Glycogen and Glycogenolysis

Glycogen Storage Disease Type I, von Gierke Disease (and Others: At Least 11 Types of Glycogen Storage Disease)

This disease occurs as a result of an inherited deficiency of an enzyme involved in the synthesis or breakdown of glycogen, the storage form of glucose. Glycogen is formed primarily in the liver and muscle and, secondarily, in many other tissues as well. There are a variety of subtypes of this disease based on the deficient enzyme that will dictate the primary organ affected, usually, as either liver or muscle. The frequency of occurrence of glycogen storage disease (GSD) is 1 baby in 20,000–40,000. The main type of GSD (~90%) is type I, von Gierke’s disease . It is transmitted from the parents to the offspring by an autosomal recessive mechanism. One example would be two unaffected “carrier” (recessive condition) parents could produce four offspring (in this example); among them are one child with the overt disease, one unaffected child with no carrier gene, and two children who are unaffected carriers (recessives). There are two genetic patterns through which GSD or carrier status are inherited ( Fig. 7.1 ).

For a child to be overtly diseased, both parents must carry the mutated gene as shown in part (A); in this case, one child in four (25%) will have the overt GSD. Unaffected children who are carriers will be two of four (50%) and a normal child will be one of four (25%). In type VI (and IX) GSD the inheritance is X-linked (B). For example, a carrier mother with a normal father will produce one normal son in four, one normal daughter in four, one carrier daughter in four, and one affected son in four. This pattern of inheritance to generate overt GSD only affects the male offspring . Females carry the mutated gene on one of their two X chromosomes. The overt disease generated by the mutated gene on one X chromosome is masked by the normal gene on the other X chromosome. However, the male has only one X chromosome so that a mutated gene on that chromosome will be expressed as overt disease.

In GSD, there is an accumulation of glycogen that may be specific for one primary organ or another. The types of GSD, the mutated enzyme involved, and the symptoms for each type are displayed in Table 7.1 .

For most situations the aim of treatment is to stabilize blood glucose levels as circulating glucose will be low (hypoglycemia) in many of the forms of GSD. This can be accomplished by supplementing glucose or cornflower (starch). Sometimes, a high-protein diet is helpful. Those patients who do not benefit from supplements may require a liver transplant. When the immune system is compromised in some patients, antibiotics are indicated. One type of GSD has been cured in a mouse model by gene therapy where the gene for the mutated enzyme was replaced with the gene for the normal enzyme. To a certain extent, this approach has worked in patients with GSD type II ( Pompe’s disease ); however, infantile Pompe’s disease is difficult to treat and can limit life expectancy. GSD can affect muscles, including the heart and liver. Consequent breathing problems and heart disease in children sometimes can lead to death.

In von Gierke’s disease the most prevalent form of GSD, the loss of the activity of glucose-6-phosphatase (G-6-Pase), results in many metabolic disruptions that are captured in Fig. 7.2 .

Glycogen—The Storage Carbohydrate

Just as starch is the carbohydrate storage form in plants and is composed of linear amylose and the branched polysaccharide, amylopectin , glycogen is the carbohydrate storage form in eukaryotes, including humans. Like amylopectin, glycogen is a branched polysaccharide. Glycogen forms granules in the cytoplasm of different cells, especially liver and skeletal muscle. It can reach levels of over 100 g (c. 8%) in the liver cells of an adult human but reaches lower levels in muscles, to about 1.5%. Because the muscle mass is much greater, the total amount of glycogen in muscle is greater than the total amount in liver. Only liver glycogen can be made available to other tissues via the bloodstream in the form of glucose . However, the level of glycogen depends on many factors, including metabolic rate, food intake, and exercise. Some glycogen is found in red blood cells as well as in the heart and smaller amounts in the kidney and brain glia.

The ingestion of a meal results in carbohydrate breakdown in the gut, absorption of glucose into the bloodstream, and utilization of glucose for energy through glycolysis and the citric acid cycle . In the fed state, glucose is taken up by the liver cell through the glucose transporter-2 ( GLUT2) , and converted to glycogen. When energy is required sometime after a meal, the water-insoluble liver glycogen is broken down to glucose that enters the bloodstream and is utilized by other tissues, such as muscles or brain. The storage and release of glucose from glycogen are processes that are under hormonal control. Glycogen stored in muscle, in contrast to the liver, is not made available to other tissues because muscle cells do not express G-6-Pase that permits transfer of glucose into the bloodstream. This enzyme catalyzes the following reaction:

While free glucose is taken up by the liver through the GLUT2 transporter, it is also exported from the liver cell by the same transporter (GLUT2) in the cell membrane .

Glucose Metabolism in Muscle

Glucose metabolism in the muscle cell is summarized in Fig. 7.3 .

Glycogenin and Formation of Glycogen

Glycogenin is an enzyme, classified as a glycosyltransferase . It is a homodimer composed of two 37-kDa subunits. It has the initial role in the formation of glycogen from uridine diphosphate (UDP)-glucose in muscle and liver. There are two isoforms of the enzyme: in muscle, it is glycogenin-1 encoded by the gene, GYG1, and in liver and cardiac muscle, it is glycogenin-2 , encoded by the gene, GYG2. Linear chains of glucose are attached to glycogenin and up to 10 glucose molecules can be added. At this level, glycogen synthase and the branching enzyme take over to complete the synthesis of glycogen that may contain as many as 30,000 glucose residues. Glycogenin is essential to this process and a patient who has a defective gene (encoding the mRNA for glycogenin) cannot store glycogen and will display muscle weakness and cardiac disease. In the initial reactions the glycogenin dimer reacts with linear chains of glucose molecules. Then, the glycogenin–(glucose) n complex reacts with UDP-glucose to form glycogen through the actions of glycogen synthase and the branching enzyme. Glucose-1-phosphate (G-1-P) is converted to UDP-glucose by the action of UDP-glucose pyrophosphorylase, a reaction that is powered by the hydrolysis of inorganic pyrophosphates as shown in Fig. 7.4 .

A partial structure of glycogen is shown in Fig. 7.5 , showing a straight chain of glucose residues and a branched chain.

In Fig. 7.6 a partial structure of the glycogen particle is shown. Five layers are shown here but the completed glycogen particle has 12 layers.

The fully formed glycogen particle resembles the structure shown in Fig. 7.7 .

UDP-glucose interacts with each glycogenin monomer through a manganese ion and amino acid residues in the active site of glycogenin ( Fig. 7.8 ).

Glycogenin forms the center of the growing glycogen molecule and attaches UDP-glucose molecules to itself, a step in which glycogenin acts as a primer. Exactly how the enzyme catalyzes the addition of glucose is not completely clear, although the UDP-glucose is bound to the hydroxyl group of tyrosine-194 before seven more glucose residues can be added to the chain. When about eight residues are extended in a chain, glycogen synthase takes over to extend the chain further and the branching enzyme creates the side chains. The transfer of the first glucose residues is intermolecular and subsequent glucose molecules are attached by intramolecular reaction within the glycogenin dimer . The straight chains of glycogen are made up of α-1,4 linkages between the glucose units, except for the branches that are made through α-1,6 linkages. Glycogen synthase catalyzes the reaction: UDP-glucose+glycogen ( n glucose units) →UDP+glycogen ( n +1 glucose units) . In this way, glucose molecules are added to nonreducing ends of glycogen ( Fig. 7.9 ).

A nonreducing end of a sugar is one that contains an acetal group , whereas a reducing sugar end is either an aldehyde or a hemiacetal group ( Fig. 7.10 ).

Glycogenolysis (Releasing Glucose from Glycogen)

The enzyme, phosphorylase , catalyzes the following phosphorolysis reaction:

Phosphorolysis occurs to within four residues of a branch point to produce one molecule of G-1-P for each glucose unit released. The debranching enzyme ( transglucosylase activity , α-1,4 to α-1,4) then catalyzes the transfer of a trimer from a branch to the free end of the glycogen molecule. The α-1,6-glucosidase activity of the debranching enzyme then cleaves the α-1,6 glucose at the branch point of glycogen ( Fig. 7.11 ). Glucose units of 11–12 are released from glycogen in the form of G-1-P by phosphorolysis, and these can enter glycolysis through the phosphoglucomutase reaction that yields glucose-6-phosphate (G-6-P) .

G-6-P is in a key position between glycogen, free glucose, ribose-5-phosphate (can be incorporated into ribonucleotides), and pyruvate as shown in Fig. 7.12 .

In the liver, G-6-Pase converts G-6-P back into free glucose. A mutation in this enzyme can lead to a GSD . For export of free glucose into the bloodstream for use by other tissues, glucose moves out of the liver through the transporter, GLUT2 . This occurs in fasting or in the postabsorptive state (after a meal has been absorbed) when the concentration of glucose is higher in the liver than it is in the blood.

Phosphoribosylpyrophosphate in this pathway is used in the formation of nucleotides, as will be discussed later. The pentose phosphate pathway ( phosphogluconate pathway or hexose monophosphate shunt ) generates NADPH during the oxidative phase of the pathway. One molecule of NADPH is generated from NADP ⁺ in the following reactions of the pathway: conversion of G-6-P to 6-phosphogluconate by G-6-P dehydrogenase ; conversion of 6-phosphogluconate to ribulose-5-phosphate by phosphogluconate dehydrogenase , and conversion of ribulose-5-phosphate to ribose-5-phosphate by ribose-5-phosphate isomerase (2 NADPH). A hemolytic anemia is produced in patients that have a genetic deficiency of G-6-P dehydrogenase (G6PDH) , resulting in an inadequate supply of NADPH to the red blood cell . This is the most common defect resulting from an enzyme deficiency . This disorder involves 400 million people worldwide (especially in Africa, Asia, Mediterranean countries, and South America), approximately the number of persons affected with the malaria parasite. The mutation is X-recessive-linked and is polymorphic with more than 300 variants. Neonatally, the disorder can present as hyperbilirubinemia. Some individuals with this mutation can be asymptomatic, whereas others have episodes of hemolysis (destruction of red blood cells), varying in intensity, as there is a range of severity. G6PDH deficiency, however, is protective against malaria . Thus the mutation is dangerous if antimalarial drugs (e.g., primaquine) are used because of the loss of protection in the red blood cell against hemolyzing reactive oxygen species (H ₂ O ₂ ) that are induced by the drug. Consequently, it is vital to test for G6PDH deficiency before administering the antimalarial drug. Also in the red blood cell, the enzyme, glutathione (GSH) peroxidase , functions to degrade hydroperoxides that arise because of the oxygen-rich environment. This enzyme reduces the hydroperoxides by the use of two molecules of reduced GSH, converting it to oxidized glutathione (GSSG):

To regenerate GSH from GSSG, NADPH is required:

In the absence of sufficient NADPH, GSSG would accumulate, and there would be insufficient GSH to remove harmful hydroperoxides.