3. METABOLIC
PATHWAYS
CATABOLIC PATHWAYS
Are involved in oxidative
breakdown of larger
complexes.
They are usually
exergonic in nature
ANABOLIC PATHWAYS
Are involved in the
synthesis of
compounds.
They are usually
endergonic in nature.
4. CHARACTERISTICS OF METABOLISM
1. Metabolic pathways are mostly
irreversible
2. Every metabolic pathway has a
committed first step.
3. All metabolic pathways are regulated.
4. Metabolic pathways in eukaryotic cells
occur in specific cellular locations.
5. GLYCOLYSIS
Glycolysis comes from a merger of two Greek words:
ďGlykys = sweet
ďLysis = breakdown/ splitting
It is also known as Embden-Meyerhof-Parnas pathway
or EMP pathway.
6. INTRODUCTION
⢠GLYCOLYSIS is the sequence of 10 enzyme-catalyzed
reactions that converts glucose into pyruvate with
simultaneous production on of ATP.
⢠In this oxidative process, 1mol of glucose is partially
oxidised to 2 moles of pyruvate.
⢠This major pathway of glucose metabolism occurs in
the cytosol of all cell.
⢠This unique pathway occurs aerobically as well as
anaerobically & doesnât involve molecular oxygen.
7. ⢠It also includes formation of Lactate from Pyruvate.
⢠The glycolytic sequence of reactions differ from
species to species only in the mechanism of its
regulation & in the subsequent metabolic fate of
the pyruvate formed.
⢠In aerobic organisms, glycolysis is the prelude to
Citric acid cycle and ETC.
⢠Glycolysis is the central pathway for Glucose
catabolism.
8. Glucose
Extracellular
matrix & cell wall
polysachharide.
Glycogen,
Starch,
Sucrose
Pyruvate
Ribose-5-
phosphat
e
Oxidation via
pentose phosphate
pathway
Synthesis of
structural polymers
storage
Oxidation
via glycolysis
Major pathways of
glucose utilization.
9.
10. TWO PHASES OF GLYCOLYSIS
⢠Glycolysis leads to breakdown of 6-C glucose
into two molecules of 3-C pyruvate with the
enzyme catalyzed reactions being bifurcated
or categorized into 2 phases:
1. Phase 1- preparatory phase
2. Phase 2- payoff phase.
11. PREPARATORY PHASE
⢠It consists of the 1st 5 steps of glycolysis in which
the glucose is enzymatically phosphorylated by ATP
to yield Fructose-1,6-biphosphate.
⢠This fructuse-1,6-biphosphate is then split in half to
yield 2 molecules of 3-carbon containing
Glyceraldehyde-3-phosphate/ dihyroxyacteone
phosphate.
12. ⢠Thus the first phase results in cleavage of the
hexose chain.
⢠This cleavage requires an investment of 2 ATP
molecules to activate the glucose mole and prepare
it for its cleavage into 3-carbon compound.
13.
14. PAYOFF PHASE
⢠This phase constitutes the last 5 reactions of
Glycolysis.
⢠This phase marks the release of ATP molecules
during conversion of Glyceraldehyde-3-phosphtae
to 2 moles of Pyruvate.
⢠Here 4 moles of ADP are phosphorylated to ATP.
Although 4 moles of ATP are formed, the net result
is only 2 moles of ATP per mole of Glucose oxidized,
since 2 moles of ATP are utilized in Phase 1.
15.
16. STEP 1: PHOSPHORYLATION
⢠Glucose is phosphorylated by ATP to form sugar
phosphate.
⢠This is an irreversible reaction & is catalyzed by
hexokinase.
⢠Thus the reaction can be represented as follows:
Glucose
Glucose-6-phosphate
Hexokinase
ATP
ADP
17. STEP 2: ISOMERIZATION
⢠It is a reversible rearrangement of chemical structure of
carbonyl oxygen from C1 to C2, forming a Ketose from the
Aldose.
⢠Thus, isomerization of the aldose Glucose6-phosphate
gives the ketose, Fructose-6-phoshphate.
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6-phosphate
18. STEP 3: PHOPHORYLATION
⢠Here the Fructose-6-phosphate is phosphorylated
by ATP to fructose-1,6-bisphosphate.
⢠This is an irreversible reaction and is catalyzed by
phosphofructokinase enzyme.
Fructose-6-phosphate
Fructose-1,6-bisphosphate
ATP
ADP
Phosphofructokinase
19. STEP 4: BREAKDOWN
⢠This six carbon sugar is cleaved to produce two 3-C
molecules: glyceradldehyde-3-phosphate (GAP) &
dihydroxyacetone phosphate(DHAP).
⢠This reaction is catalyzed by Aldolase.
Glyceraldehyde-3-
phosphate
Dihydroxyacetone
phosphate
Triose phosphate
isomerase
Fructose-1,6-
bisphosphate
Aldolase
20. STEP 5: ISOMERIZATION
⢠Dihydroxyacetone phosphate is oxidized to form
Glyceraldehyde-3-phosphate.
⢠This reaction is catalyzed by triose phosphate
isomerase enzyme.
Glyceraldehyde-3-phosphate
Dihydroxyacetone phosphate
Triose phosphate
isomerase
2
2
21. STEP 6
⢠2 molecules of Glyceraldehyde-3-phosphate are
oxidized.
⢠Glyceraldehyde-3-phosphate dehydrogenase
catalyzes the conversion of Glyceraldehyde3-
phosphate into 1,3-bisphosphoglycerate.
Aldehyde Carboxylic acid
Carboxylic
acid
Ortho-
phosphate
Acyl-
phosphate
product
Joining)
23. STEP 7
⢠The transfer of high-energy phosphate group that
was generated earlier to ADP, form ATP.
⢠This phosphorylation i.e. addition of phosphate to
ADP to give ATP is termed as substrate level
phosphorylation as the phosphate donor is the
substrate 1,3-bisphosphoglycerate (1,3-BPG).
⢠The product of this reaction is 2 molecules of
3-phosphoglycerate.
25. STEP 8
⢠The remaining phosphate-ester linkage in 3-
phosphoglycerate, is moved from carbon 3 to
carbon 2 ,because of relatively low free energy of
hydrolysis, to form 2-phosphoglycerate(2-PG).
3-phosphoglycerate
2-phosphoglycerate
Phosphoglycerate
mutase
2
2
26. STEP 9: DEHYDRATION OF 2-PG
⢠This is the second reaction in glycolysis where a
high-energy phosphate compound is formed.
⢠The 2-phosphoglycerate is dehydrated by the action
of enolase to phosphoenolpyruvate(PEP). This
compound is the phosphate ester of the enol
tautomer of pyruvate.
⢠This is a reversible reaction.
28. STEP 10: TRANSFER OF PHOSPHATE
FROM PEP to ADP
⢠This last step is the irreversible transfer of high
energy phosphoryl group from
phosphoenolpuruvate to ADP.
⢠This reaction is catalyzed by pyruvate kinase.
⢠This is the 2nd substrate level phosphorylation
reaction in glycolysis which yields ATP.
⢠This is a non-oxidative phosphorylation reaction.
30. OVERALL BALANCE SHEET OF
GLYCOLYSIS
⢠Each molecule of glucose gives 2 molecules of
Glyceraldehyde-3-phosphate. Therefore , the total
input of all 10 reactions can be summarized as:
Glucose + 2ATP+ 2Pi+ 2NADâş+ 2Hâş+ 4ADP
2Pyruvate+ 2Hâş+ 4ATP+ 2HâO+ 2NADH+ 2ADP
On cancelling the common terms from the above
equation, we get the net equation for Glycolysis:
31. Glucose+ 2Pi+ 2ADP+ 2NADâş
2Pyruvate+ 2NADH+ 2ATP+ 2Hâş + 2HâO
THUS THE SIMULTANEOUS REACTIONS INVOLVED IN
GLYCOLYSIS ARE:
ďGlucose is oxidized to Pyruvate
ďNADâş is reduced to NADH
ďADP is phosphorylated to ATP
32. ENERGY YIELD IN GLYCOLYSIS:
STEP NO. REACTION CONSUMPTION of ATP GAIN of ATP
1 1 -
3
Glucose glucose-6-phosphate
Fructose-6-phosphate
fructose-1,6-biphosphate
1 -
7 - 1x2=2
10
1,3-diphosphoglycerate
3-phosphoglycerate
Phosphoenolpyruvate pyruvate - 1x2=2
2 4
Net gain of ATP=4-2= 2
33.
34.
35.
36.
37.
38.
39. TCA Cycle
âŤAlso known as Krebs cycle
âŤTCAcycle essentially involves the oxidation of
acetyl CoAto CO2 and H2O.
âŤTCAcycle âthe central metabolic pathway
âŤThe TCA cycle is the final common oxidative
pathway for carbohydrates, fats, amino acids.
40. âŤTCA cycle supplies energy & also provides many
intermediates required for the synthesis of amino
acids, glucose, heme etc.
âŤTCAcycle is the most important central pathway
connecting almost all the individual metabolic
pathways.
41. âŤDefinition
âŤCitric acid cycle or TCAcycle or tricarboxylic acid
cycle essentially involves the oxidation of acetyl
CoAto CO2 & H2O.
âŤLocation of the TCAcycle
âŤReactions of occur in mitochondrial matrix, in
close proximity to the ETC.
44. Reactions of TCA cycle
âŤOxidative decarboxylation of pyruvate to acetyl
CoAby PDH complex.
âŤThis step is connecting link between glycolysis and
TCAcycle.
45. Reactions of TCA Cycle
âŤStep:1 Formation of citrate
âŤOxaloacetate condenses with acetyl CoA to form
Citrate, catalysed by the enzyme citrate synthase
âŤInhibited by:
âŤATP, NADH, Citrate - competitive inhibitor of
oxaloacetate.
46. Steps 2 & 3
Citrate is isomerized to isocitrate
âŤCitrate is isomerized to isocitrate by the enzyme
aconitase
âŤThis is achieved in a two stage reaction of
dehydration followed by hydration through the
formation of an intermediate -cis-aconiase
47. Steps 4 & 5
Formation of ďĄ-ketoglutarate
âŤIsocitrate dehydrogenase (ICDH) catalyses the
conversion of (oxidative decarboxylation) of isocitrate
to oxalosuccinate & then to ďĄ-ketoglutarate.
âŤThe formation of NADH & the liberation of CO2
occure at this stage.
âŤStimulated (cooperative) by isocitrate, NAD+, Mg2+,
ADP, Ca2+ (links with contraction).
âŤInhibited by NADH &ATP
48. Step: 6Conversion of ďĄ-ketoglutarate
to succinyl CoA
âŤOccurs through oxidative decarboxylation,
catalysed by ďĄ-ketoglutarate dehydrogenase
complex.
âŤďĄ-ketoglutarate dehydrogenase is an multienzyme
complex.
âŤAt this stage of TCAcycle, second NADH is
produced & the second CO2 is liberated.
49. Step: 7
Formation of succinate
âŤSuccinyl CoAis converted to succinate by
succinate thiokinase.
âŤThis reaction is coupled with the phosphorylation
of GDPto GTP.
âŤThis is a substrate level phosphorylation.
âŤGTPis converted toATPby the enzyme nucleoside
diphosphate kinase.
50. Step: 8
Conversion of succinate to fumarate
âŤSuccinate is oxidized by succinate dehydrogenase
to fumarate.
âŤThis reaction results in the production of FADH2.
âŤStep: 9 Formation of malate: The enzyme
fumarase catalyses the conversion of fumarate to
malate with the addition of H2O.
51. Step:10
Conversion of malate to
oxaloacetate
âŤMalate is then oxidized to oxaloacetate by malate
dehydrogenase.
âŤThe third & final synthesis of NADH occurs at this
stage.
âŤThe oxaloacetate is regenerated which can
combine with another molecule of acetyl CoA&
continue the cycle.
52. Regeneration of
oxaloacetate
âŤThe TCAcycle basically involves the oxidation of
acetyl CoA to CO2 with the simultaneous
regeneration of oxaloacetate.
âŤThere is no net consumption of oxaloacetate or any
other intermediate in the cycle.
53. Significance of TCA
cycle
âŤComplete oxidation of acetyl CoA.
âŤATPgeneration.
âŤFinal common oxidative pathway.
âŤIntegration of major metabolic pathways.
âŤFat is burned on the wick of carbohydrates.
âŤExcess carbohydrates are converted as neutral fat
âŤNo net synthesis of carbohydrates from fat.
âŤCarbon skeleton of amino acids finally enter the TCAcycle.
54. Requirement of O2 by TCA
cycle
âŤThere is no direct participation of O2 in TCAcycle.
âŤOperates only under aerobic conditions.
âŤThis is due to, NAD+ & FAD required for the
operation of the cycle can be regenerated in the
respiratory chain only in presence of O2.
âŤTherefore, citric acid cycle is strictly aerobic.
55. Energetics of TCA
Cycle
âŤOxidation of 3 NADH by ETC coupled with
oxidative phosphorylation results in the synthesis of
9ATP.
âŤFADH2 leads to the formation of 2ATP.
âŤOne substrate level phosphorylation.
âŤThus, a total of 12ATPare produced from one
acetyl CoA.
57. ⢠HMP pathway or HMP shunt is also called as
pentose phosphate pathway or phosphogluconate
pathway.
⢠This is an alternative pathway to glycolysis and
TCAcycle for the oxidation of glucose.
⢠HMPshunt is more anabolic in nature.
58. ⢠It is concerned with the biosynthesis of NADPH &
pentoses.
⢠About 10% of glucose entering in this
pathway/day.
⢠The liver & RBC metabolise about 30% of glucose
by this pathway.
59. Location of the pathway
⢠The enzymes are located in the cytosol.
⢠The tissues such as liver, adipose tissue, adrenal
gland, erythrocytes, testes & lactating mammary
gland, are highly active in HMPshunt.
⢠Most of these tissues are involved in biosynthesis of
fatty acids and steroids which are dependent on the
supply of NADPH.
60. HMP shunt-unique multifunctional
pathway
⢠It starts with glucose 6-phosphate.
⢠NoATPis directly utilized or produced in HMP
shunt
⢠It is multifunctional pathway, several
interconvertible substances produced, which are
proceed in different directions in the metabolic
reactions
61. Reactions of the pathway
⢠Reactions of the pathway:
⢠Divided into Two phases oxidative & non â oxidative.
⢠Oxidative phase
⢠Step:1
⢠Glucose 6- phosphate is oxidised by NADP- dependent
Glucose 6- phosphate dehydrogenase (G6PD), 6-
phosphogluconolactone is formed.
⢠NADPH is formed in this reaction and this is a rate limiting
step.
62. ⢠Step:2
⢠6-phosphogluconolactone is hydrolysed by glucono lactone
hydrolase to form 6-phosphogluconate.
⢠Step : 3
⢠The next reaction involving the synthesis of NADPH and is
catalysed by 6 â phosphogluconate dehydrogenase to
produce 3 keto 6 â phosphogluconate which then undergoes
decarboxylation to give ribulose 5 â phosphate.
63. Non-Oxidative Phase
⢠Step: 4
⢠The ribulose -5-phosphate is then isomerized to
ribose -5-phosphate or epimerised to xylulose -5-
phosphate
⢠Step: 5 Transketolase reaction
⢠Transketolase is a thiamine pyrophosphate (TPP)
dependent enzyme.
64. ⢠It transfers two-carbon unit from xylulose 5-
phosphate to ribose 5-phosphate to form a 7-
carbon sugar, sedoheptulose 7-phosphate and
glyceraldehyde 3 â phosphate.
65. ⢠Step: 6 Transaldolase reaction
⢠Transaldolase brings about the transfer of a 3 â
carbon fragment from sedoheptulose 7-phosphate
to glyceraldehyde 3-phosphate to give fructose 6-
phosphate & 4 â carbon erythrose 4 â phosphate.
66. ⢠Step: 7 Second transketolase Reaction
⢠In another transketolase reaction a 2 â carbon unit
is transferred from xylulose 5 â phosphate to
erythrose 4 â phosphate to form fructose 6 â
phosphate & glyceraldehyde 3 â phosphate.
⢠Fructose 6 â phosphate & glyceraldehyde 3 â
phosphate are further metabolized by glycolysis &
TCAcycle.
69. Significance of HMP Shunt
⢠HMPshunt is unique in generating two important products-
pentoses and NADPH
⢠Importance of pentoses:
In HMPshunt, hexoses are converted into pentoses, the
most important being ribose 5 â phosphate.
⢠This pentose or its derivatives are useful for the synthesis of
nucleic acids (DNA & RNA)
⢠Many nucleotides such asATP, NAD+, FAD & CoA
70. Importance of NADPH
⢠NADPH is required for the bio synthesis of fatty
acids and steroids.
⢠NADPH is used in the synthesis of certain amino
acids involving the enzyme glutamate
dehydrogenase.
⢠Free radical Scavenging
⢠The free radicals (super oxide, hydrogen peroxide)
are continuously produced in all cells.
71. ⢠These will destroy DNA, proteins, fatty acids & all
biomolecules & in turn cells are destroyed.
⢠The free radicals are inactivated by the enzyme
systems containing SOD, POD & glutathione
reductase.
⢠Reduced GSH is regenerated with the help of
NADH.
72. ⢠Erythrocyte Membrane intigrity
⢠NADPH is required by the RBC to keep the
glutathione in the reduced state.
⢠In turn, reduced glutathione will detoxify the
peroxides & free radicals formed within the RBC.
⢠NADPH, glutathione & glutathione reductase
together will preserve the intigrity of RBC
membrane.
73. ⢠Prevention of Met-Hemoglobinemia
⢠NADPH is also required to keep the iron of
hemoglobin in the reduced (ferrous) state & to
prevent the accumulation of met-hemoglobin.
⢠Met-hemoglobin cannot carry the oxygen.
74. ⢠Detoxification of Drugs
⢠Most of the drugs and other foreign substances are
detoxified by the liver microsomal P450 enzymes,
with the help of NADPH.
⢠Lens of Eye:
⢠Maximum concentration of NADPH is seen in lens
of eye.
⢠NADPH is required for preserving the
transparency of lens.
75. ⢠Macrophage bactericidal activity:
NADPH is required for the production of reactive
oxygen species (ROS) by macrophases to kill
bacteria.
⢠Availability of Ribose:
Ribose & Deoxy â ribose are required for DNA&
RNAsynthesis.
76. ⢠Ribose is also necessary for nucleotide co â
enzymes.
⢠Reversal of non â oxidative phase is present in all
tissues, by which ribose could be made available.
⢠What aboutATP
ATP is neither utilized nor produced by the HMP
shunt.
⢠Cells do not use the shunt pathway for energy
production.
77. Regulation of HMP Shunt
âŤThe entry of glucose 6-phosphate into the pentose
phosphate pathway is controlled by the cellular
concentration of NADPH
âŤNADPH is a strong inhibitor of glucose 6-phosphate
dehydrogenase (G6PD)
âŤNADPH is used in various pathways, inhibition is
relieved & the enzyme is accelerated to produce
more NADPH
78. âŤThe synthesis of glucose 6-phosphate
dehydrogenase is induced by the increased
insulin/glucagon ratio after a high carbohydrate
meal.
79. Glucose-6-phosphate dehydrogenase deficiency (G6PD)
⢠It is an inherited sex â linked trait.
⢠It is more severe in RBC.
⢠Decreased activity of G6PD impairs the
synthesis of NADPH in RBC.
⢠This results in the accumulation of met
hemoglobin & peroxides in erythrocytes
leading to hemolysis.
80. ⢠The deficiency is manifested only when exposed to
certain drugs or toxins, e.g.intake of antimalarial
drug like primaquine & ingestion of fava
beans(favism) & sulpha drugs also parecipitate the
hemolysis
81. G6PD deficiency & malaria
⢠G6PD deficiency is associated with resistance to malaria
(caused by plasmodium infection)
⢠The parasite requires reduced glutathione for its survival,
which will not be available in adequate amounts in
deficiency of G6PD.
⢠Met â hemoglobinemia
⢠G6PD deficient persons will show increased Met â
hemoglobin in circulation, even though cyanosis may not
be manifested.
82. References
⢠Textbook of Biochemistry â U Satyanarayana
⢠Textbook of Biochemistry â DM Vasudevan
