1. Sugar nucleotides and glycogen metabolism
Ferchmin 2019
Sugar nucleotides and glycogen metabolism
Index
3) Introduction to subject
4-7) Sugar nucleotides
8-9) Galactose and lactose
10-11) Aldose reductase, sorbitol, fructose and cataracts.
13-17) Synthesis and breakdown of glycogen
18-19) Glycogen storage diseases (GSD)

2. Objectives or what you ought to learn from this file
Know the ≠ forms o activated sugars and their metabolic roles.
Be able describe the metabolism of lactose & galactose
as well as lactose intolerance and the 3 types of galactosemias.
Be able to distinguish the ≠ between fructose and aldoses in the genesis of cataracts.
Understand the structure of the granule of glycogen.
Learn the synthesis and breakdown of glycogen including regulation and the genetic deficiencies associated.
Understand the GSD and the enzymes involved in each case
In general terms the objectives of all my lectures are explained in the slide 4 of this PPTX.
That is:
For every pathway you ought to know:
1) Purpose of the pathway. (Adaptive value for the organism).
2) Molecules going in and coming out? (The starting metabolites and the final products).
3) Place where it happens (organs, types of cell, subcellular compartments).
4) Regulatory enzymes. (Metabolic conditions that stimulate or inhibit the pathway).
5) Organization of the pathway and the formulas of the compounds involved. (The map of the pathway).
6) Relationship with other pathways. (Shared metabolites, enzymes and regulations).
7) Later, you will have to visualize each pathway interacting with other pathways in normal and in pathological conditions.

3. Introduction
The intermediates in glycolysis, pentose shunt, and gluconeogenesis are sugar-phosphate. However, in many metabolic steps, sugars are activated as nucleotides where the sugar is bound to a nucleotide through the anomeric hydroxyl as a phosphate ester.
 Sugar nucleotides are the substrates for polymerization into disaccharides, glycogen, and complex homo- or hetero-polysaccharides. They are also key intermediates in the metabolism of some sugars.
 Sugar nucleotides not only have the energy of the pyrophosphate bond that is needed to bind the monosaccharide covalently to another molecule but also the nucleotide is a label that defines a different sugar pool with a specific function. Glucose-6-P is a different pool of glucose than UDP-Glucose. Different enzymes recognize as substrate either the free monosaccharide or the nucleotide. In other words, free glucose, phosphorylated glucose, and UDP-glucose are different molecules destined to perform a particular function.
 The role of sugar nucleotides (specifically UDP-glucose) in the biosynthesis of glycogen and many other carbohydrate derivatives was discovered by Noble Laureate Dr. Luis Federico Leloir and his collaborators in Buenos Aires.

4. Several nucleotides can activate sugars
Several nucleotides can activate sugars. In the case of glucose for glycogen synthesis it is UTP.
The first reaction a), is reversible but it is coupled to b) that releases -5 kcal/mol and favors reaction a).
The first nucleotide-activated sugar was identified by Leloir, who established UDP-Glc as substrate for the polymerization of Glc to glycogen. UDP is the most common nucleotide used to activate sugars in animals, as it is found linked to Glc, Gal, GlcNAc, GalNAc, GlcA and Xyl. Only two additional nucleotides are used in animals, GDP that is linked to Man and Fucose, and CMP that is linked to Sialic acid in complex lipids like gangliosides or glycoproteins.

5. Structure of UDP-α-D-Gal and list of nucleotide-activated sugars found in animal cells.
You do not have to memorize the all the possible nucleotides of UDP ,GDP and CMP.
Nucleotide-activated sugars are involved in the biosynthesis of large molecules like glycans, oligosaccharides and polysaccharides. Furthermore, the nucleotide-activated sugar UDP-Glucuronic acid is also widely used for the detoxification of xenobiotics by UDP-glucuronosyltransferases (UGT), which are mainly expressed in the liver in humans. The addition of Glucuronic acid to apolar molecules bilirubin increases their solubility and thereby their elimination from the body.

6. Sugar nucleotides of glucose or of any other carbohydrate are called activated because sugars bound to nucleotides can be transferred by specific enzymes to proteins and other molecules or can be subjected to enzymatic modifications possible only with activated sugars.
UDP-glucuronic is important for detoxification of many drugs and metabolites. Below is shown the formula of conjugated bilirubin:
The synthesis of glucuronic acid is a minor route of glucose metabolism that generates less notorious sugars. The first step is the activation of glucose-1-phosphate to form UDP-glucose. UDP-glucose is then oxidized to UDP-glucuronic acid by NAD+ and UDP-glucose dehydrogenase.
UDP-glucuronic acid participates in biosynthetic reactions that involve condensation of glucuronic acid with a variety of molecules to form a glycoside, an ester, or an amide, depending on the nature of the acceptor molecule. The high-energy bond between UDP and glucuronic acid provides the energy to form the new bond in the product. UDP is hydrolyzed to UMP (uridine monophosphate) and inorganic phosphate, ensuring the irreversibility of the reaction.
Because glucuronic acid is highly polar, its conjugation with less polar compounds such as steroids, bilirubin, and many xenobiotics makes them more water-soluble and easier for renal excretion.
Glucuronic acid is a component of structural polysaccharides like hyaluronic acid and other connective tissue polysaccharides.
Some sulfated mucopolysaccharides  contain L-iduronate, a 5-epimer of D-glucuronate. D-Glucuronate already incorporated to the chain of a heteropolysaccharide is epimerized in carbon 5 by UDP-glucuronate-5-epimerase giving L-iduronic is synthesized that had UDP-glucuronate-5-epimerase activity—i.e., a fraction that converted the substrate to UDP-L-iduronate. There is also a UDP-N-acetyl-D-glucosamine-4-epimerase that make galactosamine. There are a plethora of other biosynthetic pathways that start with glucose which are minor in quantity but not in importance, among them the synthesis of inositol (not shown here).
We, primates, lost the ability to make ascorbate. Therefore we are genetically deficient.

8. Galactose metabolism Leloir pathway
Fructose is an ketose, therefore it
is not a substrate of aldose
reductase and causes no cataracts.
Glucose and galactose are aldoses
and they do cause cataracts
• Deficiency of galactokinase causes a mild form of galactosemia that causes cataracts.
• Deficiency of hexose-1-phosphate-uridylyltransferase causes the sever type of galactosemia with liver failure, mental retardation, and cataracts. The name galactose-1-phosphate-uridylyltransferase (in the drawing) is wrong but commonly used including in the NBE. The correct name is hexose-1-phosphate uridylyltransferase because it transfers equally well glucose as it does galactose. Therefore, the name in the fig is not in agreement with the commission for enzyme nomenclature none the less is used in most books and the NBE.
• The 4 epimerase uses NAD+ as cofactor, and the transition state is 4 keto hexose.
• Type III galactosemia is caused by mutations in the gene encoding UDP-glucose-4-epimerase. A variety of different point mutations located throughout the gene is responsible. The principal, disease-causing effects of these mutations appear to be a reduction in the catalytic rate constant and instability of the protein. Galactosemia III is very rare, and the symptoms vary depending on the mutation(s).
Why the complete loss of function of UDP-glucose-4-epimerase does not exist?

9. Lactose synthesis is mediated by a galactosyl transferase that binds galactose with N-acetyl-glucosamine to give N-acetyllactosamine, a constituent of several oligosaccharides.
At the onset of lactation a protein specific to the mammary gland increases the affinity of this enzyme for glucose from 1 molar to 1 mM. Lactose is metabolized by LACTASE or β-galactosidase. This enzyme should be lost after weaning. However, many ethnic groups keep β-galactosidase for life. The biology of lactose and lactase is very interesting and clinically relevant.
Lactose intolerance is the “normal condition of mature humans”. Lifelong lactose tolerance was acquired by ethnic groups that domesticated milk giving animals.
β1,4 Galactosyltransferase is unique among all glycosyltransferases in that its substrate specificity can
be modified by α-lactalbumin. Together, ß1,4 galactosyltransferase and α-lactalbumin form
the lactose synthase complex. Because α-lactalbumin is only expressed in the mammary gland,
lactose synthesis only occurs in the mammary gland. In addition, expression of the α-lactalbumin
gene is closely regulated by hormones, so that lactose synthesis only occurs during the lactating
state of the tissue and lactation fails if α-lactalbumin is not express.

10. Excess of glucose can deplete NADPH+H+ and cause opacity of the lens (cataracts).
Conversion of glucose to sorbitol can cause a problematic osmotic disturbance.  The polyol pathway occurs physiologically in the male reproductive tract by the seminal vesicles.  In the first step, glucose is converted to sorbitol by aldose reductase.  The sorbitol formed is then oxidized to fructose.
A problem arises when sorbitol production occurs in tissues by reduction of hexoses At high glucose levels, like in diabetes, aldose reductase activity in organs which lack sorbitol dehydrogenase leads to an accumulation of sorbitol. In the lens of the eye, the built up of sorbitol contributes to cataract formation due to osmotic disturbance. In the lens even more damaging is the depletion of NADPH by the synthesis of sorbitol.
The sperm gets its fructose through this pathway.
Why the sperm needs fructose and not glucose?
Sorbitol (and galactitol) cause cataracts and osmotic disturbances

11. Similarities between galactosemia and diabetes mellitus
Galactosemia causes several alterations in metabolism. Phosphate is sequestered in the form of galactose-1-phosphate and causes hypophosphatemia. The unavailability of phosphate inhibits glycogenolysis, respiratory chain, and glycolysis. In addition to galactose-1-P, free galactose accumulates too. All ALDOSES are substrates of aldose reductase (fructose is a ketose; therefore, it is not a substrate of aldose reductase). The products of aldose reductase are the corresponding sugar alcohols. In galactosemia, galactitol accumulates and in diabetes, sorbitol (also known by a more rationally but seldom used name of glucitol). Both sugar alcohols, when in excess, cause similar damage. Aldose reductase consumes NADPH+H+ needed to maintain the transparency of the lens; therefore, the synthesis of sorbitol contributes to cataract formation. Sorbitol per se alters the osmotic balance and promotes the formation of cataracts. Aldose reductase is a target of new drugs for diabetic patients. Sorbitol is needed for osmotic compensation in kidneys and participates in the metabolism of fructose in the male seminal track, but it causes untoward effects in the lens and peripheral nerves. In liver and seminal vesicles, there is sorbitol dehydrogenase (NAD-dependent) that forms fructose. Fructose and citrate are the primary sources of energy for the sperm. The cyclic form of glucose or any other aldose is an unreactive hemiacetal. However, through mutarotation, aldoses are in equilibrium with the open aldehyde. The aldehyde reacts with the (epsilon) ε-amino of lysine and the free amino-terminal of proteins. Glycosylation of proteins by glucose is initiated by a Schiff base formation followed by Amadori (reportedly you saw this with Dr. Hann) rearrangement with the regeneration of the carbonyl group in a different carbon. Hemoglobin HbA1c is glycosylated hemoglobin. In the blood of none diabetics, it accounts for about 6 % of total hemoglobin. In diabetic patients, HbA1c reflects the degree of glucose control over the last 120 days. The initial glycosylated protein could cross­ link proteins by a regeneration of the carbonyl group followed by sequential reaction with other proteins forming a long half-life molecule.

12. Glycogen Metabolism

13. Synthesis of uridine diphosphoglucose or UDPGlu
Synthesis of Glycogen
Synthesis of uridine diphosphoglucose or UDPGlu
Do you remember phosphoglyceromutase? Any similarities with phosphoglucomutase?
Understand the role of UDP-glucose pyro-phosphorylase in the synthesis of UDP-Glucose, the immediate precursor of glycogen synthesis.
This slide was already explained at the beginning of the presentation.
PPi is hydrolyzed by a pyrophosphorylase in a reaction coupled with the pyrophosphorylase
to dissipate energy as heat thus making the synthesis of UDP-Glu thermodynamically favorable.

14. Why do we need to waste 2 ATPs and make glycogen?
Deficiency of branching enzyme gives long branches. Causes death at about to years of age. Andersen’s disease
The content of glycogen is about 10 % of the wet weight of the liver and 2% of muscle.
The synthesis of glycogen starts from UDP-glucose and a glycogenin attached primer. Glycogenin is a glycosyltransferase that using UDP-glucose as glucose donor initiates glycogen biosynthesis by first binding glucose from UDP-glucose to the hydroxyl of Tyr-194 by glycogenin's glucosyltransferase activity. Glycogenin grows an oligosaccharide of 8 glucose which is then farther elongated by glycogen synthase and the branching enzyme.
Glycogenin covalently linked to the reducing end of glycogen remains in the core of the glycogen granule until it is degraded in a lysosome. 
The immediate precursor of glycogen elongation is UDP-Glu, not free glucose, glucose-1-P or 6-P.

15. Glycogen degradation
In the absence of glucose-6-phosphatase, neither Glc-6P nor free Glc leaves the liver, and a Glc-6-P accumulates causing the von Gierke disease.
The breakdown of glycogen and entry into glycolysis as glucose-6-P is achieved by three enzymes: glycogen phosphorylase, debranching enzyme and phosphoglucomutase.
Glycogen phosphorylase produces glucose-1-P plus limit dextrin.
The debranching enzyme has a transferase and glycosidase (hydrolase) activities.
Hexokinase is bypassed when glucose comes from glycogen!
Deficiency of phosphorylase (Mc Ardle’s) causes muscle cramps and no lactate formation during exercise.
Diseases related to this pathway:
Glucose-6-P phosphatase causes von Gierke. von Gierke causes hypoglycemia, liver damage and related health issues.
Why dosing with glucagon, fructose, galactose or glycerol does not significantly alleviate the hypoglycemia?
Deficiency of debranching enzyme causes accumulation of limit dextrin (Cori disease)
In the next slide, we will discuss the regulation of glycogen phosphorylase and synthase by allosteric interactions with metabolites and Ca+2. Later we will see how epinephrine and glucagon activate protein kinase A (PKA) and regulate glycogen synthesis and breakdown by phosphorylation.

16. Glycogen phosphorylase and synthase are also regulated by allosteric interactions with metabolites
The positive and negative modulators of glycogen metabolism in liver and muscle are easy to understand because of their adaptive value. No memorization is needed if the rationale is understood.

17. The granule of glycogen
The carbon 4 (non-reducing) terminals are the sites involved in both synthesis and phosphorolysis of glycogen. The degree of branching is important for rapid release of glucose from glycogen. Glycogen forms a granule that can be seen with a simple microscope. The glycogen granule has all the enzymes needed for the synthesis, break down and regulation of glycogen metabolism.
Starch, the “plant glycogen” is less branched than glycogen because grass does not have the capacity to run away from the cow.
Notice the glycogenin in the center. Drawing from Wikipedia.

19. There are nine glycogen storage diseases (GSD) although there is no consensus about the exact number of them.
I will not lecture specifically about the GDS but you are responsible to study them. Last time I checked the most clinically oriented and stress-free site to study GSDs is the webpage of the Association for Glycogen Storage Diseases. The link below will take you to relevant information.
In the next “handout” we will begin with regulatory mechanisms involved in glycogen metabolism
We did review the following GSDs: Andersen’s, von Gierke, McArdle’s, and Cori diseases.