Glycogen Breakdown Oxidative/Reductive Roles Relative to Cellular Energy
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Chapter 7 Metabolism II In this second section of metabolism, we cover metabolic pathways that do not have a strong emphasis on oxidation/reduction. Metabolism II Carbohydrate Storage and Breakdown In the last chapter, we focused on metabolic pathways that played important Glycogen Breakdown oxidative/reductive roles relative to cellular energy. In this chapter, the pathways Regulation of Glycogen Metabolism that we cover have lesser roles from an energy perspective, but important roles, GPa/GPb Allosteric Regulation nonetheless, in catabolism and anabolism of building blocks of proteins and GPa/GPb Covalent Conversion nucleic acids, nitrogen balance, and sugar balance. In a sense, these might be Turning Off Glycogen Breakdown thought of as the “kitchen sink” pathways, but it should be noted that all cellular Glycogen Synthesis pathways are important. Regulation of Glycogen Synthesis Maintaining Blood Glucose Levels Carbohydrate Storage/Breakdown Pentose Phosphate Pathway Carbohydrates are important cellular energy sources. They provide energy quickly Calvin Cycle through glycolysis and passing of intermediates to pathways, such as the citric C4 Plants acid cycle, amino acid metabolism (indirectly), and the pentose phosphate Urea Cycle pathway. It is important, therefore, to understand how these important molecules Nitrogen Fixation are made. Amino Acid Metabolism Plants are notable in storing glucose for energy in the form of amylose and Amino Acid Catabolism amylopectin (see HERE) and for structural integrity in the form of cellulose (see Nucleotide Metabolism HERE). These structures differ in that cellulose contains glucoses solely joined by Pyrimidine de novo Biosynthesis beta-1,4 bonds, whereas amylose has only alpha1,4 bonds and amylopectin has Purine de novo Biosynthesis alpha 1,4 and alpha 1,6 bonds. Deoxyribonucleotide de novo Biosynthesis 173 Animals store glucose primary in liver and muscle in the form of a Breakdown of glycogen involves 1) release of glucose-1- compound related to amylopectin known as glycogen. The phosphate (G1P), 2) rearranging the remaining glycogen (as necessary) to permit continued breakdown, and 3) conversion of G1P to G6P for further metabolism. G6P can be 1) broken down in glycolysis, 2) converted to glucose by gluconeogenesis, and 3) oxidized in the pentose phosphate pathway. Just as in gluconeogenesis, the cell has a separate mechanism for glycogen synthesis that is distinct from glycogen breakdown. As noted previously, this allows the cell to separately control the reactions, avoiding futile cycles, and enabling a process to occur efficiently (synthesis of glycogen) that would not occur if it were The Repeating Structure of Cellulose simply the reversal of glycogen breakdown. structural differences between glycogen and amylopectin are Synthesis of glycogen starts with G1P, which is converted to an solely due to the frequency of the alpha 1,6 branches of glucoses. 'activated' intermediate, UDP-glucose. This activated In glycogen they occur about every 10 residues instead of every intermediate is what 'adds' the glucose to the growing glycogen 30-50, as in amylopectin. Glycogen provides an additional source of glucose besides that produced via gluconeogenesis. Because glycogen contains so many glucoses, it acts like a battery backup for the body, providing a quick source of glucose when needed and providing a place to store excess glucose when glucose concentrations in the blood rise. The branching of glycogen is an important feature of the molecule metabolically as well. Since glycogen is broken down from the "ends" of the molecule, more branches translate The Repeating Unit of Glycogen to more ends, and more glucose that can be released at once. 174 chain in a reaction catalyzed by the enzyme known as glycogen synthase. Once the glucose is added to glycogen, the glycogen molecule may need to have branches inserted in it by the enzyme known as branching enzyme. Glycogen Breakdown Glycogen phosphorylase (sometimes simply called phosphorylase) catalyzes breakdown of glycogen into Phosphorolysis of Glycogen Glucose-1-Phosphate (G1P). The reaction, (see above enzyme also catalyzes the hydrolysis of the remaining glucose at right) that produces G1P from glycogen is a phosphorolysis, not a the 1,6 branch point. Thus, the breakdown products from hydrolysis reaction. The distinction is that hydrolysis glycogen are G1P and glucose (mostly G1P, reactions use water to cleave bigger molecules into however). Glucose can, of course, be converted to smaller ones, but phosphorolysis reactions use See Kevin’s YouTube lectures Glucose-6-Phosphate (G6P) as the first step in phosphate instead for the same purpose. Note that on Glycogen Metabolism glycolysis by either hexokinase or glucokinase. the phosphate is just that - it does NOT come from HERE, HERE, and HERE G1P can be converted to G6P by action of an ATP. Since ATP is not used to put phosphate on enzyme called phosphoglucomutase. This reaction G1P, the reaction saves the cell energy. is readily reversible, allowing G6P and G1P to be interconverted Glycogen phosphorylase will only act on non-reducing ends of a as the concentration of one or the other increases. This is glycogen chain that are at least 5 glucoses away from a branch important, because phosphoglucomutase is needed to form G1P point. A second enzyme, Glycogen Debranching Enzyme (GDE), for glycogen biosynthesis. is therefore needed to convert alpha(1-6) branches to alpha(1-4) Regulation of Glycogen Metabolism branches. GDE acts on glycogen branches that have reached Regulation of glycogen metabolism is complex, occurring both their limit of hydrolysis with glycogen phosphorylase. GDE acts to allosterically and via hormone-receptor controlled events that transfer a trisaccharide from a 1,6 branch onto an adjacent 1,4 result in protein phosphorylation or dephosphorylation. In order branch, leaving a single glucose at the 1,6 branch. Note that the to avoid a futile cycle of glycogen synthesis and breakdown 175 simultaneously, cells have evolved an elaborate set of controls GPa and GPb can each exist in an 'R' state and a 'T' state. For that ensure only one pathway is primarily active at a time. both GPa and GPb, the R state is the more active form of the enzyme. GPa's negative allosteric effector (glucose) is usually not Regulation of glycogen metabolism is managed by the enzymes abundant in cells, so GPa does not flip into the T state often. glycogen phosphorylase and There is no positive allosteric glycogen synthase. Glycogen effector of GPa, so when glucose phosphorylase is regulated by is absent, GPa automatically flips both allosteric factors (ATP, G6P, into the R (more active) state. AMP, and glucose) and by covalent modification GPb can convert from the T state (phosphorylation/ to the GPb R state by binding dephosphorylation). Its regulation AMP. Unless a cell is low in is consistent with the energy energy, AMP concentration is low. needs of the cell. High energy Thus GPb is not converted to the substrates (ATP, G6P, glucose) R state very often. On the other allosterically inhibit GP, while low hand, ATP and/or G6P are usually energy substrates (AMP, others) present at high enough allosterically activate it. concentration in cells that GPb is readily flipped into the T state. GPa/GPb Allosteric Regulation GPa/GPb Covalent Regulation Glycogen phosphorylase exists in two different covalent forms – one Because the relative amounts of Regulation of Glycogen Phosphorylase form with phosphate (called GPa GPa and GPb largely govern the here) and one form lacking overall process of glycogen phosphate (GPb here). GPb is converted to GPa by breakdown, it is important to understand the controls on the phosphorylation by an enzyme known as phosphorylase kinase. enzymes that interconvert GPa and GPb. This is accomplished by 176 the enzyme Phosphorylase Kinase, which transfers phosphates can remove phosphates from phosphorylase kinase (inactivating from 2 ATPs to GPb to form GPa. Phosphorylase kinase has two it) AND from GPa, converting it to the much less active GPb. covalent forms – phosphorylated (active) and dephosphorylated (inactive). It is phosphorylated by the enzyme Protein Kinase A Glycogen Synthesis (PKA). Another way to activate the enzyme is with calcium. The anabolic pathway contrasting with glycogen breakdown is Phosphorylase kinase is dephosphorylated by the same enzyme, that of glycogen synthesis. Just as cells reciprocally regulate phosphoprotein phosphatase, that removes phosphate from GPa. glycolysis and gluconeogenesis to prevent a futile cycle, so too do cells use reciprocal schemes to regulate glycogen breakdown PKA is activated by cAMP, which is, in turn produced by and synthesis. Let us first consider the steps in glycogen adenylate cyclase after activation by a G-protein. G-proteins are synthesis. 1) Glycogen synthesis from glucose involves activated ultimately by binding of ligands to specific 7-TM phosphorylation to form G6P, and isomerization to form G1P receptors, also known as G-protein coupled receptors. These are (using phosphoglucomutase common to glycogen breakdown). discussed in greater detail in Chapter 8. Common ligands for G1P is reacted with UTP to form UDP-glucose in a reaction these receptors include epinephrine (binds beta-adrenergic catalyzed by UDP-glucose pyrophosphorylase. Glycogen receptor) and glucagon (binds glucagon receptor). Epinephrine synthase catalyzes synthesis of glycogen by joining carbon #1 of exerts it greatest effects on muscle and glucagon works the UDPG-derived glucose onto the carbon #4 of the non- preferentially on the liver. reducing end of a glycogen chain. to form the familiar alpha(1,4) Turning Off Glycogen Breakdown glycogen links. Another product of the reaction is UDP. Turning OFF signals is as important, if not more so, than turning It is also worth noting in passing that glycogen synthase will only them ON. The steps in the glycogen breakdown regulatory add glucose units from UDPG onto a preexisting glycogen chain pathway can be reversed at several levels. First, the ligand can that has at least four glucose residues. Linkage of the first few leave the receptor.