1. Carbohydrate Metabolism
Lai-Chu Wu, D. Phil. Department of Molecular and Cellular Biochemistry
for profile

2. Lecture Outline
Part One Digestion of carbohydrates and glycolysis. The pathway, key enzymes, energy, and regulation.
Part Two Gluconeogenesis. The key steps and enzymes between pyruvate and glucose, Cori cycle and the regulation of glycolysis and gluconeogenesis.
Part three Glycogen metabolism. Important facts about glycogen, its structure, synthesis and degradation.

3. Carbohydrate Metabolism and Glycolysis
Part I
Carbohydrate Metabolism and Glycolysis

4. Objectives
Know the general structure of glucose, describe the pathway of glycolysis and the general chemical structures (that is functional groups) involved.
Understand how glycolysis requires an initial investment of energy, but results in a net gain of energy.
Explain how a molecule of glucose can produce two molecules of pyruvate.
Give how availability of oxygen determines the conversion of pyruvate into lactic acid or acetyl coenzyme A.
Know how glycolysis is regulated.

5. Carbohydrates: Introduction
Carbohydrates are the most abundant organic molecule in nature.
Major sources of dietary carbohydrates
Starch, from ingested plant cells
Glycogen, major form of carbohydrate storage in animals
Lactose (disaccharide of glucose and galactose) in milk and fructose & sucrose in fruits
Function:
Provide energy for our body
Give structure to cell walls and cell membranes
Serve as metabolic intermediates to amino acids and lipids
Components of nucleotides

6. Major pathways in carbohydrate metabolism
Citric acid cycle
acetyl-coenzyme A
When energy is needed, glucose proceeds through glycolysis to pyruvate, which yields a small amount of energy
Pyruvate can be converted to acetyl-coenzyme A, enters the citric acid cycle and yields a large amount of energy.
Under anaerobic conditions, pyruvate is converted into lactate
Fate of glucose 6-phosphate
When the cells are well supplied with glucose and energy, glucose is stored by conversion
to glycogen (glycogenesis),
to fatty acids or nonessential amino acids.
To ribose and NADPH for nucleotide synthesis
When the cells need energy, glucose is converted
to pyruvate (glycolysis) and then
enters the citric acid cycle.

7. Major pathways in carbohydrate metabolism continued
Citric acid cycle
acetyl-coenzyme A
Glycogen, the storage form of glucose in animals, is synthesized by glycogenesis when glucose levels are high and degraded by glycogenolysis when glucose is in short supply.
Glucose can also be synthesized from noncarbohydrate precursors (amino acids from proteins and glycerol from lipids) by gluconeogenesis.
Glucose can also enter the pentose phosphate pathway, which yields NADPH (for reductive biosynthesis) and ribose 5-phosphate (for synthesis of nucleic acids).
Fate of glucose 6-phosphate
When the cells are well supplied with glucose and energy, glucose is stored by conversion
to glycogen (glycogenesis),
to fatty acids or nonessential amino acids.
To ribose and NADPH for nucleotide synthesis
When the cells need energy, glucose is converted
to pyruvate (glycolysis) and then
enters the citric acid cycle.

8. Digestion of Carbohydrates
Carbohydrate (CHO) digestion begins in the mouth where salivary α-amylase breaks down polysaccharides into smaller polysaccharides and disaccharides.
Limited CHO digestion occurs in the stomach as α-amylase is inactivated by stomach acid.
Continues in the small intestine where pancreatic α-amylase together with maltase, sucrase and lactase released from the mucous lining hydrolyzes polysaccharides into monosaccharides (mainly glucose, fructose and galactose), which enter the bloodstream and transport to the cells.

9. Glycolysis - Overview
Glycolysis, an ancient and linear metabolic pathway, is found in almost all organisms and occurs in the cytosol of every cell in our body.
In glycolysis, a glucose molecule is split by a series of 10 enzymatic reactions into two molecules of pyruvate, and results in the net production of ATP, which fuels the body’s cells.

10. Glucose Activation Conversion of glucose into glucose-6-phosphate
When glucose enters a cell from the bloodstream, it is immediately converted to glucose 6-phosphate.
Glucose 6-phosphate is trapped within the cell because phosphorylated molecules are negatively charged cannot cross the cell membrane.
The formation of glucose-6-phosphate is highly exergonic and is not reversible.
Conversion of glucose into glucose-6-phosphate

11. The glycolytic pathway
Overview of the glycolytic pathway
Glycolysis is a series of 10 enzyme-catalyzed reactions.
Steps 1-5 of glycolysis break one glucose molecule down into two glyceraldehyde 3-phosphate. An investment of 2 ATP molecules is required.
Steps 6-10 end at pyruvate and yield 4ATPs and 2 NADH

12. Step 1 – conversion of glucose to G6P
Glucose reacts with ATP to yield glucose 6-phosphate and ADP in a reaction catalyzed by hexokinase.
You do not need to memorize all the structures. I will highlight the functional groups involved for you to follow the reactions.
Glucose reacts with ATP to yield glucose 6-phosphate and ADP in a reaction catalyzed by hexokinase.

13. Hexokinase versus Glucokinase
There are four mammalian hexokinase isoforms that have different enzyme kinetics with respect to different substrates and conditions, and functions
Some isoforms have high affinity to glucose allowing cells such as brain and muscle to obtain glucose even at low glucose concentrations
Glucokinase displays positive cooperativity with glucose, and functions when glucose concentration is high.

14. Hexokinase
In most tissues, the phosphorylation of glucose to G6P is catalyzed by hexokinase, one of the three regulatory enzymes in glycolysis
inhibited by its product G6P
Has a low Km (0.1 mM, i.e. high affinity) for glucose, thus it is operating at Vmax under physiological blood glucose of ~5 mM
Activity does not change with blood glucose levels

15. Glucokinase Glucokinase is an isoform of hexokinase
In liver parenchymal cells and islet cells of the pancreas, glucokinase is the predominant enzyme responsible for the phosphorylation of glucose.
has a high Km of 10 mM and Vmax which enables it to handle the high concentration of glucose that is present in the portal venous blood after a meal
Not inhibited by G6P, allowing continue removal and “trap” glucose from blood as G6P.

16. Step 2 – Isomerization of G6P to F6P
Glucose 6-phosphate (aldose/aldohexose) is converted to fructose 6-phosphate (ketose/ketohexose).
The result is conversion of the six-membered glucose ring to a five-membered ring with a CH2OH group, which prepares the molecule for addition of another phosphate group in the next step.
Aldehyde
alcohol
Alcohol
ketone
Isomers are compounds with the same molecular formula but different structures
Glucose and fructose have the same formula C6H12O6

17. Addition of PO4 to –OH in the next step
Structures of G6P to F6P
aldehyde
Isomers are compounds with the same molecular formula but different structures
Glucose and fructose have the same formula C6H12O6
ketone
Addition of PO4 to –OH in the next step

18. Step 3 - Phosphorylation of F6P by phosphofructokinase
Fructose 6-phosphate reacts with ATP to yield fructose 1,6-bisphosphate plus ADP. Phosphofructokinase (PFK), the enzyme for step 3, provides a major control point in glycolysis.
When the cell is short of energy, [AMP/ADP] builds up and activates phosphofructokinase.
When energy is in good supply, [ATP] and [citrate] build up and inhibit this enzyme.
Fructose 6-phosphate reacts with ATP to yield fructose 1,6-bisphosphate plus ADP. Phosphofructokinase, the enzyme for step 3, provides a major control point in glycolysis.

19. Energy investment stage of glycolysis
Stage 1 of glycolysis is sometime called the energy investment phase in which 2 ATPs are consumed.
Can you recall what are the two reactions that take up an ATP? the name of the enzymes, and what is common in the names of these enzymes?
Stage stage 1

20. Steps 4 and 5 – Cleavage and Isomerization
Step 4. The six-carbon chain of fructose 1,6- bisphosphate is cleaved between C3 and C4 into two three-carbon pieces by the enzyme aldolase.
Step 5. Dihydroxyacetone phosphate is isomerized by the enzyme triose phosphate isomerase to glyceraldehyde 3-phosphate. 4 5
Step 4. The six-carbon chain of fructose 1,6- bisphosphate is cleaved into two three-carbon pieces by the enzyme aldolase.
Step 5. Dihydroxyacetone phosphate is isomerized by the enzyme triose phosphate isomerase to glyceraldehyde 3-phosphate.

21. Stage I of glycolysis - Recap
Glucose is phosphorylated twice and cleaved to form two molecules of glyceraldhyde-3-phosphate (G3P)
Two ATP are consumed, and hence stage 1 is called the “investment” phase
G3P is a high energy molecule “trapped” inside cell, ready for stage 2 of glycolysis (the energy generating stage)

22. Step 6 -10, stage 2 and energy generating phase - Overview
Starting from G3P and through 5 steps generate pyruvate
It is called the energy generating phase because 2 ATPs and a NADH/H+ are produced.
Since one glucose generates two G3P, in all stage 2 generates 2 pyruvate, 4 ATP, and 2 NADH/H+ 7

23. Step 6 - Oxidation & Phosphorylation by glyceraldehyde 3-phosphate dehydrogenase
G3P is converted to 1,3BPG
Two reactions actually occur:
aldehyde group in G3P is first oxidized to a carboxylic acid and
phosphorylated by glyceraldehyde 3-phosphate dehydrogenase.
The oxidation reaction (G3P to 1,3BPG) is coupled to the reduction of NAD+ to NADH/H+
the source of phosphate is inorganic phosphate, but not ATP, so this phosphorylation step does not consume ATP
acid
Glyceraldehyde 3-phosphate forms 1, 3-bisphosphoglycerate the enzyme glyceraldehyde 3-phosphate dehydrogenase. The coenzyme nicotinamide adenine dinucleotide (NAD+) and inorganic phosphate ion (Pi) are required.

24. Step 7 - Phosphorylation by Phosphoglycerate Kinase
A phosphate group from 1,3-BPG is transferred to ADP, resulting in the synthesis of ATP.
It is catalyzed by phosphoglycerate kinase.
First energy releasing step in glycolysis

25. Step 8 - Isomerization by phosphoglycerate mutase
A phosphate group is transferred from 3-Phosphoglycerate at carbon 3 to carbon 2, generating
2-Phosphoglycerate
The shift of the PO4 group from C3 to C2 by phosphoglycerate mutase is reversible.
The carbon in COOH is labeled as carbon #1
A phosphate group is next transferred from carbon 3 to carbon 2 of phosphoglycerate in a step catalyzed by the enzyme phosphoglycerate mutase.

26. Step 9 - Dehydration by Enolase
Dehydration reaction resulting in loss of water from 2-phosphoglycerate producing phosphoenolpyruvate (PEP), which contains a high energy enol phosphate group
Loss of water from 2-phosphoglycerate produces phosphoenolpyruvate (PEP). The dehydration reaction is catalyzed by the enzyme enolase.

27. Step 10 - Phosphorylation by Pyruvate Kinase
Transfer of the phosphate group from PEP to ADP yields pyruvate and generates ATP
Second energy releasing step in glycolysis
Transfer of the phosphate group from phosphoenolpyruvate to ADP yields pyruvate and generates ATP. This reaction is catalyzed by pyruvate kinase, and is not reversible.

28. Energy Production In Glycolysis
Starting from glucose, glycolysis uses 2 ATP and 2 NAD, and produces 4 ATP and 2 NADH.
ATP is consumed in the reactions catalyzed by hexokinase (step 1) and phosphofructokinase (step 3).
ATP is produced in the reactions catalyzed by phosphoglycerate kinase (step 7) and pyruvate kinase (step 10).
NAD is converted to NADH in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase (step 6).
Stage stage 1
Energy Production In Glycolysis
Starting from glucose, glycolysis uses 2 ATP and 2 NAD, and produces 4 ATP and 2 NADH.
ATP is consumed in the reactions catalyzed by hexokinase (step 1) and phosphofructokinase (step 3).
ATP is produced in the reactions catalyzed by phosphoglycerate kinase (step 7) and pyruvate kinase (step 10).
NAD is converted to NADH in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase (step 6). 7

29. Tricarboxylic acid cycle
The Fate of Pyruvate
The fate of pyruvate depend on metabolic conditions.
glycolysis
Anaerobic
condition
pyruvate
lactate
aerobic
condition
Protein metabolism
acetyl-SCoA
lipid metabolism
Tricarboxylic acid cycle

30. Entry of Other Sugars into Glycolysis
Fructose is converted to glycolysis intermediates in 2 ways:
In muscle cells, it is phosphorylated to F6P and enters in step 3 of glycolysis.
In the liver, it is converted to G3P and enters in step 6.
Galactose is converted to G6P by a five-step pathway

31. A couple of fun facts about the final step of glycolysis
In the final step, step 10, of glycolysis, PEP is converted into pyruvate by pyruvate kinase. In this reaction, a PO4 is removes, so the enzyme was actually named for the reversed reaction, which does not exist 
Have you ever wonder why you got hot when you run? Step 10 releases lots of energy, almost enough to generate 2 ATP. Only one ATP is produced and the remaining energy is lost as heat.

32. Carbohydrate Metabolism Part 1 Quiz

33. Part II
Gluconeogenesis

34. Objectives Know the gluconeogenesis pathway
Evaluate the pathway and key steps of glycolysis and gluconeogenesis to appreciate their reciprocal regulation
Know how pyruvate, lactate and alanline enter gluconeogenesis

35. Gluconeogenesis - Introduction
The anabolic counterpart to glycolysis is gluconeogenesis, which occurs mainly in the liver (~90%) and kidney (~10%).
Gluconeogenesis is the synthesis of glucose typically from 3-carbon precursors such as lactate, pyruvate, alanine and glycerol formed by metabolism in the peripheral tissues.
The purpose of this pathway is to provide the body with glucose under physiological conditions when storage of glucose (in the form of glycogen) is depleted and there is no glucose available from the gut.
This pathway becomes critical during the early stages of starvation and fasting. Failure of gluconeogenesis is usually fatal.

36. Substrates for gluconeogenesis
Pyruvate (not from glycolysis)
Lactate, glucose-lactate cycle (AKA Cori cycle, discovered by Carl and Gertrude Cori)
Amino acids (glucose-alanine cycle)
Glycerol

37. Anaerobic metabolism of glucose forms lactate
The anaerobic metabolism of glucose by red blood cells, skeletal muscle and other peripheral tissues leads to the formation of lactate.
The reaction involves the reduction of the ketone group at C2 of pyruvate to an alcohol group forming lactate
In this reaction, NADH/H+ is oxidized to NAD+
This reaction is important because it replenishes the NAD+ used in step 6 of glycolysis. Without NAD+, glycolysis will stop.
Under anaerobic conditions (absence of oxygen) the electron transport stops, NADH concentration increases, NAD+ is in short supply and glycolysis cannot continue.
NAD+ is essential because glycolysis, the only available source of fresh ATP, must continue or death occurs. The reduction of pyruvate to lactate solves the problem by generating NAD+.

38. Lactate as a substrate for gluconeogenesis: Glucose-lactate (Cori) cycle
Lactate produced in muscle under anaerobic conditions and carried by blood is converted to glucose in the liver.
The liver releases glucose, which is carried by the blood to the skeletal muscle.
Cori cycle is the shuttling of glucose from the liver to skeletal muscle and of lactate from skeletal muscle back to the liver, where it is resynthesized to glucose.

39. Alanine as a substrate for gluconeogenesis: The glucose- alanine cycle
During starvation, skeletal muscle protein are degraded to yield amino acids and then ammonia and glutamate.
The transamination of pyruvate with glutamate yields alanine which is transported through the blood to the liver.

40. Glucose- alanine cycle (continued)
In the liver, transamination reforms pyruvate and glutamate.
Pyruvate is converted to glucose, which can be transported back to the skeletal muscle and used in glycolysis to yield ATP.
Function:
Provide carbon as a precursor of glucose synthesis in the liver.
Transport nitrogen atoms to the liver for excretion as urea.

41. Glycerol as a substrate for gluconeogenesis
liver
adipocyte
triacylglycerol
glycerol
Glycerol-3-P
DHAP
G3P
Fatty acids
glucose
Glycerol is formed in the adipose tissues by lipolysis of triacylglycerol when metabolic fuel is scarce
Glycerol is delivered to the liver through blood
In the liver, glycerol is phosphorylated to form glycerol-3-phosphate, an intermediate of glycolysis (step 7).

42. The pathway of gluconeogenesis largely is the reversed reaction of glycolysis
The gluconeogenesis pathway starts with pyruvate, ends with glucose, and consists of 11 reactions.
Of the 11 reactions in gluconeogenesis, 7 are catalyzed by the same enzymes used as in glycolysis, but for the reversed reactions
three steps in glycolysis releases are highly exergonic (release lots of energy) and therefore are not reversible.

43. The three irreversible steps in glycolysis
Three steps in glycolysis are not reversed by gluconeogenesis:
glucose to glucose 6-phosphate
fructose 6-phosphate to fructose 1,6-bisphosphate
phosphoenolpyruvate to pyruvate

44. Enzymes specific for gluconeogenesis or glycolysis
Substrate/product
glycolysis
gluconeogenesis
Glucose to G6P
hexokinase/glucokinase (GK)
Glucose-6-phosphatase (G6Pase)
F6P to F1,6BP
Phosphofructose kinase (6PF-1K or PFK)
fructose-1,6-bisphosphatase
PEP to pyruvate
Pyruvate kinase (PK)
NIL
Pyruvate to oxaloacetate (OAA)
pyruvate
Carboxylase (PC)
OAA to PEP
phosphoenolpyruvate carboxykinase
(PEPCK)
Note that conversion of PEP to pyruvate releases lots of energy and therefore two steps are needed to convert pyruvate back to PEP.

45. Reactions specific for gluconeogenesis
Seven reactions in gluconeogenesis are the reversed reactions of glycolysis (will not be further discussed)
Reactions to be discussed:
1. Pyruvate to PEP
2. F 1,6 BP to F6P
3. G6P to glucose

46. Conversion of pyruvate into phosphoenolpyruvate
Conversion of pyruvate to PEP involves two steps.
Pyruvate is first converted to oxaloacetate. In this reaction, pyruvate, carbon dioxide and ATP are converted to oxaloacetate, ADP and inorganic phosphate
Oxaloacetate is then converted to PEP, using GTP
Two steps are required, utilizing two enzymes and the energy provided by ATP and GTP.
Step 1: pyruvate, carbon dioxide, and ATP are converted to oxaloacetate, ADP and inorganic phosphate.
This reaction is :
catalyzed by pyruvate carboxylase
activated by acetyl CoA
occurs in the mitochondrial matrix.
Step 2: oxaloaceate and GTP are converted to PEP, GDP and carbon dioxide.
This reaction is:
catalyzed by PEP carboxykinase (PEPCK)
occurs in both mitochondria and the cytosol
one ATP and one GTP is used to form PEP from pyruvate. An equivalent of 2 ATP is used. Because 2 molecules of pyruvate make one molecule of glucose, a total of 4 ATP are used. 46

47. Conversion of F1,6-bisphosphate to fructose-6-phosphate
Hydrolysis of F1,6BP by F1,6 phosphatase (F1,6P) to F6P bypasses the PFK reaction.
This reaction is an important regulation in gluconeogenesis
high ATP and low AMP (=energy rich state) stimulates F1,6P (low ATP & high AMP=energy poor state)
F2,6BP inhibits F1,6P (F2,6BP activates PFK; hence reciprocal control of glucose synthesis and breakdown)
Steps 9 to 11 - Conversion of F1,6-bisphospahtre to glucose
Step 9: conversion of F1,6BP to F6P
This reaction is the reverse of the reaction catalyzed by phosphofructokinase during glycolysis
The gluconeogenic reaction is catalyzed by fructose-1,6-bisphosphatase, which is the major regulatory enzyme in gluconeogenesis
The enzyme is activated by citrate, inhibited by AMP and Fructose 2,6 bisphosphate
Step 10: conversion of fructose-6-phosphate to glucose-6-phosphate
This reaction is the reverse of the conversion of G6P to F6P in glycolysis and it is catalyzed by the same enzyme phosphoglucose isomerase
Step 11: conversion of G6P to glucose
This is the reverse of the reaction catalyzed by hexokinase during glycolysis
In gluconeogenesis, this reaction is catalyzed by glucose-6-phosphatase, which is only present in gluconeogenic tissues:
a. liver
b. kidney
c. epithelial cells of the small intestine

48. Conversion of glucose-6-phosphate to glucose
Hydrolysis of G6P by glucose-6-phosphate bypasses the hexokinase reaction in glycolysis.
Liver and kidney are the only organs contains the glucose-6-phosphatase
Muscle lacks this enzyme, so G6P formed by gluconeogenesis can only be used by muscle.

49. Energy in Gluconeogenesis
Gluconeogenesis is an endergonic reaction that requires energy.
In gluconeogenesis, the conversion of two moles of pyruvate to 1 mole of glucose uses 4 moles of ATP and 2 moles of GTP
The conversion of 2 moles of pyruvate to 1 mole of glucose uses 4 moles of ATP and 2 moles of GTP. 49

50. Carbohydrate Metabolism Part 2 Quiz

51. Part III
Glycogen metabolism

52. Objectives
Know the general structure of glycogen and discuss the metabolic advantages of such structure
Know how the glycogen molecule is synthesized and degraded
Know the different ways by which energy is generated from glycogen in the liver and muscle

53. Structure of Glycogen
Glycogen is a polymer of glucose (up to 120,000 glucose residues) and is a primary carbohydrate storage form in animals.
The polymer is composed of units of glucose linked by alpha(1-4) glycosidic bonds in the linear molecule and alpha(1-6) glycosidic bonds at branch points. 1
Glycogen is the storage form of glucose
Provide an immediate source of blood glucose for use as a metabolic fuel
Glycogen is a highly branched, very large polymer of glucose linked along its main chain with -1,4 glycosidic linkage, and branches arise by -1,6 glycosidic linkage at about every 10 residues
Major sites of glycogen storage are liver and muscle.
In humans, liver glycogen stores can only last for ~12 hours.
Unlike glycolysis and gluconeogenesis, glycogen synthesis and break down occur in separate pathways.
Liver and skeletal muscle are the primary sites in the body where glycogen is found

54. Glycogen as the storage of glucose
The end of the molecule containing a free carbon number one on glucose is called a reducing end. The other ends are all called non-reducing ends.
Because glycogen contains so many glucoses, it provides a quick source of glucose when needed and 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 to more ends, and more glucose that can be released at once.
Glycogen is the storage form of glucose
Provide an immediate source of blood glucose for use as a metabolic fuel
Glycogen is a highly branched, very large polymer of glucose linked along its main chain with -1,4 glycosidic linkage, and branches arise by -1,6 glycosidic linkage at about every 10 residues
Major sites of glycogen storage are liver and muscle.
In humans, liver glycogen stores can only last for ~12 hours.
Unlike glycolysis and gluconeogenesis, glycogen synthesis and break down occur in separate pathways.
glucose

55. Synthesis of glycogen - glycogenesis
Glycogenesis occurs when glucose concentrations are high.
Glycogenesis involves 4 steps:
Glucose is phosphorylated to Glucose 6-Phosphate (G6P)
G6P is isomerized to G1P
UDP is added to G1P to form Glucose-UDP
Glucose-UDP is added to the growing glycogen chain 1 2 3
Glucose activation – synthesis of uridine diphosphate glucose
Step 1: Glucose is phosphorylated to glucose 6-phosphate by hexokinase and ATP (as in glycolysis)
Step 2: G6P is isomerized to glucose 1-phosphate by phosphoglucomutase.
Step 3: The glucose residue is then attached to uridine diphosphate (UDP) to produce UDP-glucose. This reaction is catalyzed by UDP-glucose pyrophosphorylase, which is reversible. However, the quick hydrolysis of pyrophosphat2 draws the reaction forward (essentially irreversible)
glucose-UDP, the activated carrier of glucose, transfers glucose to a growing glycogen chain. 4

56. Glycogen synthesis Synthesis of the glycogen molecule
Biosynthesis of a molecule of glycogen starts with the protein glycogenin, in which glucose is attached to Tyr 194OH group.
Glycogenin is a self-glucosylating enzyme that can create short chains of glucose attaching to itself.
UDP-glucose + glycogenin    UDP + glucosylglycogenin
Once the chains contain more than 10 glucose residues, the chain act as a primer for glycogen synthase to further elongate them by transferring a new glucose residue onto the free reducing end of the chain..
UDP-glucose + glycogen (n)  glycogen (n+1) + UDP
Chain branching
The elongated chains of glycogen can have branches. Branches in the glycogen molecule help keep the polymer soluble. The branching also creates lots of ends for enzyme attack and provides for rapid release of glucose when it is needed, and for rapid re-synthesis of glycogen, for storage of available glucose.
Branching enzyme forms branched glucose chains by transferring oligomers of around seven glucose residues from one chain onto another glucose residue on a different chain. The attachment of the oligomer is onto position 6 of the receiving glucose, thus forming a 1,6 linkage.

57. Addition of one glucose to the glycogen chain
UDP-glucose + glycogen (n) glycogen (n+1) + UDP
Non-reducing end
a-1,4-glycosidic bond
Addition of one glucose to existing glycogen chain
Once molecules of polyglycosylated glycogenin are established in a cell, further synthesis is controlled by glycogen synthase. This enzyme elongates existing glucose chains by transferring a new glucose residue onto the free reducing end of the chain.

58. Formation of Branch chains in glycogen
Segments of the glycogen chain are transferred onto the C-6 hydroxyl of neighboring chain, forming a-1,6-linkage.
The branching enzyme is called glycosyl-4:6 transferase
Defect of branching enzyme causes type IV glycogen storage (also called Andersen’s disease).
As glucose subunits are added to the ends of a glycogen chain, segments of the glycogen chain can be transferred onto the neighboring chain, forming a-1,6-linkage.

59. Glycogen degradation (Glycogenolysis)
glycogen phosphorylase
Occurs in muscle cells when energy needed.
Also occurs in liver when blood glucose is low.
Glycogen degradation requires 3 enzymes:
Glycogen phosphorylase (phosphorylase) catalyzes glycogen phosphorylysis (bond cleavage by the substitution of a phosphate group) and yields glucose-1-phosphate (G1P)
Glycogen debranching enzyme removes glycogen’s branches, allowing glycogen phosphorylase to complete it’s reactions. It also hydrolyzes a(16)-linked glucosyl units to yield glucose. 92% of glycogen’s glucose residues are converted to G1P and 8% to glucose.
Phosphoglucomutase converts G1P to G6P-can either go through glycolysis (muscle cells) or converted to glucose (liver).
Glycogenolysis
Glycogen phosphorylase
Glucose 1-phosphate is formed from one glucose residue of glycogen, in a reaction catalyzed by glycogen phosphorylase.
Glycogen (n) + Pi  glycogen (n-1) + G1P
Next, glucose 1-phosphate is converted to glucose 6-phosphate and then to glucose.
Occurs in muscle cells when energy needed and in liver when blood glucose is low.
Glycogen phosphorylase catalyzes glycogen phosphorylysis (bond cleavage by the substitution of a phosphate group) and yields glucose-1-phosphate (G1P) 59

60. Fate of glucose-1-phosphate
liver
muscle
Fate of G1P:
Glycogenolysis occurs in muscle cells when energy needed.
Occurs in liver when blood glucose is low.
The enzyme glucose 6-phosphatase is present in liver but absent in other tissues. It de-phosphorylates glucose 6-phosphate to glucose, and release glucose into the bloodstream. The phosphorylated (and ionic, highly charged) sugar cannot pass through the plasma membrane of the liver cells, while the de-phosphorylated (and neutral) sugar can. Thus liver cells help to regulate the blood glucose level by degrading liver glycogen stores.
Muscle cells do not produce glucose-6-phosphatase. Therefore, the "trap" G-6-P will be used to generate energy by glycolysis and through the TCA cycle.

61. Glycogen degradation - Debranching
Non-reducing
end
Phosphorylase removes glucose one at a time from the non-reducing end, but stops at a point four residues away from the branch.
A transferase enzyme moves the outer three residues from a branch to of the adjacent branch, and leaves one at the branch point.
Then a second enzyme, alpha-1,6-glucosidase, clips the alpha-1,6 bond, releasing the last residue of the stub and converting the chain into a linear polymer that phosphorylase can continue to act on.
"Debranching" glycogen
Phosphorylase removes glucose one at a time from the non-reducing end but it cannot cleave past a branch in glycogen.
It stops at a point four residues away from the branch.
A transferase enzyme first moves the outer three residues from a from a limit branch to a nonreducing end of the adjacent branch, which forms a new (14) linkage with three more units available for phosphorylase, and leaves one residue (the one with the 1,6 linkage) at the branchpoint. Then a second enzyme, alpha-1,6-glucosidase, clips the alpha-1,6 bond, releasing the last residue of the stub and converting the chain into a linear polymer that phosphorylase can continue to act on.

62. Regulation of glycogen metabolism
Because of the importance of maintaining blood glucose levels, the synthesis and degradation of glycogen are tightly regulated
Under well fed state, G6P stimulates glycogen synthase, converting G1P into glycogen, whereas glucose, ATP and G6P inhibit glycogen phosphorylase, preventing the breakdown of glycogen
Glycogen metabolism is also under allosteric controls and hormonal regulation.

63. Summary -Metabolic Pathways of Glucose
function
Glycolysis
Converts glucose to pyruvate
gluconeogenesis
Synthesis of glucose from noncarbohydrate sources (pyruvate, lactate, alanline and glycerol)
glycogenogenesis
Synthesizes of glycogen from glucose
glycogenolysis
Degradation of glycogen to G1P

64. Carbohydrate Metabolism Part 3 Quiz

65. References
Principles of Medical Biochemistry, Meisenberg and Simmons, 3rd edition, chapter 22.
Lippincott’s Illustrated Reviews: Biochemistry, Champe, Harvey, and Ferrier, Chapter 13

66. Thank you Thank you for completing this module.
If you have any questions or comments, please contact me.