Storage Mechanisms and Control of Carbohydrate …Learning Objectives 1. How Is Glycogen Produced...

Storage Mechanisms and Control of Carbohydrate Metabolism

Learning Objectives

1. How Is Glycogen Produced and Degraded?

2. How Does Gluconeogenesis Produce Glucose from Pyruvate?

3. How Is Carbohydrate Metabolism Controlled?

4. Why Is Glucose Sometimes Diverted through the Pentose Phosphate Pathway?

• Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids?

• Why not store energy as free glucose?

Adult Human 70 kg

Triacylglyceride: 100.000 kcal Protein (muscle): 25.000 kcal Glycogen: 600 kcal Glucose: 40 kcal Triacylglyceride: approx 11 kg of body weight glycogen storage instead of fat: increase in weight: 55 kg!!

Glycogen Breakdown ”glycogenolysis”

• 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

HO(Glucose) nOH HO- PO32 -+

HO(Glucose) n-1OH

gl ycogenphosphorylase

+

OPO32 -

OH

HOHO

CH2 OHO

Glycogen

a-D-Glucose-1-phosphate

+ H2 O

• Enzyme-catalyzed isomerization converts the

1-phosphate to the 6-phosphate

OPO32 -

OH

HOHO

CH2 OHO


phospho-gl ucomutase

OH

OH

HOHO

CH2 OPO32 -

O


glycolysis Note: more ATP is produced from glucose of glycogen

–Glycogen transferase enzyme transfers three glucose residues from (limit branch) to another branch, where they are removed by glycogen phosphorylase

–Glycogen debranching enzyme then hydrolyzes the a(1,6) glycosidic bond of the last glucose residue remaining at the point of branching.

Glycogenesis glycogenin

• Glucose 1-phosphate reacts with uridine

triphosphate to give UDPG and pyrophosphate

Glucose- 1-phosphate + UTP UDP G + PP i

UDP-glucose pyrophosphorylase

O - P - O - P

O H

H O H O

C H 2 O H

O

O

O H O H

H H

H

C H 2

H

O

H N

N

O

O

O -

O

O -

Uridine diphosphate glucose (UDPG )

O

• Coupling of UDPG formation with hydrolysis of pyrophosphate drives formation of UDPG to completion

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

– Uridine diphosphate glucose (UDPG) then adds its glucose unit to the growing glycogen chain

– Exchange of phosphate from ATP regenerates UTP

UDP + ATP UTP + ADP

nucleosidephosphate kinase

HO(Glucose) n OH +

Glycogen

UDP G

glycogen synthase

HO-Glucose- O(Glucose) n OH + UDP new glucose unit added α (1-4 bond)

Glycogenesis

• Branching enzyme transfers about seven glucose residue-long segment from growing branch to a new branch point via α(1-6) glycosidic bond

Control of Glycogen Metab

Glycogen phosphorylase - a major control point

Glycogen Glycogen

(Dephosphorylated form( (Phosphorylated form)

Coordinate Control of Glycogen Metabolism

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

Control of Glycogen Metab • 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/

Glycogen storage diseases.

• Type I Von Gierke’s disease

• Deficiency of glucose-6-phosphatase

Liver cells and renal tubule cells loaded

with glycogen. Hypoglycemia, lactic acidemia,

ketosis, hyperlipemia.

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

Gluconeogenesis

• The synthesis of glucose from none carbohydrate sources like lactate, glycerol and amino acids. – 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 occur in the cytosol & mitochondria

– gluconeogenesis takes place in the liver 90%

and in kidneys 10%

Gluconeogenesis

– 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)

+ 2 ATP - 6 ATP

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

– requires biotin

– pyruvate carboxylase is subject to allosteric control; it is activated by acetyl-CoA

Gluconeogenesis

CH3 CCOO-

CH2 CCOO-

O

COO-

+ CO2+ ATP

+ ADP + Pi

Pyruvate

Oxaloacetate

bi oti n

pyruvatecarboxyl ase

O

+ 2 H+

Biotin

• Biotin is a carrier of CO2 (carboxylation)

NHHN

S

O

H H

COO-

Biotin

NHN

S

O

H H

C

O

NH-enz yme

C

O

-O

1 . H2 N- enzyme

2 . CO2 + AT P

– 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: G0’ = 2.1 kJ•mol-1

Gluconeogenesis

CH2 = CCOO-

OPO32 -

CH2 CCOO-

O

CO2-

+ CO2+ GTP

PhosphoenolpyruvateOxaloacetate

+ GDP

+ ATPPyruvate + GT P

Phosphoenolpyruvate + ADP + GDP + Pi + 2 H+

Gluconeogenesis

• Second different reaction (2nd bypass) in gluconeogenesis

– G° = -16.7•kJ mol-1

– fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and F2,6P and activated by ATP

HO

CH2 OPO32 -

CH2 OHO

OH

HH

HO

a-D-Fructose-6-phosphate

H

Mg 2 +

HO

CH2 OPO32 -

CH2 OPO32 -

O

OH

H

H

HO

a-D-Fructose-1,6-bisphosphate

H

+ H2 O

fructose1,6-bi sphosphatase

+ Pi

Gluconeogenesis

• Third different reaction (3rd bypass) in gluconeogenesis

G°’ = -13.8 kJ•mol-1

OHOH

HOHO

CH2 OHO

a-D-Glucose

OHOH

HOHO

CH2 OPO32 -

O


gl ucose-6-phosphatase

H2 O+ + Pi

The Cori Cycle

• The Cori cycle

– 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

The Cori Cycle

Control of carbohydrate metabolism

How ?


• 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

p. 520

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


Allosteric

Coval entmodification

Substrate cycles

Genetic

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

Inhibiti on or activation of an enzyme depends on formati on or breaki ng of a covalent bond, often by phosphorylation or dephosphorylati on

Two opposi ng reacti ons (such as formati on or breakdown of a substance) are catalyzedby di fferent enzymes, which are activated or inhibited separately

The amount of enzyme present is increased by protein synthesis


• Substrate cycling

– opposing reactions can be catalyzed by different enzymes and each opposing enzyme or set of enzymes can be regulated independently

Fructose-6-phosphate

+ ADPFructose 1,6-bi sphosphate

+ ATP

+Fructose 1,6-bi sphosphate H2 O

Fructose-6-phosphate + Pi

G0'

(kJ•mol -1)

-25.9

-8.6

phosphofructo-ki nase

fructose-1,6-bi sphosphatase

Major Control Points in Carbohydrate Metabolism

• Three steps in glycolysis are major control points in glucose metabolism

• Hexokinase – Inhibited by high levels of glucose 6-phosphate

Phosphofructokinase, – When glycolysis is inhibited through glucose 6-phosphate builds up, shutting

down hexokinase

• Pyruvate kinase (PK) is an allosteric enzyme – Inhibited by ATP and alanine

– Activated by fructose-1,6-bisphosphate

• PK has 3 different isoenzymes – M predominates in muscle, L in liver, and A in other tissues

– Native PK is a tetramer

– Liver isoenzymes are subject to covalent modification

Control of Pyruvate Kinase

Summary

• A number of control mechanisms operate in carbohydrate metabolism. These include allosteric effectors, covalent modification, substrate cycles, and genetic control

• In the mechanism of substrate cycling, the synthesis and the breakdown of a given compound are catalyzed by two different enzymes

Pentose Phosphate Pathway

– As the name implies, five-carbon sugars, including ribose, are produced from glucose

– The oxidizing agent is NADP+; it is reduced to NADPH, which is a reducing agent in biosyntheses e.g. lipid

– PPP is composed from two reactions:

1. Oxidative reactions: begins with two oxidation steps (using NADP+) to give ribulose-5-phosphate

2. Non-oxidative reactions: a series of carbon-shuffling steps during which three-, four-, five-, six-, and seven-carbon monosaccharide phosphates are produced

– ATP production is not an important concern

PPP oxidative reactions

C H O

C H 2 O P O 3 2 -

O H H

H H O

O H H

O H H

Glucose-6-phosphate

C O O -

C H 2 O P O 3 2 -

O H H

H H O

O H H

O H H

C O O -

C

C H 2 O P O 3 2 -

O H H

O

O H H

O H H

6-Phosphogluconate

NADP + NADPH NADP + NADPH

C H 2 O H

C

C H 2 O P O 3 2 -

O

O H H

O H H

+ C O 2

Ribulose-5-phosphate

Non-oxidative reactions

C H 2 O H

C

C H 2 O P O 3 2 -

O

O H H

O H H

Ribulose-5- phosphate

C H 2 O H

C

C H 2 O P O 3 2 -

O

H H O

O H H

Xylulose-5-phosphate

C H O

C H 2 O P O 3 2 -

O H

O H H

O H H

Ribose-5-phosphate

C H O

C H 2 O P O 3 2 -

O H H

C H 2 O P O 3 2 -

O H

O H H

O H H

H

C

C H 2 O H

H O H O

Sedoheptulose- 7-phosphate

Glyceraldehyde- 3-phosphate

H

RNA

CHO

CH2 OPO32 -

OHH

OHH

Erythrose-4-phosphate

CHO

CH2 OPO32 -

OHH

CH2 OPO32 -

OH

OHH

H

C

CH2 OH

HO H

O

Sedoheptul ose-7-phosphate

Glyceraldehyde-3-phosphate

CH2 OPO32 -

OH

OHH

H

C

CH2 OH

HO H

O


OHH

CHO

CH2 OPO32 -

OHH

Glyceraldehyde-3-phosphate

CH2 OPO32 -

OH

OHH

H

C

CH2 OH

HO H

O


CH2 OH

C

CH2 OPO32 -

O

HHO

OHH

Xyl ulose-5-phosphate

CHO

CH2 OPO32 -

OH

OHH

H

Erythrose-4-phosphate

To glycolysis


– 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-PD

• 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 affect an estimated 400 million people.

• Hemolysis, abdominal pain, dizziness, headache, dyspnea, palpitation, neonatal jaundice

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.


• Summary of oxidative reactions • Glucose-6-phosphate + 2 NADP

Ribulose-5-phosphate + CO2 + 2 NADPH

• Summary of non-oxidative reactions Reactant Enzyme Products

C 5 + C 5 Transketolase

C 7 + C 3

C 7 + C 3 C 6 + C 4

C 5 + C 4 C 6 + C 3

Transaldolase

Transketolase

3 C 5 2 C 6 + C 3 Net:

Relationship between PPP and Glycolysis

End Chapter 18

Storage Mechanisms and Control of Carbohydrate …Learning Objectives 1. How Is Glycogen Produced...

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Transcript of Storage Mechanisms and Control of Carbohydrate …Learning Objectives 1. How Is Glycogen Produced...