CAROHYDRATE METABOLISM

PART 2

BY

PROF.DR. SOUAD ABOAZMA

OXIDATION OF GLUCOSE

The pathways for oxidation of glucose are classified into two main

groups:

a- The major pathways for complete oxidation of glucose

into CO2, H2O and energy are:

1- Glycolysis → convert one molecule of glucose into 2 mol of

pyruvic acid + 2 NADH.H+.

2- Oxidative decarboxylation of pyruvic to acetyl CoA +

NADH.H++CO2

3- Complete oxidation of acetyl CoA in Kerb’s cycle into CO2, H2O

and energy .

b- The minor pathways for oxidation, which are not for energy

production.

1- Hexose monophosphate pathway (HMP).

2- Uronic acid pathway.

GLYCOLYSIS

EMBDEN-MEYERHOF PATHWAY

acid in presence of pyruvicoxidation of glucose to give Def.:O2 and lactic acid in absence of mitochondria (RBCs) and in

absence of O2 .

Site: Cytoplasm of all cells especially muscles and RBCs.

Steps:H – C = O

H C – OH

OH – C – H

H – C – OH

H – C – OH

CH2OH

H – C = O

H – C – OH

OH – C – H

H – C – OH

H – C – OH

CH2O-P

Mg

ATP ADP

Hexokinase, glucokinase

G-6-PD-Glucose

Mechanism of oxidation of glyceraldehydes 3-phosphate. Enz: glyceraldehydes 3-P dehydrogenase which is inhibited by the –SH poison iodoacetate, thus able to inhibit glycolysis.

:GLYCOLYSISENERGY PRODUCTION FROM

A. glycolysis in presence of O2 (Aerobic glycolysis):

Reaction catalyzed by ATP production

Stage I

1. Hexokinase/Glucokinase reaction (for

phosphorylation)

-1 ATP

2. Phosphofrutokinase-1 (for phosphorylation) -1 ATP

Stage III

3. Glyceraldehyde-3-P dehydrogenase (oxidation of

2 NADH in electron transport chain)

+ 6 or +4 ATP

4. Phosphoglycerate kinase (substrate level

phosphorylation)

+2 ATP

Stage IV

5. Pyruvate kinase (substrate level phosphorlyation) +2 ATP

Net gain = 10 or 8 - 2 =

8 or 6ATP

):glycolysis(Anaerobic 2in Absence of OGlycolysisB.

•In absence of O2 re-oxidation of NADH at glyceraldehyde-3-P-

dehydrogenase stage cannot take place in electron-transport

chain.

But the cells have limited coenzyme. Hence to continue the

glycolytic pathway NADH must be oxidized to NAD+. This is

achieved by reoxidation of NADH by conversion of pyruvate

to lactate (without producing ATP) by the enzyme lactate

dehydrogenase. Occurs in cells with no mitochondria as RBCs

(mature) ,or under low O2 supply as intensive muscular exercise.

In anaerobic glycolysis per molecule of glucose oxidation 4 - 2 = 2 ATP

will be produced.

REGULATION OF GLYCOLYSIS

There are 3 types of mechanisms responsible for regulation of

the enzyme activity which are:

1- Changed in rate of enzyme synthesis that affect the

quantity of enzyme as:

* Induction →↑ rate of enzyme synthesis at gene expression

→↑ mRNA synthesis →increase enzyme concentration.

* Repression →↓ rate of enzyme synthesis at gene expression

→↓ mRNA synthesis →decrease enzyme concentration.

2- Covalent modification by reversible phosphorylation

dephosphorylation.

3- Allosteric regulation by allosteric activator or inhibitor that

affect the quality of the enzymes.

A- Allosteric regulation of glycolysis:

irreversible reaction in 3 regulatory enzymes which responsible for 4 There are

.glycolysis

Hexokinase1.It is found in most tissues to give G-6-P when blood glucose level is low.

2.Acts on glucose and other hexoses to give hexose-6-P.

3.It has low km and Vmax→ acts maximally at fasting bl. glucose level.

4.It is inhibited by its products, which is G-6-P → allosteric feedback inhibition.

Glucokinase1.It is found in liver and acts maximally after meal.

2.Acts only on glucose.

3.It has a high km and high Vmax → so it is active when bl. glucose level is high (after meal).

4.It is induced (↑its rate of synthesis) by insulin.

5.It is not inhibited by G-6-P.

Phosphofructokinase1.It is the major regulatory enzyme in most tissues.

2.It is allosterically activated by F-6-P, AMP and inhibited by ATP, citrate, and H+.

Pyruvate kinase1.It is allosterically inhibited by ATP, fatty acids, alanine, and acetyl CoA. And activated by

F-1-6 diphosphate.

2. It is phosphorylated by cAMP dependent protein kinase, which becomes inactive

and dephosphorylated by phosphatase enzyme, which becomes active.

•Hormonal regulation:-B

Insulin/glucagons ratio is the main hormonal regulation of glucose

utilization; it increases during glucose feeding and decreases during

fasting.

A.Glucagons: it is secreted in case of carbohydrates deficiency or in

response to low blood glucose level (hypoglycemia). It affects liver

cells mainly as follows:

1.It acts as repressor of glycolytic key enzymes except hexokinase.

2.Through cAMP-dependent protein kinase A, it produces

phosphorylation of specific protein enzymes that lead to

inactivation of glycolytic key enzymes( only for pyruvate kinase).

B.Insulin: it is secreted after feeding of carbohydrate or in response to

high blood glucose level (hyperglycemia). It stimulates all pathways of

glucose utilization. Insulin binds to a specific cell membrane receptors

and produces certain signal cascade, which results in the following:

1.It acts as inducer for glycolytic key enzymes.

2.It activats phosphodiesterase enzyme(decreases cAMP that

leads to inhibition of protein kinase A).

3.It activats protein phosphatase-1 that produces

dephosphorylation of glycolytic key enzymes and their activation.

INHIBITORS OF GLYCOLYSIS:

1- Aresnate : which used in oxidative step

insted of Pi→ so glycolysis proceeds in

presence of arsenate but ATP, which formed

from 1-3 diphosphoglycerate is lost.

2- Iodoacetate produces inhibition of

glyceraldehydes-3-P dehydrogenase (inhibitor

of SH group).

3- Flouride inhibits enolase →↓↓ glycolysis in

bacteria →no production of lactic acid produced

by bacteria, which cause dental caries. It used

as anticoagulant in blood sample used for

estimation of blood glucose →↓↓ glycolysis in

RBCs .

:IN RBCSDIPHOSPHOGLYCERATE3 ,2FORMATION OF

2:3 diphosphoglycerate has an effect on O2 binding power of

haemoglobin→ It lowers O2 affinity by haemoglobin →↑ dissociation of

O2 to the peripheral tissues as in cases of high altitude.

CLINICAL SIGNIFICANCE OF 2,3 DIPHSOPHOGLYCERATE:

1- Persons who live at high altitude undergo state of low

O2 affinity for HB due to simultaneous increase of 2,3

diphosphoglycerate. This increase can be reversed on

returning to sea level.

2- Fetal HB has less 2,3 diphosphoglycerate than adult

HB, so fetal HB has high O2 affinity.

3- During storage of blood in blood banks, there is

decrease in 2,3 diphosphoglycerate so, stored blood has

high O2 affinity, which is not suitable for blood transfusion

especially to ill patients. If 2,3 diphosphoglycerate is

added to stored blood, it can’t penetrate RBCs wall. So, it

is advisable to add insoine, which is a substance that can

penetrate RBCs wall and change it into 2,3

diphosphoglycerate through HMP shunt.

GLYCOLYSISDIFFERENCES BETWEEN AEROBIC AND ANAEROBIC

Aerobic glycolysis Anaerobic glycolysis

- Site Cytoplasm of all

tissues

RBCs and skeletal muscle

during muscular ex.

- End products Pyruvic acid +

NADH.H+

Lactic acid + NAD+

- Energy production 6 OR, 8 ATP 2 ATP

- Lactate dehdyrogenase Not needed Needed

GLYCOLYSISDISEASES ASSOCIATED WITH IMPAIRED

:deficiency Hexokinase-1•In patients with inherited defects of hexokinase activity, the red blood cells contain low

concentrations of the glycolytic intermediates including the precursor of 2,3-DPG.

•In consequence, the hemoglobin of these patients has an abnormally high oxygen affinity.

•The oxygen saturation curves of red blood cells from a patient with hexokinase deficiency

are shifted to the left, which indicates that oxygen is less available for the tissues.

2- Pyruvate kinase deficiency (hemolytic anemia):•All red blood cells are completely dependent upon glycolytic activity for ATP production.

•Failure of the pyruvate kinase reaction, the production of ATP will decrease leading to

hemolysis of red cells.

•Inadequate production of ATP reduces the activity of the Na+ - and K+ -stimulated ATPase

ion pump.

3- Lactic acidosis:-•Blood levels of lactic acid are normally less than 1.2 mM. In lactic acidosis, the values for

blood lactate may be 5 mM or more.

•The high concentration of lactate results in lowered blood pH and bicarbonate levels.

•High blood lactate levels can result from increased formation or decreased utilization of

lactate.

•Common cause of hyperlacticidemia is anoxia.

•Tissue anoxia may occur in shock and other conditions that impair blood flow, in respiratory

disorders, and in severe anemia.

AEROBIC AND ANAEROBIC EXERCISE USE DIFFERENT FUELS

distance running, while -Aerobic exercise is exemplified by longanaerobic exercise by sprinting or weight lifting.

organ -there is really very little interanaerobic exercise During

cooperation. The vessels within the muscles are compressed during peak

contraction, thus their cells are isolated from the rest of the body. Muscle

largely relies on its own stored glycogen and phosphocreatine.

energy phosphate for ATP -serves as a source of highPhosphocreatineare glycolysisand glycogenolysisuntil seconds 5 -4for first synthesis

becomes the primary source of ATP for want of Glycolysis. stimulatedoxygen.

is metabolically more interesting. For moderate Aerobic exercise . of muscle glycogen.glycolysisenergy is derived from thetexercise, much of

However, a well-fed individual doesn't store enough glucose and glycogen to

provide the energy needed for running long distances. The respiratory

quotient, the ratio of carbon dioxide exhaled to oxygen consumed, falls

progressive switch from glycogen during distance running. This indicates the

gradually increases as glucose Lipolysisto fatty acid oxidation during a race. stores are exhausted, and, as in the fast state, muscles oxidize fatty acids in

. preference to glucose as the former become available

Marathon runner (42.2 KM or 26 Miles)

•In the marathon runner predominantly red fibres (oxidative)

are used.

•Red fibres contain myoglobin and mitochondria.

•The major sources of energy in marathon runner are:-

•Aerobic metabolism is the principal source of ATP.

•Blood glucose.

•Hepatic glycogen is degraded to maintain the level of blood

glucose.

•Muscle glycogen is also a source of fuel but it is degraded

slowly than in sprinter .

•Free fatty acids derived from the triacylglycerol of adipose

tissue. Sources of fuel used by marathon runner and there duration of action

Source of fuel Duration of action by minutes

Blood glucose

Liver glycogen

Muscle glycogen

Triacylglycerol

4

18

70

4000

MAJOR FEATURES OF SKELETAL MUSCLE S METABOLISM

1.Skeletal muscle functions under both aerobic (resting) and anaerobic (eg, sprinting)

conditions, so both aerobic and anaerobic glycolysis operate, depending on conditions.

2.Skeletal muscle contains myoglobin as a reservoir of oxygen.

3.Insulin acts on skeletal muscle to increase uptake of glucose.

4.In the fed state, most glucose is used to synthesize glycogen, which acts as a store of glucose for

use in exercise, 'preloading' with glucose is used by some long-distance athletes to build up stores

of glycogen.

5.Epinephrine stimulates glycogenolysis in skeletal muscle, whereas glucagon does not because of

absence of its receptors.

6.Skeletal muscle cannot contribute directly to blood glucose because it does not contain glucose-6-

phosphatase.

7.Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to

synthesize glucose, which can then return to muscle, (the cori cycle).

8.Skeletal muscle contains phosphocreatine, which acts as an energy store for short-term (seconds)

demands.

9.Free fatty acids in plasma are a major source of energy, particularly under marathon conditions

and in prolonged starvation.

10.Skeletal muscle can utilize ketone bodies during starvation.

11.Skeletal muscle is the principle site of metabolism of branched chain amino acids, which are

used as energy source.

12.Proteolysis of muscle during starvation supplies amino acids for gluconeogenesis.

13.Major amino acids emanating from muscle are alanine (destined mainly for gluconeogenesis in

liver and forming part of the glucose-alanine cycle) and glutamine (destined mainly for the gut and

kidneys).

Significance of glycolysis•Glycolysis is the principle route for glucose

metabolism for the production of ATP molecules

•It also provide pathway for the metabolism of

fructose and galactose derived from diet.

•It represent the only source of energy for RBCs and

contracting muscle.

•It provide mitochondria with pyruvic which give

acetyle CoA (Kreb's Cycle)

•Glycolysis give DHAP which reduced to α

glycerophosphate which form backbone of

triacylglycerol in lipogenesis

•In erythrocytes glycolysis supplies 2,3 DPG which

is required for haemoglobin function in transport of

oxygen.

ACIDPYRUVICOF DECARBOXYLATIONOXIDATIVE

keto-αacid and other pyruvicIt is conversion of :Def.acids into CoA derivatives, CO2 & NADH.H+.

RBCsIn mitochondrial matrix of all tissues except : SiteCoAacid into acetyl pyruvicThe conversion of : Steps

is catalyzed by pyruvate dehydrogenase complex, which

composed of 3 enzymes act cooperative with each other

in presence of 5 co-enzymes: TPP, lipoic acid, FAD,

NAD+, and CoASH.

Pyruvate +TTP + Lipoic acid + CoA +FAD+ NAD+ --→ CO2 + Acetyl-CoA + NADH + H+

acid:pyruvicof decarboxylationSteps of oxidative •Pyruvate is decarboxylated to form a hydroxyethyl

derivative bound to the reactive carbon of thiamine

pyrophosphate, the coenzyme of pyruvate decarboxylase.•The hydroxyethyl intermediate is oxidized by transfer to the

disulfide form of lipoic acid covalently bound to

dithydrolipoyl transacetylase.•The acetyl group, bound as a thioester to the side chain of

lipoic acid, is transferred to CoA.

•The sulfhydryl form of lipoic acid is oxidized by FAD-dependent dihydrolipoyl dehydrogenase, leading to the

regeneration of oxidized lipoic acid.

•Reduced flavoprotein is reoxidized to FAD by dihydrolipoyl

dehydrogenase and NAD+.

OF DECARBOXYLATIONREGULATION OF OXIDATIVE

ACID :PYRUVIC

1- Product inhibition : The enzyme complex is inhibited by acetyl CoA, which

accumulates when it is produced faster than it can be oxidized by

citric acid cycle. The enzyme is also inhibited by elevated levels of

NADH+.H, which occure when the electron transport chain is

overloaded with substrate and oxygen is limited.

2- Covalent modification:The pyruvate dahydrogenase complex exists in two forms: an active

nonphosphorylated form and an inactive phosphorylated form.Phosphorylated

and nonphosphorylated pyruvate dehydrogenase can be interconverted by

two separate enzymes, a kinase and a phosphatase. The kinase is activated

by increase in the ratio of acetylCoA/ CoA or NADH/ NAD+. An increase in the

ratio of ADP/ATP, which signals increased demand for energy production ,

inhibits the kinase and allows the phosphatase to produce more of the active

,nonphosphorylated enzyme.

:METABOLISMPYRUVATECLINICAL ASPECTS OF

metabolism leads to lactic pyruvateInhibition of :acidosis, which may be due to

1- Arsenite or mercuric ions complex the –SH

group of lipoic acid.

2--Dietary deficiency of thiamin as in alcoholics.

These two factors lead to inhibition of pyruvate

dehydrogenase complex.

3- Inherited pyruvate dehydrogenase deficiency,

which may be due to defects in one or more of

the components of the enzyme complex.

CITRIC ACID CYCLE

CYCLE) KREB’SACID CYCLE (TRICARBOXYLIC

Def.:

It is the series of reactions in mitochondria, which

oxidized acetyl CoA to CO2, H2O & reduced H2 carriers

(as NADH.H+ & FADH2) that oxidized through

respiratory chains for ATP synthesis.

Site:

Mitochondria of all tissue cells except RBCs, which

not contain mitochondria. The enzymes of the cycle

are present in mitochondrial matrix except succinate

dehydrogenase, which is tightly bound to inner

mitochondrial membrane.

Steps:

ENERGY PRODUCTION:

:ENERGY PRODUCTION FROM OXIDATION OF ONE MOLECULE OF GLUCOSE

REGULATION OF KREB’S CYCLE:

1- As the primary function of TCA cycle is to provide energy, respiratory

control via the E.T.C and oxidative phosphorylation exerts the main control.

2- In addition to this overall and coarse control, several enzymes of TCA

cycle are also important in the regulation.

Three key enzymes are:

(a)Citrate synthase. (b)Mitochondrial isocitrate dehydrogenase.

(c)α-ketoglutarate dehydrogenase.

These enzymes are responsive to the energy status as expressed by the [ATP]/[ADP] ratio and [NADH]/[NAD+] ratio.

(a)Citrate synthase enzymes is allosterically inhibited by ATP and long-chain

acyl CoA.

(b)NAD+-dependent mitochondrial iso-citrate dehydrogenase (ICD) is activated

allosterically by ADP and is inhibited by ATP and NADH.

(c)α-ketoglutarate dehyrogenase complex which allosterically inhibited by succinyl

CoA, NADH-H+ and ATP.

3- In addition to above succinate dehydrogenase enzyme is inhibited by oxaloacetate

(OAA) and the avability of OAA is controlled by malate dehydrogenase, which

depends on [NADH]/[NAD+] ratio.

CYCLEKREB’SFUNCTIONS OF •lipids CHO,It is the final pathway for complete oxidation of all foodstuffs -1

and protein, which are converted to acetyl CoA.

2- It is the major source of energy for cells except cells without mitochondria

as RBCs.

3- It is the major source of succinyl CoA, which used for:

1.Porphyrine and HB synthesis.

2.Ketone bodies activation.

3.Converted to OAA → glucose.

4. Detoxication by conjugation

•4- Synthetic functions of Kreb’s cycle:

a- Amphibolic reactions.

Some components of Kreb’s cycle are used in synthesis of other

substances as:

In fasting state, oxaloacetic acid is used for synthesis of glucose

by gluconeogenesis.

In fed state, citric acid is used for synthesis of fatty acids.

Reactions of Kreb’s cycle are used for synthesis of amino acid

(transamination into non essential amino acids) eg:

-OAA + glutamic acid aspartic acid + α-ketoglutarate.

-Pyruvic acid + glutamic acid alanine + α-ketoglutarate.

A anabolic role of the citric acid cycle

•b- Anaplerotic reactions.

Synthesis of one or more component of Kreb’s cycle from outside

the cycle:

O.A.A. can be synthesized from pyruvic acid by pyruvate

carboxylase, and from aspartic acid by transamination.

Fumarate can be synthesized from phenylalanine and tyrosine.

Succinyl CoA can be synthesized from valine, isoleucine,

methionine, and threonine.

α-ketogluterate can be synthesized from glutamic acid by

transamination.

Inhibitors of Citric Acid Cycle

1-Flouro-acetate reacts with oxalacetate forming

flourocitrate, which inhibits the aconitase enzyme.

2-Arsenite inhibits α-ketogluterate dehydrogenase.

3-Malonate acts as competitive inhibitor for succinate

dehydrogenase.

Major anaplerotic pathways of citric acid cycle

ROLES OF VITAMINS IN CITRIC ACID CYCLE

Four of the soluble vitamins of B complex have

important roles in cirtic acid cycle. They are:

-αin the form of FAD, a cofactor in ,riboflavin-1

ketogluterate dehydrogenase complex and in

in the form of ,niacin-2; dehydrogenasesuccinate

NAD, the coenzyme for three dehydrogenases in the

cycle, isocitrate dehydrogenase, α-ketogluterate

thiamin-3; dehydrogenasemalateand dehydrogenase

(vitamin B1), as TPP, the coenzyme for decarboxylation

in α-ketogluterate dehyrdogenase reaction; and

as part of coenzyme A, which , pantothenic acid-4

present in the form of acetyl-CoA and succinyl-CoA.

-4- Pyruvic acidMalic enzym

Biotin NADPH-H CO2Malic acid

1-Pyruvic acidPyruvate carboxylase

ATPCO2,++ biotin, Mn

ADP

O.A.A

3- Acetyl CoAAcetyl CoA carboxylase

ATPCO2, Mn++, biotin

ADP

Malonyl CoA

2-PropionylCOA

Propionyl CoA carboxylase

ATP

CO2, Mn++, biotin

ADP

D- methylmalonyl CoA → L-MMCoA

CAC ← Succinyl CoA

carboxylationfixation or 2COIt is an addition of CO2 to the molecule in presence of CO2, biotin, Mn++, ATP,

and specific carboxylase

- CO2 is produced by α – ketoglutarate dehydrogenase , isocitrate

dehydrogenase and pyruvate dehydrogenase complex examples for

carboxylation :-

5- Synthesis of carbomyl phosphate of urea cycle and pyrimidine.

6- Formation of Carbon number 6 of purine.

7- Synthesis of H2CO3/NaHCO3 buffer system

GLUCONEOGENESIS

It is the formation of glucose from non CHO sources. Its main function is to supply blood glucose in cases of carbohydrate deficiency (fasting, starvation, and low carbohydrate diet). Sites:Cytoplasm and mitochondria of liver and kidney due to presence of glucose-6-phophatase and fructose-1, 6-biphosphatase.

Steps:A.These mechanisms are concerned with conversion of glucogenic amino acids., lactate, glycerol, and propionic acid to glucose, which are reverse to glycolytic pathway (except for three irreversible kinases) to supply erythrocytes, skeletal muscles, nervous system, and mammary glands with their need of glucose.B.Energy barrier will obstruct reversal of glycolysis at the following sites:

1- Conversion of pyruvate to phosphoenol pyruvateThis can be overcomed by dicarboxylic acid shuttle, as the

following :-1.In mitochondria, pyruvic acid is converted to oxalacetatein presence of pyruvate carboxylase, ATP, biotin, Mn++ and CO2.2.Oxalacetate can not diffuse outside mitochondria →converted into compounds that can diffuse from mitochondria as malate, aspartate, and citrate → they are converted once again to oxalacetate in extramitochondriaportion of the cells.

-Oxalacetate reformed in extramitochondriacompartment is converted to phosphoenol pyruvatein presence of phosphoenol pyruvatecarboxykinase enzyme and GTP.

Citrate

Phosphoenol

pyruvateCarbocxykinase

Oxalacetate

MalateAspartate

Pyruvic acid

Pyruvate

Pyruvate

carboxylase

CO2

ATP

Mn++

ADP

Oxalacetic acid

Citrate

Aspartic acid

MalateReduction H2

NH2

Transamination

Attachment to acetyl CoA

Mitochondria

2- Conversion of fructose 1:6 biphosphate to F-6-P: -

This occurs by fructose 1:6 biphosphatase, which present in liver and kidneys.

3- Conversion of Glucose-6-P to glucose:This is catalysed by another enzyme, which is

G-6-Phosphatase that is present in liver, intestine, and kidney.

Carbon sources for glucoeogenesis:

1- Propionic acid:1.It is the product of odd number fatty acid oxidation by βoxidation.2.It is converted into succinyl CoA, which converted into oxalacetic acid → phosphoenol pyruvic →→Glucose.2- Glycerol:

glycerol-3-P converted into dihydroxy acetone-P, which can be converted by trio’s isomerase into glyceraldehydes-3-P→→→ glucose.

Glycerokinase is present in liver and kidney mainly3- Glucogenic amino acids:

Amino acids by deamination can be converted into α-keto acids as pyruvic, α-ketoglutaric and OAA → they can be converted into glucose. Proteins are considered as one of the main sources of blood glucose especially after 18 hours due to deplation of liver glycogen.

4- Lactic acid:In vigorous skeletal muscle activity, large amount of lactic acid produced → passes to the liver through blood stream → converted in liver into pyruvic acid and lastly to glucose → reach muscle once again through blood → this cycle called Cori cycle.Importance of Cori cycle:•It prevents loss of lactate as waste products in urine.•Oxidation of reduced NAD.•It supplies red cells and contracting muscles with glucose for reutilization and ATP production.•Prevent accumulation of lactic acid, which change pH of blood.

5- Glucose-alanine cycle:During starvation there is muscle protein catabolism →in presence of NH3 and pyruvic acid (produced from glycolysis), alanine is formed →reach liver and converted into pyruvic acid, which give glucose through glucogenesis and NH3 which converted into urea →excreted in urine.Significant of glucose alanine cycle:•Disposal of NH3 produced from muscle protein catabolism through formation of urea, which excreted in urine.•Conserve NAD/NADH.H+ ratio.

REGULATION OF GLUCONEOGENESISKey enzymes which regulate gluconeogeneis are: Pyruvate carboxylase, Phospho-enol-Pyruvate carboxykinase (PEPCK), Fructose-1, 6-bi-phosphatase and Glucose-6-phosphatase.

A -short time regulation:-1- Effect of increased fatty acid oxidation on gluconeogenesis : Fasting,

low carbohydrates in diet, stress and sever muscular exercise stimulate

secretion of anti-insulin hormones which increase lipolysis in adipose

tissues. The increased fatty acids oxidation acts as a signal to inhibit

glycolysis and stimulate gluconeogenesis in the liver by the following

mechanisms:

*It increases the production of ATP which produce allosteric inhibition of

phosphofructokinase-1 (PFK-1) and pyruvate kinase.

*The excess acetyl-CoA, produced by fatty acid oxidation allosterically

stimulate pyruvate carboxylase and inhibites pyruvate dehydrogenase thus

direct pyruvate to gluconeongesis.

*Excess citrate (produced from acetyl CoA) produces allosteric inhibition of

PFK-1 and PFK-2.

.

2- Fructose 2,6-bisphosphate plays a unique role in the

regulation of glycolysis and gluconeogenesis in liver

Fructose 2,6 bisphosphate is formed by phosphorylation of fructose

6-phosphate by PFK-2, the same enzyme is also responsible for its

breakdown, since it has fructose 2,6 bisphosphatase activity.

Fructose 2,6 bisphosphatase (bifunctional enzyme ) is under the

allosteric control of fructose 6-phosphate which stimulate kinase and

inhibit phosphatase.

Carbohdrate feeding and insulin stimulate PFK-2 and inhibite

fructose 2,6 bisphosphatase, producing accumulation of fructose 2,6

bisphosphate which produce allosteric activation of glycolytic key

enzyme PFK-1 and allosteric inhibition of gluconeogenesis key

enzyme fructose 1,6 bisphosphatase.

In fasting state, glucagons stimulate production of cAMP which

activate cAMP dependent protein kinase-A which in turn inactivates

PFK-2 and activate fructose 2,6 bisphosphatase. Hence

gluconeogenesis is stimulated by decrease in the concentration of

fructose 2,6 bisphosphate which inactivate PFK-1 and relieves the

inhibition of Fructose 1,6 bisphosphate.

Fructose-6-p

F-1,6 BpasePFK-1F2-6Bp

ATP

ADP

ATP

ADP

PFK-2

+

F-1,6-bisphosphate

glycolysis

F2-6Bp

ATP

ADP

PFK-2

Gluconeogenesis favoured

P

Control of glycolysis and gluconeogenesis in liver by fructose 2,6- bisphosphate

-long time regulation:-B

Glucagon and insulin also have longterm effects on hepatic glycolysis and gluconeogenesis by induction and repression of key enzymes of both pathway. A high glucagon/insulin ratio in blood increases the capacity for gluconeogenesis and decreases that for glycolysis in liver. A low glucagon/insulin ratio has the opposite effects. Glucagon signals induction of greater quantities of PEP carboxykinase, Fructose 1,6 bisphosphatase and glucose 6-phosphatase.

Pentose phosphate pathway

Definition

•The penstose phosphate pathway is an alternative rout for

the metabolism of glucose, ATP neither produced nor utilized.

It is the pathway for formation of pentose phosphate.

•The pentose phosphate pathway is also described as a

shunt rather than pathway because it shunts (to move from

one trak to another) hexoses from glycolysis forming

pentoses, which may be cycled back into the pathway of

glycolysis by conversion into fructose 6-phosphate and

glycerldehde-3 phosphate and used for resynthesis of

glucose-6 phospahte so named hexose monophosphate

shunt is also phosphogluconate pathway.

• It is a multicyclic process in which three molecules of glucose 6-phosphate give rise to :

-Three molecules of Co2-Three molecules of pentoses-6 molecular of NADPH The pentoses are rearranged to generate two

molecules of glucose-6-phosphate and one molecule of glycerldehyde 3-phosphate .

Site: occurs in cytoplasm of many tissues including liver, adipose tissues, adrenal, ovary, testis, red cell and retina

The main functions of pentose shunt are:1-Production of NADPH-H+, which used in:Synthesis of fatty acids and cholesterol.Reduction of oxidized glutathione.Activation of folic acid.

2-Formation of pentoses in the form of ribose-5 P, which used in:Synthesis of nucleotides either free as ATP, or polynucleotides as DNA and RNA.Synthesis coenzymes as FAD, NAD+.

Steps: I- Oxidative irreversible reaction ( phase I) which include:•the oxidative portion of HMP consists of three reactions that lead to formation of ribulose-5-P, CO2 and two molecules of NADPH-H+ for each glucose-6 phosphate oxidized.2- Non oxidative revesible reaction which include:

a- Phase II:Ribulose-5-P by ribulose-5-phospho epimerase is converted into xylulose-5-P while by ribulose-5 phosphe isomerase is converted into ribose-5-P.

b- phase III :-pentose-5-P is converted into glyceraldehydes-3-P and fructose-6-P by transketolase and transaldolase. These enzymes create a reversible link between pentose phosphate pathway and glycolysis by catalyzing these 3 reactions.

.

Function of non oxidative reversible phase : Formation of different pentoses 5-PIn skeletal muscles due to deficiency of G-6-pD, so muscle can synthesize ribose 5 phosphate from fructose-6-phosphate and glyceraldehydes 3-phosphate by pathway reversible to phase II.RBCs need more NADPH it than pentose-5 so convert three pentose-5-p into glycolytic intermediate.

REGULATION OF PENTOSE PHOSPHATE PATHWAY ( PPP):1- G-6-P dehydrogenase increased in amount when diet contains excess

carbohydrates.

2- The regulatory enzymes of PPP are g-6-P dehydrogenase and

lactonase enzymes.Insulin acts as inducer for their synthesis.

3- NADPH.H+ accumulation produces feedback inhibition of g-6-

dehydrogenase.

PPP is the major sources of NADPH.H+ but there are 2 other sources

for NADPH.H+ which present in cytoplasme:

1)cytoplasmic isocitrate dehydrogenase.

2)malic enzyme which convert malic acid into pyruvic acid ,CO2 and

NADPH.H+ .

4- Depending on the needs of the cell, conversion of pentose-5-P to

hexose-6-P can occure. The reaction favour the formation of

glyceraldehydes-3-P → glycerol-3-P which used in synthesis of

phosphoglycerides as triacylglycerol. Also glyceraldehydes-3-P can be

converted to 2,3 diphosphoglycerate in RBCs which ↓↓ O2 association

with HB→↑O2 dissociation to the tissues.

Role of NADPH in Red blood cell:•Red blood cells need NADPH to maintain glutathione in reduced form to protect themselves from oxidizing agents. •The red cell is exposed to large amount of molecular oxygen. Some of the molecular oxygen is converted to superoxide and H2O2 that can cause irreversible damage to the cell. •Reduced glutathione, a strong reducing agent protects against damage by reducing H2O2 to H2O.•The role of NADPH is to maintain glutathione in reduced form which is essentially required to preserve the integrity of RBCs. It also keeps iron of the haemoglobin in reduced ferrous state and prevents formation of methaemoglobin

Role of NADPH in disposal of H2O2

Deficiency of glucose 6-phosphate dehydrogenase.•Several types of X-linked inherited deficiency of the enzyme glucose-6-phosphate dehydrogenase have been recognized. •Enzyme deficient cells have a lower rate of NADPH production, resulting in a deficiency of reduced glutathione (GSH) which is essential to maintain the integrity of erythrocyte membrane and for keeping HB in the ferrous state. •Most individuals who have G-6-pD mutation are asymptomatic. Some individuals develop haemolytic anaemia if they are exposed to : •Certain oxidant drugs e.g. Antibiotic (sulphamethoxazole), antimalarial(primaquine), Antipyretic (acetoaminiphen).•Certain type of infection: The inflammatory response of infection generate free radicals which can damage the red cells easily if G6-PD is deficient. •Favism (due to ingestion of fava beans)•Favism is the haemolytic anaemia due to ingestion of fava beans (broad beans) in individuals with G-6-pD deficiency •Fava beans contain the purine glycosides these compound react with glutathione leading to reduced level of glutathione (GSH).•Favism is not observed in all individuals with G-6-PD deficiency but all patients with favism have G-6-PD deficiency.