Enzymology of Complex Alpha-Glucans

Felix Nitschke

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781351379656
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Cell Biology
• Index: Biological Sciences

Glycogen and Starch: So Similar, yet so Different. Both carbohydrates are central to the primary metabolism of a large part of the living kingdom. Generally, animals, fungi, and bacteria store glycogen, while plants largely rely on starch. This book provides a broad and current view on both glycogen and starch, in lower and higher organisms. Beside biochemistry, physiology and regulation of glycogen and starch metabolism, the reader can expect an insight into glycogen storage diseases, select methods and relevant techniques. While significant progress has been made in both fields, this volume emphasizes an opportunity of collaboration for researchers working on a major intersection of the living world.

CHAPTER 5

Mammalian Glycogen Metabolism Enzymology, Regulation, and Animal Models of Dysregulated Glycogen Metabolism

Bartholomew A. Pederson

Glycogen synthesis and degradation require several enzymes, with glycogen synthase and glycogen phosphorylase being key enzymes in elongating and shortening, respectively, glucose chains in the glycogen macromolecule. Complex, reciprocal regulation of glycogen synthase and glycogen phosphorylase controls the amount of glycogen. Mammalian glycogen metabolism has been studied most extensively in muscle and liver. In the muscle, glycogen serves as a fuel for contraction and as a storage depot for glucose removed postprandially from the circulation. In the liver, glycogen is synthesized to remove excess postprandial glucose from the bloodstream and subsequently degraded during times of fasting to provide glucose to the body. Glycogen metabolism has also been studied to a lesser degree in other tissues, notably heart and kidney, and especially of late, the brain. The author has focused primarily on mammalian glycogen metabolism in muscle, liver, and brain. In addition to discussion of the enzymology and regulation of glycogen metabolism, select glycogen storage diseases are discussed, focusing on those where glycogen synthesis has been modulated in animal models of the disease.

5.1 Introduction

Glycogen synthesis and degradation require several enzymes, with glycogen synthase and glycogen phosphorylase being key enzymes in elongating and shortening, respectively, glucose chains in the glycogen macromolecule (Fig. 5.1). Complex, reciprocal regulation of glycogen synthase and glycogen phosphorylase controls the amount of glycogen Mammalian glycogen metabolism has been studied most extensively in muscle and liver. In the muscle, glycogen serves as a fuel for contraction and as a storage depot for glucose removed postprandially from the circulation. In the liver, glycogen is synthesized to remove excess postprandial glucose from the bloodstream and subsequently degraded during times of fasting to provide glucose to the body. Glycogen metabolism has also been studied to a lesser degree in other tissues, notably heart (Taegtmeyer 2004) and kidney (Delaval et al. 1983), and especially of late, the brain (DiNuzzo and Schousboe 2019). This chapter focuses primarily on mammalian glycogen metabolism in muscle, liver, and brain. In addition to discussion of the enzymology and regulation of glycogen metabolism, select glycogen storage diseases are discussed, focusing on those where glycogen synthesis has been modulated in animal models of the disease.

Glycogen is stored throughout the body, with the largest reserves found in liver and skeletal muscle. The metabolic pathway for the synthesis of glycogen from glucose (glycogenesis) requires the sequential actions of glucose transport, hexokinase, phosphoglucomutase, UDP-glucose pyrophosphorylase, glycogen synthase, and glycogen branching enzyme. Glycogen synthase catalyzes the transfer of glucose from UDP-glucose to a growing glycogen chain and the glycogen branching enzyme introducing branches in the glycogen molecule. Regulation of synthesis is primarily controlled by glucose transport and modulation of glycogen synthase enzymatic activity, with activity generally being highest when glucose levels are relatively high. In contrast, when glucose levels are relatively low (liver) or when energy demands increase (muscle), degradation of glycogen is favored. There are two pathways for the breakdown of glycogen. One pathway (glycogenolysis) is catalyzed by glycogen phosphorylase and the glycogen debranching enzyme. The former enzyme sequentially removes glucose residues, as glucose-1-P, from a glycogen chain The latter enzyme both remodels the degrading glycogen molecule and releases the final unit from each chain as free glucose. Glucose-1-P is then acted on by phosphoglucomutase to form glucose-6-P. In liver, and to a lesser extent in kidney and small intestine, glucose-6-phosphatase converts this metabolite to free glucose which is released into the circulation to increase blood glucose levels. In muscle, glucose-6-P is metabolized in the glycolytic pathway to generate ATP for the cell. Glycogenolysis is regulated primarily through modulation of glycogen phosphorylase activity. A second pathway (glycophagy) also exists for glycogen degradation. in this pathway, glycogen is taken up into lysosomes and degraded to free glucose by the action of alpha-glucosidase.

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Ball State University/Indiana University School of Medicine-Muncie, Muncie, IN 47303

Figure 5.1. Glycogen metabolic pathway. GBE, glycogen branching enzyme; AGL, debranching enzyme; G6P, glucose-6-phosphate; G6Pase, glucose-6-phosphatase; GP, glycogen phosphorylase; GAA, alpha-glucosidase; G1P, glucose-1-phosphate; GLUT, glucose transporter; GN, glycogenin, GS, glycogen synthase; HK, hexokinase; PGM, phosphoglucomutase; UDPG, uridine diphosphate glucose; UGPPase, UDP-glucose pyrophosphosphatase; UMP, uridine monophosphate; UP, UDP-glucose pyrophosphorylase; UTP, uridine triphosphate. Hexagons depict glucose molecules. Illustration by Arielle Payne.

5.1.1 Role of Muscle Glycogen

Blood glucose maintenance. Increases in postprandial insulin levels promote glucose uptake into muscle cells and activation of glycogen synthase. In humans, this results in 26 to 35 percent of glucose from a meal being stored as glycogen in skeletal muscle (Taylor et al. 1993).

Exercise. The importance of muscle glycogen during exercise is borne out by both physiological and pathological studies. Impaired performance is observed in many patients with glycogen storage diseases that affect availability of muscle glycogen (Preisler et al. 2015). The benefit of muscle glycogen for exercise is well established and exploited by athletes (reviewed in Hargreaves 2015, Hawley et al. 2018, Jensen and Richter 2012, Nielsen et al. 2011). Saltin and colleagues conducted seminal studies where they found that exercise reduced muscle glycogen levels in contracting muscle (Bergstrom and Hultman 1966, 1967), muscle glycogen levels prior to exercise correlated with endurance exercise capacity (Bergstrom et al. 1967), the utilization of muscle glycogen was related to the intensity and duration of exercise (Hermansen et al. 1967), and the depletion of muscle glycogen was associated with exercise fatigue (Karlsson et al. 1974). Three pools of glycogen have been observed in muscle—intermyofibrillar, intramyofibrillar, and sub sarcolemmal—with each being proposed to serve a distinct function (Ortenblad and Nielsen 2015). In addition to serving as a source of ATP to power muscle contraction, glycogen has been proposed to serve in a structural role as well as being a regulator of cell signaling (reviewed in Hawley et al. 2018, Hearris et al. 2018). The breakdown of muscle glycogen during exercise is stimulated by increases in the levels of inorganic phosphate and AMP and possibly increased calcium, all of which promote activation of glycogen phosphorylase (reviewed in Jensen and Richter 2012).

5.1.2 Role of Liver Glycogen

Blood glucose maintenance. The primary role of glycogen metabolism in the liver is in the maintenance of blood glucose levels. During times of excess glucose in the blood stream, such as after a carbohydrate-containing meal, the combination of elevated glucose and insulin levels promotes glucose transport into the liver, glucose phosphorylation, and synthesis of glycogen (Roach et al. 2001). The synthesis of liver glycogen after ingestion of a meal is either from glucose released during digestion, termed the direct pathway, or alternatively glycogen is synthesized from gluconeogenic substrates, termed the indirect pathway (Kurland and Pilkis 1989). In humans, approximately 19 percent of the glycogen synthesized after a meal is stored in the liver (Taylor et al. 1996). When blood glucose levels drop, such as after a period of fasting, insulin levels decrease, and glucagon levels increase. These changes promote liver glycogen degradation and release of glucose into the blood stream to provide fuel to other tissues of the body (Roach et al. 2001).

Exercise. While the role of muscle glycogen in exercise has received much study, the importance of glycogen in the liver for exercise has garnered less attention. Exercise-induced hepatic glucose output from gluconeogenesis and glycogenolysis is regulated by a decrease in hepatocyte energy status in conjunction with changes in levels of circulating hormones including increased glucagon, decreased insulin, and possibly elevated catecholamines (Gonzalez et al. 2016, Trefts et al. 2015). The glucose released from liver glycogen stores can be used by muscle cells to fuel contraction. Significant liver glycogen depletion impairs the body’s ability to maintain blood glucose levels and is generally accepted to be a major cause of endurance exercise fatigue (Gonzalez et al. 2016, Hearris et al. 2018).

5.1.3 Role of Brain Glycogen

As compared to muscle and liver, glycogen metabolism in brain is less well studied, due in part to the relatively low concentrations of glycogen found in this organ (Brown 2004). However, brain glycogen stores have been proposed to play a role in diverse processes such as sleep regulation (Kong et al. 2002, Petit et al. 2015), exercise (Matsui et al. 2012), and learning (Bak et al. 2018), as well as in pathological situations such as seizure (Cloix and Hevor 2009, Dalsgaard et al. 2007), ischemia (Fern 2015, Rossi et al. 2007), hypoglycemia (Brown and Ransom 2015, Suh et al. 2007b), and hypoxia (Czech-Damal et al. 2014, Lopez-Ramos et al. 2015).

Learning. Brain glycogen metabolism has been proposed to contribute to normal neural function. Notably, the majority of the glycogen is found in astrocytes with the highest levels in areas of high synaptic density (Koizumi and Shiraishi 1970a ,b, Phelps 1972), suggesting that astrocytic glycogen may be involved in neuronal activity. Brain glycogen is mobilized during memory formation in day-old chicks and is reported to be regulated by noradrenaline and serotonin (reviewed in Gibbs 2015). In addition to serving as a source of rapid ATP generation, brain glycogen may act as a preferred substrate for glutamate synthesis (Gibbs et al. 2006, Gibbs et al. 2007) Inhibition of glycogen mobilization in day-old chicks impaired learning ability (Gibbs et al. 2006). Additionally, rats treated with an inhibitor of glycogen phosphorylase had impairments in long-term memory, but not short-term memory in one study (Suzuki et al. 2011), or had impairments of both long and short term memory in a different study (Newman et al. 2011). Most recently, mice with a brain specific disruption of brain glycogen synthase (GYS1) retained the ability to learn, though at a slower pace (Duran et al. 2013). These findings suggest a role for brain glycogen in learning, but our understanding remains incomplete.

Astrocytes play an active role in the memory process, and evidence suggests there is metabolic coupling between astrocytes and neurons. Besides the direct utilization of glucose by neurons and astrocytes, it has been proposed that astrocytes can provide fuel to neurons in the form of lactate produced from the metabolism of either glucose or glycogen (Chih and Roberts 2003). The shuttling of lactate from the astrocyte to the neuron is known as the astrocyte neuron lactate shuttle hypothesis (ANLSH) (Pellerin et al. 1998). In the context of the ANLSH model, glycogen in astrocytes could provide fuel to neurons through degradation to glucose-6-P and subsequent conversion to lactate through the glycolytic pathway. Lactate could be shuttled across the extracellular space to adjacent neurons where pyruvate generated from lactate would enter the TCA cycle to generate ATP for the neuron (Fig. 5.2). Lactate serves as a fuel in the brain, being preferred over glucose when both substrates are present (Wyss et al. 2011), and disruption of brain lactate transporter expression in rats resulted in learning impairments (Suzuki et al. 2011). Additionally, intrahi...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• Table of Contents
• Morphological and Structural Aspects of α-Glucan Particles from Electron Microscopy Observations
• Polarimetric Nonlinear Microscopy of Starch Granules: Visualization of the Structural Order of α-Glucan Chains within a Native Starch Particle
• Analyses of Covalent Modifications in α-Glucans
• Storage Polysaccharide Metabolism in Micro-Organisms
• Mammalian Glycogen Metabolism Enzymology, Regulation, and Animal Models of Dysregulated Glycogen Metabolism
• The Pathologies of a Dysfunctional Glycogen Metabolism
• Reversible Phosphorylation in Glycogen and Starch
• Starch Granules and their Glucan Components
• Regulation of Assimilatory Starch Metabolism by Cellular Carbohydrate Status
• Reserve Starch Metabolism
• Heteromeric Protein Interactions in Starch Synthesis
• Index