#Storage Mechanisms and Control of

Carbohydrate Metabolism

#12

made by: Ahmad Abudayyeh

corrected by: laith sorour

date: 6/11/2016

*Pre-lecture note: High energy phosphate means ATP

يعني مثال لما يسألك

how many high energy phosphates produced by glycolysis means how many ATP

molecules produced.

*you must be careful to the question building

ex: if I ask you the total ATP (or High energy phosphate) produced during glycolysis?

The answer is 4 ATP

-if I ask you Net ATP (or High energy phosphate) produced during glycolysis? The

answer is 2 ATP

Low energy phosphate = ADP/AMP

*Glycolysis is produced in cytosol

*important for many organs especially for (RBCs, cornea of heart, muscles during

severe exercise, adrenal cortex)

*all the kinases need Mg2+

*to make the reaction exergonic you should hydrolysis the phosphate

We’ll talk about three important pathways in this lecture:

-Glycogen synthesis and degradation.

-Gluconeogenesis.

-pentose phosphate pathway.

Glycogen characteristics:

1- Highly branched. (more branched than

amylopectin)

2- Storage form of glucose in animals.

(Starch is the storage form in plants)

3- Contains α – (1→4) glycosidic linkage

between glucose subunits, Branches are

linked to the chains from which they are

branching off by α – (1→6) glycosidic bonds between the first glucose of the

new branch and a glucose on the stem chain.

4- Globular, which let it be 1) stored in large amounts, 2) easily synthesized and

easily degraded and 3) it is compact and that decreases the amount of water

between the molecules and decrease its weight.(each gram of glycogen

molecule stores 2.5 gram of water with it)

• Why do animals store any energy as glycogen? Why not convert

all excess fuel into fatty acids?

1- Because glucose is the main source of energy for the nervous system, the brain

uses glucose as the source of energy not fatty acids.

2- Fatty acids can’t be converted into glucose.(FA comes from carbohydrates

irreversibly)\ glycogen can be easily converted to glucose

3- Fatty acids need oxygen to produce energy while glycogen can produce energy

aerobically or anaerobically.

• Why not store energy as free glucose?

1- Affects the osmotic balance in the body.

2- It needs large amount of water.

3- If glucose outside the RBC has lower concentration than inside the RBC, it will

cost energy in order to transport into the cell.

Adult Human 70 kg

Triacylglyceride: 100.000 kcal11 kg

Protein (muscle): 25.000 kcal

Glycogen: 600 kcal

Glucose: 40 kcal

Triacylglyceride: approx. 11 kg of body weight

glycogen storage instead of fat will lead to:

Increase in weight: 55 kg!!

These are the storage forms of energy in the body.

100.000 kcal of triacylglyceride weighs 11 Kg of body weight, but if we use glycogen

instead of fat there will be an increase in the weight by 55 Kg of body weight, why?

Q) why we store most of energy as triglycerides (fats)?

-Because fat is stored reduced & dehydrated (without water), while glycogen is

stored with water, every 1 g of glycogen is stored with 2.5 g of water.

*Glycogen covers our need of energy for one day only.

Glycogen Breakdown (degradation) “glycogenolysis” (14:17)

*Lysis means degradation. (Proteolysis: degradation of protein, lipolysis, etc.)

• Step 1) Glycogen is cleaved by glycogen phosphorylase by adding

phosphate to give a-D-glucose-1-phosphate (phosphorolysis).

– No ATP is involved in this phosphorolysis.

– Occur in the liver maintains blood glucose.

The enzyme used is glycogen phosphorylase which give us phosphorylated glucose

directly (glucose-1- phosphate).

• Enzyme-catalyzed isomerization converts the

1-phosphate to the 6-phosphate.

Note: more ATP is produced from glucose of glycogen.

HO(Glucose) nOH HO- PO32 -+

HO(Glucose) n-1OH

glycogenphosphorylase

+

OPO32 -

OH

HOHO

CH2 OHO

Glycogen

-D-Glucose-1-phosphate

+ H2 O

OPO32 -

OH

HOHO

CH2 OHO

-D-Glucose-1-phosphate

phospho-glucomutase

OH

OH

HOHO

CH2 OPO32 -

O

-D-Glucose-6-phosphate

glycolysis

The net production of glucose if the source of glucose from glycogen (from liver) by

glycolysis is 3 ATP.

Glucose-6-phosphate is one of the key

compounds in biochemistry that is an

intermediate in many reactions.

Step 2) Glycogen transferase enzyme transfers three glucose residues

from (limit branch) to another branch, where they are removed by

glycogen phosphorylase. Until we have 7 units, works when we have 7 units

or more branches

step 3) Glycogen debranching enzyme then hydrolyzes the α-(1,6)

glycosidic bond of the last glucose residue remaining at the point of

branching.

Then in glycogenolysis we need three enzymes:

1- Glycogen phosphorylase.

2- Glycogen transferase.

3- Glycogen debranching.

Glycogenesis (19:10)

• Glucose 1-phosphate reacts with uridine triphosphate to give

UDPG and pyrophosphate.

CHECK THE PICTURE IN SLIDE 11.

In cells, we have a substance in spite of fasting or starvation it will stay, it’s a primer

called glycogenin (a few units of glucose on a protein).

UDP G: Uridine diphosphate glucose It’s the active glucose molecule.

• Coupling of UDPG formation with hydrolysis of pyrophosphate

drives formation of UDPG to completion.

PPi (pyrophosphate): pyrophosphate undergoes hydrolysis so that the

reaction goes in the right direction.

Uridine diphosphate glucose (UDPG) then adds its glucose unit to the

growing glycogen chain by Glycogen synthase enzyme.

*Synthase means there’s other unit involved in the reaction and not ATP. So it’s

different from synthetase.

CHECK THE PICTURE IN SLIDE 13.

Exchange of phosphate from ATP regenerates UTP.

Glucose-1-phosphate + UTP

2Pi

G°'(kJ•mol -1)

UDPG + PPi

PPi+ H2 O -30.5

-30.5Glucose-1-phosphate + UTP UDPG + 2Pi

­ 0

+ H2 O

UDP + ATP UTP + ADP

nucleosidephosphate kinase

UTP/ATP/CTP... etc. All are high energy phosphates.

Branching enzyme transfers

about seven glucose residue-

long segment from growing

branch to a new branch

point via α-(1-6) glycosidic

bond.

When the chain gets longer(>11) a

branching enzyme takes the

glucose subunits and make a

branch at the 1:6 bond.

Then in glycogenesis we need two

enzymes:

1- Glycogen synthase.(makes active glucose)

2- Branching enzyme.

Control of Glycogen Metabolism (22:39)

By controlling the degradation, using phosphorylase, we have two types:

phosphorylase A and phosphorylase B.

Phosphorylase b is active when it’s

dephosphorylated.

G-6-phosphate or ATP inactivates

phosphorylase b.

So, glycogen is stored when needed.

(fasting, severe exercise …)

When there’s an excess of glucose it’s

stored as glycogen, but when the glycogen stores are full, glucose is converted into fat.

*In human metabolism, you can’t have two pathways (synthesis & degradation) running

at the same time.

Inactive forms are shown in red, and active ones in green.

Epinephrine or cortisol is one of the fight and flight factors, so when we need energy

degradation pathway(by glycogen phosphorylase) is active and the synthesis pathway

(by glycogen synthase) is inactive.

•

The activity of glycogen synthase is subject to the same type of

covalent modification as glycogen phosphorylase.

– The response, however, is opposite.

– Hormonal signals (glucagon or epinephrine) stimulate its

phosphorylation.

– Once phosphorylated, glycogen synthase becomes inactive at

the same time the hormonal signal is activating glycogen

phosphorylase.

– Glycogen synthase can be phosphorylated by several other

enzymes including glycogen synthase kinase.

– dephosphorylation is by phosphoprotein phosphatase.

Glycogen Loading:

http://runnersconnect.net/running-nutrition-articles/carbohydrate-loading-

marathon/

read it ^.^ ///glycogen is the first energy used

Glycogen storage diseases.

• Type I Von Gierke’s disease.

• Deficiency of glucose-6-phosphatase.

Liver cells and renal tubule cells loaded with glycogen.

Hypoglycemia(because no glucose), lactic acidemia, ketosis,

hyperlipemia.

Kinase: adds phosphate to glucose.

Phosphatase: removes phosphate from glucose.

Summary

• Glycogen is the storage form of glucose in animals, including

humans. Glycogen releases glucose when energy demands are high

• Glucose polymerizes to form glycogen when the organism has no

immediate need for the energy derived from glucose breakdown

• Glycogen metabolism is subject to several different control

mechanisms, including covalent modification and allosteric effects.

Gluconeogenesis (29:30)

• The synthesis of glucose from non-carbohydrate sources like

lactate, glycerol and amino acids.

18 amino acids are glucogenic amino acids gives glucose and 2 amino acids are

ketogenic (Leucine, Lysine) gives ketone bodies (degraded to acetyl-CoA which is

precursor of ketone bodies).

– gluconeogenesis is not the exact reversal of glycolysis; that is,

pyruvate to glucose does not occur by reversing the steps of

glucose to pyruvate

– It is impossible to reverse any kinase reaction under

physiological conditions.

– gluconeogenesis occurs in the cytosol & mitochondria

– gluconeogenesis takes place in the liver 90%

and in kidneys 10%.

Gluconeogenesis occurs between cytosol and mitochondria.

Glycogenesis/ glycogenolysis/ glycolysis occurs in the cytosol.

There are three irreversible steps in glycolysis:

--- phosphoenolpyruvate to pyruvate + ATP

--- fructose-6-phosphate to fructose-1,6-

bisphosphate

--- glucose to glucose-6-phosphate

– the net result of gluconeogenesis is reversal of these three

steps, but by different reactions and using different enzymes

(bypassing).

In gluconeogenesis, we need new enzymes to bypass the irreversible steps in

glycolysis.

Glycolysis gives in Net 2 ATP.

Gluconeogenesis uses 6 ATP.

• Step 1: carboxylation of pyruvate (1st bypass)

– requires biotin

– pyruvate carboxylase is subject to allosteric control; it is

activated by acetyl-CoA.

+ 2 ATP - 6 ATP

CH3 CCOO-

CH2 CCOO-

O

COO-

+ CO2+ ATP

+ ADP + Pi

Pyruvate

Oxaloacetate

biotin

pyruvatecarboxylase

O

+ 2 H+

• Biotin is a carrier of CO2 (carboxylation).

*Oxaloacetate doesn’t leave the mitochondria.

*we need it in citric acid cycle

*malate can leave mitochondria\malate decarboxylation

gives oxaloacetate

– decarboxylation of oxaloacetate is coupled with

phosphorylation by GTP to give PEP

– the net reaction of carboxylation/decarboxylation is

– net reaction is close to equilibrium: DG0’ = 2.1 kJ•mol-1

-glycolysis gives 2 pyruvate molecules

Here we double the reactions so that we get 2 molecules of pyruvate, like glycolysis.

Second different reaction (2nd bypass) in gluconeogenesis

– DG° = -16.7•kJ mol-1

+ ATPPyruvate + GTP

Phosphoenolpyruvate + ADP + GDP + Pi + 2 H+

HO

CH2 OPO32 -

CH2 OHO

OH

HH

HO

-D-Fructose-6-phosphate

H

Mg 2 +

HO

CH2 OPO32 -

CH2 OPO32 -

O

OH

H

H

HO

-D-Fructose-1,6-bisphosphate

H

+ H2 O

fructose1,6-bisphosphatase

+ Pi

CH2 = CCOO-

OPO32 -

CH2 CCOO-

O

CO2-

+ CO2+ GTP

PhosphoenolpyruvateOxaloacetate

+ GDP

– fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited

by AMP and F2,6P and activated by ATP

Gluconeogenesis is not the opposite of glycolysis. Because gluconeogenesis is

formation of glucose from non-carbohydrate molecules using different enzymes\only

reversible steps are the same

• Third different reaction (3rd bypass) in gluconeogenesis

DG°’ = -13.8 kJ•mol-1

Why gluconeogenesis doesn’t occur in the muscles or brain and occurs only in kidney

and liver?

Because glucose-6-phophatase is absent in muscles and brain.

*Liver acts as a glucostat, because it supplies the whole body with glucose.

• The Cori cycle (37:05)

– under vigorous anaerobic exercise, glycolysis in muscle tissue

converts glucose to

pyruvate; NAD+ is

regenerated by reduction

of pyruvate to lactate

-lactate from muscle is

transported to the liver

where it is reoxidized to

pyruvate and converted

to glucose

– thus, the liver shares the

stress of vigorous exercise.

OHOH

HOHO

CH2 OHO

-D-Glucose

OHOH

HOHO

CH2 OPO32 -

O

-D-Glucose-6-phosphate

glucose-6-phosphatase

H2 O+ + Pi

Cori cycle between muscles and liver, lactate produced in the muscle is transported to

the liver which converts it to glucose, so in the muscle glycolysis is running and in the

liver gluconeogenesis.

Control of carbohydrate metabolism (37:45)

• Allosteric: fructose-2,6-bisphosphate (F2,6P)

– high concentration of F2,6P stimulates glycolysis; a low

concentration stimulates gluconeogenesis

– concentration of F2,6P in a cell depends on the balance

between its synthesis (catalyzed by phosphofructokinase-2)

and its breakdown (catalyzed by fructose bisphosphatase-2)

– AMP inhibits FBPase and stimulates PFK

– each enzyme is controlled by phosphorylation/

dephosphorylation.

Fructose-2,6-bisphosphate

Fructose-2,6-bisphosphate is an allosteric

activator of phosphofructokinase (a glycolytic

enzyme) and an allosteric inhibitor of fructose

bisphosphate phosphatase (an enzyme in the

pathway of gluconeogenesis).

Reciprocal Regulation of Gluconeogenesis and Glycolysis in the Liver

Citrate is derived from citric acid cycle.

Control of carbohydrate metabolism

Slide 38 + 39 the doctor read them.

Allosteric

Covalentmodification

Substrate cycles

Genetic

Effectors (substrates, products, or coenzymes) of a pathway inhibit or activate an enzyme

Inhibition or activation of an enzyme depends on formation or breaking of a covalent bond, often by phosphorylation or dephosphorylation

Two opposing reactions (such as formation or breakdown of a substance) are catalyzedby different enzymes, which are activated or inhibited separately

The amount of enzyme present is increased by protein synthesis

Control of Pyruvate Kinase (40:36)

It’s more active when it’s

dephosphorylated.

ATP alanine is an indicator

on protein degradation in

muscles.

When there’s an excess in

ATP then we don’t need

energy so we inhibit

production of ATP.

But when there’s excess of

AMP then we need energy.

Slide 41 was read.

Pentose Phosphate Pathway (in cytosol)

– As the name implies, five-carbon sugars, including ribose,

are produced from glucose

– The oxidizing agent is NADP+; it is reduced to NADPH

(electron donor), which is a reducing agent in biosynthesis

e.g. lipid (like cholesterol synthesase\steroid hormone

synthases\prostoglandin)

– PPP is composed from two reactions:

1. Oxidative reactions: begins with two oxidation steps

(using NADP+) to give ribulose-5-phosphate No ATP included.

2. Non-oxidative reactions: a series of carbon-shuffling steps

during which three-, four-, five-, six-, and seven-carbon

monosaccharide phosphates are produced We need two enzymes: transketolase (transport ketone group) and

transaldolase (transport aldehyde group) both need thiamine

pyrophosphate (TPP).

-ATP production is not an important concern.

Occurs in the cytosol.

In mammary gland, fatty tissues.

Importance: We use it to produce hexoses and electron carriers (NADPH which is an

electron donor used in reductive biosynthesis such as fat synthesis).

CHECK THE PICTURES IN SLIDES 43-47.

The carbon-shuffling reactions are catalyzed by

---transketolase for the transfer of two-carbon units requires thiamine

pyrophosphate as a coenzyme

---transaldolase for the transfer of three-carbon units.

• Control of the pentose phosphate pathway

– glucose-6-phosphate (G6P) can be channeled into either

glycolysis or the pentose phosphate pathway

– if ATP needed, G6P is channeled into glycolysis

– if NADPH or ribose-5-phosphate are needed, G6P is

channeled into the pentose phosphate pathway.

-G-6-P dehydrogenase deffecincy

• More than 400 variants of G-6-PD have been characterized,

which show less activity than normal.

• G-6-PD is the most common human enzyme deficiency in the

world. It affects an estimated 400 million people.

• Hemolysis, abdominal pain, dizziness, headache, dyspnea,

palpitation, neonatal jaundice.

Causes anemia. Drug-induced hemolytic anemia (antibiotics and anti-malarelial

drugs for people with susstiability to deficiency )

-phavism leads to this anemia too

-because of G6DHase deficiency no NADH is produces so glutaithione doesn’t

work(no oxi-redo reaction) so accumulation of H2O2 & radicals happen and makes

hemolysis for cells which leads to anemia

Precipitating Factors

• Infection & other ac. Illness (diabetic ketoacidosis)

• Drugs: Antimalarials, Antipyretics or Antibiotics

• Fava beans “favism”

• Neonatal jaundice: due to decrease hepatic catabolism or increase

production of bilirubin.

CHECK SLIDE 51 + 52, the doctor just read them.

بل انظر نحو السماء ال تبكي يا صغيري

من قلبك الحريري ال ال تقطع الرجاء

في العمل جهد العمل

;p

Best wishes