate is well organized.

Phosphorylase Kinase Is Activated by Phosphorylation and Calcium Ions

Phosphorylase kinase activates phosphorylase b by attaching a phosphoryl group. The subunit composition of phosphorylase kinase in skeletal muscle is (αβγδ)4, and the mass of this very large protein is 1300 kDa. The enzyme consists of two (αβγδ)2 lobes that are joined by a β4 bridge that is the core of the enzyme and serves as a scaffold for the remaining subunits. The γ subunit contains the active site, while all of the remaining subunits (≈90% by mass) play regulatory roles. The δ subunit is the calcium-binding protein calmodulin, a calcium sensor that stimulates many enzymes in eukaryotes. The α and β subunits are targets of protein kinase A. The β subunit is phosphorylated first, followed by the phosphorylation of the α subunit.

Activation of phosphorylase kinase is initiated when Ca2+ binds to the δ subunit. This mode of activation of the kinase is especially noteworthy in muscle, where contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum (Figure 24.9). Maximal activation is achieved with the phosphorylation of the β and α subunits of the Ca2+-bound kinase. The stimulation of phosphorylase kinase is one step in a signal-transduction cascade initiated by signal molecules such as glucagon and epinephrine.

Figure 24.9: The activation of phosphorylase kinase. Phosphorylase kinase, an (αβγδ)4 assembly, is partly activated by Ca2+ binding to the δ subunit. Activation is maximal when the β and α subunits are phosphorylated in response to hormonal signals. When active, the enzyme converts phosphorylase b into phosphorylase a.

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!clinic! CLINICAL INSIGHT: Hers Disease Is Due to a Phosphorylase Deficiency

Hers disease, a hereditary disorder, is a glycogen-storage disease that emphasizes the importance of the isozymes of glycogen phosphorylase. Hers disease is caused by a deficiency in the liver isozyme of glycogen phosphorylase. Because glycogen cannot be degraded, it accumulates, leading to an enlargement of the liver (hepatomegaly) that, in some cases, causes the abdomen to protrude. In extreme cases, liver damage may result. Patients with Hers disease display a low concentration of blood glucose (hypoglycemia) because their livers are not able to degrade glycogen. Clinical manifestations of Hers disease vary widely. In some patients, the disease is undetectable and, in others, it causes liver damage and growth retardation. However, in most cases the prognosis is good, and the clinical manifestations improve with age. See problem 6 to learn the effects of a deficiency of muscle glycogen phosphorylase.