1. Sugar nucleotides and glycogen metabolism
Ferchmin 2018
Sugar nucleotides and glycogen metabolism
Index by slide numbers
1) Title and index.
2) Introduction to subject
3-5) Sugar nucleotides
6-7) Galactose and lactose
8-10) Aldose reductase, sorbitol, fructose and cataracts.
11-14) Synthesis and breakdown of glycogen
15-16) Glycogen storage diseases (GSD)

2. 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 bond 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 covalently the monosaccharide to a different 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. Enzymes will 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.

3. 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).

4. 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:
We, primates, lost the ability to make ascorbate. Therefore we are genetically deficient.

6. 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-uridylyl transferase causes the sever type of galactosemia with liver failure, mental retardation and cataracts. The name galactose-1-phosphate-uridylyl transferase (in drawing) is simply wrong but commonly used including in the NBE. The correct name is hexose-1-phosphate uridylyl transferase. This is so 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 but it is used in most books and in 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 main, 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).

7. 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 is very interesting
and clinically relevant. A clinical exercise about lactose intolerance used to be done before .You will have to do it as a self study.
ß1,4 Galactosyltransferase is unique among all glycosyltransferases in that its substrate specificity can
be modified by addition of a-lactalbumin. Together, ß1,4 galactosyltransferase and a-lactalbumin form
the lactose synthase complex. Because a-lactalbumin is only expressed in the mammary gland,
lactose synthesis only occurs in the mammary gland. In addition, expression of the a-lactalbumin
gene is closely regulated by hormones, so that lactose synthesis only occurs during the lactating
state of the tissue.

8. Excess of glucose can deplete NADPH+H+ and cause opacity of the lens (cataracts).
Conversion of glucose to sorbitol through the polyol pathway can cause a problematic osmotic disturbance.  The polyol pathway occurs typically in the male reproductive tract by the seminal vesicles but not in other tissues.  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 other than seminal vesicles . 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

9. Glycosylation (glycation) of hemoglobin and formation of HbA1c
Glycation (sometimes called non-enzymatic glycosylation) is the result of the covalent bonding of a sugar molecule,
such as glucose or fructose, (with a potentially free carbonyl grop) to a protein or lipid molecule, without the controlling
action of an enzyme.
The degree of glycation of the body's proteins is related to blood glucose levels.  Hemoglobin has a half-live of about 100 days.  The degree of glycation of hemoglobin gives, therefore, a picture of average blood glucose levels for the previous three month period.   Glycated hemoglobin is known as HbA1c.  Normally, HbA1c accounts for approximately 5-6 % of the total hemoglobin.  Diabetic patients often have HbA1c in excess of 8-10%.  It has been usually assumed that glycation of hemoglobin followed reaction with glucose, as shown in the next figure.  Current studies have, as noted above, indicated that other reactive species, especially methylglyoxal, may be involved in this process.  Note that the degree of glycation of hemoglobin is an indication of the extent of glycation of many of the body's proteins and possible ensuing cellular damage. 
How is it possible for a
diabetic patient to have abnormally low HbA1C?
Will you be good at making diagnosis?

10. Similarities between galactosemia and diabetes mellitus
Galactosemia causes several alterations in the metabolism. Phosphate is sequestered in the form of galactose-1-phosphate. This 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 helps to 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 regeneration of the carbonyl group in a different carbon. Hemoglobin HbA1c is glycosylated hemoglobin. In normal blood, it accounts for about 6 % of total hemoglobin. In diabetic patients, it 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.
Glucose, generates no-enzymatically glyoxal and methylglyoxal with two carbonyl groups which react with proteins, RNA and DNA to form stable addition products. This is non-enzymatic glucosylation (or glycation) is a normal process which follows reaction of the carbonyl compounds with.  The process and the rate of reaction is proportional to the concentration of glucose in the blood.   The glycosylated proteins or nucleic acids undergo cross-linking, forming "AGE" or advanced glycation end-products.  There are several enzymes that protect the cell by removing the glycosylation groups and there seem to be a race between the glycosylation and the removal of the bound harmful groups. This is a hot topic for diabetes, aging, and other medical problems but I do not know whether it is reflected in the NBE

11. Glycogen Metabolism

12. 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.
For the sake of reviewing the synthesis of glycogen starts from glucose and a glycogenin attached primer
Glycogenin is a glycosyl transferase that have the ability of autolocalization using UDP-glucose and initiates glycogen biosynthesis by first binding glucose from UDP-glucose to the hydroxyl of Tyr-194 from UDP-glucose, by glycogenin's glucosyltransferase. Glycogenin growth an oligosaccharide made by 8 glucoses which is elongated by glycogen synthase.
The immediate precursor of glycogen elongation is UDP-Glu.  

13. Glycogen degradation
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 Adler’s) causes muscle cramps and no lactate formation during exercise.
Diseases related to this pathway:
Glucose-6-P phosphatase causes von Gierke. Why dosing with glucagon, fructose, galactose or glycerol does not significantly increase glycemia?
Deficiency of debranching enzyme causes accumulation of limit dextrin

14. We will see later that PKA activation by epinephrine and glucagon regulates glycogen synthesis and breakdown by phosphorylation. However, glycogen phosphorylase and synthase are also regulated by allosteric interactions with metabolites.

16. There are 10 (ten) 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. The most clinically oriented and stress-free site to study GSDs is the webpage of the Association for Glycogen Storage Disease. The link takes you directly to relevant information.
In the next “handout” we will begin with regulatory mechanisms involved in glycogen metabolism