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CHAPTER 2:
MAJOR METABOLIC
PATHWAYS
ERT 317 BIOCHEMICAL ENGINEERING SEM 1 2012/13
Course details
Credit hours/Units : 4
Contact hours : 3 hr (L), 3 hr (P) and 1 hr (T) per week
Evaluations
Final Exam – 50%
Midterm Tests – 20%
Course works – 30%
Laboratories – 15%
Assignments – 15%
CARRY MARKS – 50%
Course details
Course Outcome (COs) will be covered:
CO2 – Ability to evaluate microbial system based on its
metabolic pathways and kinetics study in batch and
continuous cultures.
Course works
Assignments
Quizzes
Midterm test 1 – 22.10.2012 (Mon)
Class participations – Max. of 3 points
Important reminder
Attendance should not less than 80%, or else you will be barred from taking final examination.
Plagiarism and copying other students’ work is strictly prohibited especially in doing assignments and lab reports, or else both parties will get zero.
Cheating in quizzes and examinations is also prohibited, or else both parties will get zero.
Therefore, study hard and smart. Take note of the important chapters or things that will be highlighted throughout lectures.
Week 4-5 (01 - 12 Oct 2012)
Reading assignment:
1. Chapter 5, Bioprocess Engineering basic
Concepts. Shuler and Kargi (Main)
Major Metabolic Pathways C2
Introduction
Metabolism is the collection of enzyme catalyzed
reactions that convert substrates that are external to the
cell into various internal products.
Characteristics of Metabolisms
1. Varies from organisms to organism
2. Many common characteristics
3. Affected by environmental conditions
a) O2 availability: Saccharomyces cerevisiae
i. Aerobic growth on glucose → more yeast cells
ii. Anaerobic growth on glucose → ethanol
b) Control of metabolism is important in bioprocesses
ER 211/1 - Chapter 3
Types of Metabolism
Catabolism
Metabolic reactions in the cell that degrade a substrate
into smaller / simpler products.
Glucose → CO2
Anabolism
Metabolic reactions that result in the synthesis of larger
/ more complex molecules.
ERT 211/1 - Chapter 3
Figure 3.1: Classes of Reactions
Classification based on Metabolism
Where microbes get their energy?
Sunlight vs. Chemical
Photo- vs. Chemo- trophs
How do they obtain carbon?
Carbon Dioxide (or inorganic cmpds.) vs. Organic Compounds (sugars, amino acids)
Auto- vs. Hetero- trophs
Examples
Photoautotrophs vs. Photoheterotrophs
Chemoautotrophs vs. Chemoheterotrophs
ERT 211/1 - Chapter 3
Table 3.1:Types of -trophs
Type Energy C source Example
Photoauto- Sun CO2 Purple and green
sulfur bacteria
Photohetero- Sun Organic
compounds
Purple and green
non-sulfur bacteria
Chemoauto- Chemical
bonds
CO2 H, S, Fe, N bacteria
Chemohetero- Chemical
bonds
Organic Most bacteria,
fungi, protozoa,
animals
ERT 211/1 - Chapter 3
Bioenergetics
The source of energy to fuel cellular metabolsim is
“reduced” forms of carbon (sugars, hydrocarbons, etc.)
The Sun is the ultimate source via the process of
Photosynthesis in plants:
CO2 + H2O + hv → CH2O + O2
ERT 211/1 - Chapter 3
ATP - Adenosine Triphosphate
Catabolism of carbon-containing substrates
generates high energy biomolecules
ERT 211/1 - Chapter 3
Thermodynamic principles
Free-energy change ∆G’ of single chemical reaction:
αA+βB→γC+δD (3.1)
∆G’= ∆G°’ +RT ln (cγdδ/aαbβ) (3.2)
∆G°’ = -RT ln K’eq (3.3)
K’eq = (cγdδ/aαbβ) (3.4)
ERT 211/1 - Chapter 3
Example:
Consider a typical cell with [ATP] = 3.0 mM, [ADP] = 0.8 mM, and [Pi] = 4.0 mM. Free energy at 37°C.
Solution:
∆G’= ∆G°’ +RT ln ([ADP][Pi]/[ATP])
= -30.5 kJ/mol + (8.3145 J/Kmol)(310 K) ln (0.8 E-3 M x 4.0 E-3 M/3.0 E-3 M)
= -30.5 KJ/mol – 17.6 kJ/mol
=-48.1 kJ/mol
ERT 211/1 - Chapter 3
Metabolic Reaction Coupling: ATP
and Other Phosphate Compounds
Enzymatic hydrolysis of ATP yielding ADP and
inorganic phosphate (Pi) as well as releasing (-) large
free-energy and reversing the reaction with addition
of phosphate to ADP, free energy can be stored (+)
for later use.
ERT 211/1 - Chapter 3
Metabolic Reaction Coupling: ATP and
Other Phosphate Compounds
ERT 211/1 - Chapter 3
Metabolic Reaction Coupling: Oxidation and
reduction coupling via NAD
Recall
Oxidation = Loses electrons e.g dehydrogenation
Reduction = Addition electrons e.g hydrogenation
Example: Reduction of pyruvic acid and oxidation of lactic acid
ERT 211/1 - Chapter 3
Metabolic Reaction Coupling: Oxidation and
reduction coupling via NAD
Pairs of hydrogen atoms freed during oxidation or required in reduction are carried by nucleotide derivatives, NAD and phophorylated form NADP
ERT 211/1 - Chapter 3
NAD+ and NADP +
Nucleotide
derivatives
that accept
H+ and e
during
oxidation
/reduction
reactions
ERT 211/1 - Chapter 3
Thermodynamic Principle
Oxidation-reduction reaction:
Aox + Bred Aox + Bred
∆E°’= E°’(Aox/Ared) - E°’(Box/Bred) (3.5)
Standard half-cell potential
Aox + 2e → Ared (3.6)
The hydrogen half cell (pH=0)
2H + 2e → H2 E0 = 0.00 V (3.7)
Free-energy:
∆G°’ = -n F ∆E’ (3.8)
ERT 211/1 - Chapter 3
Carbon Catabolism
Embden- Meyerhof-Parnas (EMP) Pathway
Pentose Phosphate Pathway
Entner Doudoroff (ED) Pathway
Overview of Glucose Metabolism
• Under aerobic conditions glucose is converted to pyruvate by glycolysis while generating two ATPs
• Under aerobic conditions, pyruvate is further oxidized by the citric acid cycle and oxidative phosphorylation to generate CO2 and H2O
• Under anaerobic conditions, pyruvate is instead converted to a reduced end product, that is,
In muscle: lactate - homolactic fermentation
In yeast: ethanol + CO2 - alcoholic fermentation
(note: fermentation is an anaerobic biological reaction process)
ERT 211/1 - Chapter 3
Glucose
Metabolism
ERT 211/1 - Chapter 3
Glycolysis
• Glycolysis is the breakdown (catabolism) of glucose to pyruvate under aerobic conditions
To understand the pathway at 4 levels:
1. The chemical interconversion steps or the sequence of reactions by which glucose is converted to the pathway end’s product , i.e., pyruvate
2. The mechanism of the enzymatic conversion of each pathway intermediates and its successor
3. The energetics of the conversion, ∆G and ∆
4. The mechanisms controlling the flux (rate of flow) of metabolites through the pathway
• Glycolysis converts C6 glucose to two C3 pyruvate
• The free energy released in this process is harvested to synthesise ATP from ADP and Pi
• They are 10 reactions involved in the glycolysis, all catalaysed by 10 specific enzymes
• The enzymes of glycolysis are located in the cytosol where they are only loosely
associated
Glycolysis can be divided into two stages:
Stage 1:
Energy investment
Reactions 1-5 of the pathway are involved
Hexose (C6)glucose is phosphorylated and cleaved to yield 2 molecules of triose
(C3) glyceraldehydes -3-phosphate (GAP)
The process consumes 2 ATPs
Stage 2:
Energy recovery
Reactions 6-10 of the pathway are involved
The two molecules of GAP are converted to pyruvate
The process generates 4 ATPs
Glycolysis
ERT 211/1 - Chapter 3
Slide 6
The reactions of glycolysis
ERT 211/1 - Chapter 3
• There are 10 reactions catalyzed by 10 different enzymes
1. Hexokinase: First ATP Utilization
2. Phosphoglucose Isomerase
3. Phosphofructokinase: Second ATP Utilization
4. Aldolase
5. Triose Phosphate Isomerase
6. Glyceraldehyde-3-Phosphate Dehydrogenase: First “High Energy” Intermediate Formation
7. Phosphoglycerate Kinase: First ATP Generation
8. Phosphoglycerate Mutase
9. Enolase: Second “High energy” Intermediate Formation
10. Pyruvate Kinase: Second ATP Generation
Glycolysis: Reaction 1
• The reaction is the conversion of
glucose to form glucose-6-phosphate
(G6P)
• First ATP Utilization
• The reaction involves the transfer of a
phosphoryl group from ATP to glucose
to form glucose-6-phosphate (G6P)
• The reaction is catalyzed by
hexokinase (HK)
• The reaction needs Mg2+ ion
• Uncomplex ATP is inhibitory
Note: Kinase enzyme catalyzes the transfer of
phosphoryl groups between ATP and other
molecules
ERT 211/1 - Chapter 3
Why is Mg2+ required in any kinase enzyme activity?
Mg2+ complexed with ATP at the phosphate oxygen atoms
This shield their negative charges, making the γ-phosphorous atom of ATP more
accessible for the nucleophilic attack by the C6-OH group of glucose
Other divalent ions which can replace Mg2+ is Mn2+
α β γ 1
2 3
4
5
6
ERT 211/1 - Chapter 3
Glycolysis: Reaction 2
• The reaction is the conversion of
glucose-6-phosphate (G6P) to form
fructose-6-phosphate (F6P)
• The reaction is catalyzed by
phosphoglucose isomerase (PGI)
• This reaction is the isomerization of
an aldose to a ketose
• The proposed mechanism for the PGI
reaction involves the general acid-
base catalysis by the enzyme
• Involves ring opening and ring
closure between C1 and O
aldose
ketose
ERT 211/1 - Chapter 3
Glycolysis: Reaction 3
• The reaction is the conversion of fructose-6-phosphate
(F6P) to form fructose-1,6-bisphosphate (FBP)
• The reaction is catalyzed by phosphofructokinase (PFK)
• Second ATP Utilization
• Reaction is similar to hexokinase
reaction, catalyzing the nucleophilic attack by C1-OH
group of F6P on the electrophilic γ-phosphorous atom of
Mg2+-ATP complex
• PFK catalyzes one of the pathway’s rate-determining
reaction, it function with large negative free energy
changes
• One of the 3 non-equilibrium rxn in glycolysis (others
are HK and PK)
Mechanisms that control PFK activity:
ATP is both a substrate and also an allosteric inhibitor of
enzyme
ADP, AMP reverse the inhibitory effect of enzyme,
therefore they are activators
ERT 211/1 - Chapter 3
Glycolysis: Reaction 4
• The reaction is the cleavage of fructose-1,6-
bisphosphate (FBP) to form the two trioses,
glyceraldehyde -3-phosphate (GAP) and
dihydroxyacetone phosphate (DHAP)
• Bisphosphate, not diphosphate as the
phosphoryl gp are not linked
• The reaction is catalyzed by aldolase
• This reaction is an aldol cleavage between
C3 and C4 which requires carbonyl at C2
and hydroxyl at C4
• The two trioses are interconvertible and thus
can enter a common degradative path
• Each trioses has a phosphoryl gp
• Carbon atoms of 1, 2 and 3 of glucose
become atom 3, 2 and 1 of DHAP and 4, 5
and 6 of glucose become 1, 2 and 3 of
GAP
ERT 211/1 - Chapter 3
Mechanism for base-catalysed aldol cleavage
The presence of enolate intermediates
ERT 211/1 - Chapter 3
Glycolysis: Reaction 5
• Reaction 4 results in the
production of glyceraldehyde -3-
phosphate (GAP) and
dihydroxyacetone phosphate
(DHAP), which are ketose-aldose
isomers
• Only GAP will continue to be
degraded in the glycolysis
• Reaction 5 is the interconversion
of GAP to DHAP, occurs via an
enediol or enediolate
intermediate
• This reaction is catalyzed by
triose phosphate isomerase
(TIM) ERT 211/1 - Chapter 3
Glycolysis: Reaction 6
• Involves the oxidation and
phosphorylation of glyceraldehyde -3-
phosphate (GAP) by NAD+ and Pi to
form 1,3-bisphosphoglycerate (1,3-BPG)
• The reaction is catalyzed by
glyceraldehyde-3-phosphate
dehydrogenase
• The reaction is very exergonic and this
drives the synthesis of “high energy”
compound, 1,3-BPG
• 1,3-BPG is one of the phosphate
compounds with high standard free
energy, higher than ATP
ERT 211/1 - Chapter 3
Slide 15
ERT 211/1 - Chapter 3
Table 3.2: Standard Free Energies of Phosphate Hydrolysis of some biological
compound
Glycolysis: Reaction 7
• The reaction is the conversion of 1,3
bisphosphoglycerate (1,3-BPG)
to form 3-phosphoglycerate (3PG)
• The reaction is catalyzed by
phosphoglycerate kinase (PGK)
• The reaction results in a generation
of its first ATP molecules in the
presence of ADP and Mg2+
ERT 211/1 - Chapter 3
Mechanism of phosphoglycerate kinase (PGK) reaction
ERT 211/1 - Chapter 3
Glycolysis: Reaction 8
• The reaction is the conversion of
3-phosphoglycerate (3PG) to
form 2-phosphoglycerate (2-PG)
• The reaction is catalyzed by
phosphoglycerate mutase
(PGM)
• Note: a mutase catalyzes the
transfer of a functional group
from one position to the another
on a molecule
• The reaction is a necessary
preparation for the next reaction
which generates a “high energy”
phosphoryl compound for use in
the ATP synthesis
ERT 211/1 - Chapter 3
Proposed reaction mechanism for phosphoglycerate mutase (PGM)
•The active form of the enzyme
contains Phos-His residue at the
active site
Step 1: 3-PG binds to PGM in
which His is phosphorylated
Step 2: The phosphoryl gp is
transferred to the substrate,
resulting in an intermediate 2,3-
PG.enzyme complex
Steps 3 and 4: The complex
decomposes to form 2-PG with
regeneration of the
phosphoenzyme
•Occasionally, 2,3-BPG
dissociates from enzyme leaving
an inactive dephosphoenzyme
like in Step 5
ERT 211/1 - Chapter 3
Glycolysis: Reaction 9
• The reaction is the conversion
of 2-phosphoglycerate (2-
PG) to phosphoenolpyruvate
(PEP)
• This reaction is catalyzed by
enolase
• The reaction generate a
second “High energy”
intermediate compound, PEP
(refer to Table 13-2)
ERT 211/1 - Chapter 3
Remember
Table 13-2, Page 362-Standard Free Energies of Phosphate Hydrolysis of some
biological cpds
ERT 211/1 - Chapter 3
Glycolysis: Reaction 10
• The final reaction in glycolysis is the
conversion of phosphoenolpyruvate
(PEP) to form pyruvate
• The reaction is catalyzed by pyruvate
kinase (PK)
• One of the 3 non-equilibrium rxn in
glycolysis (others are HK and PFK)
• This reaction involves the coupling of
PEP hydrolysis to pyruvate to the
synthesis of ATP from ADP
• Thus the reaction generates the second
ATP molecules
ERT 211/1 - Chapter 3
Mechanism of the Reaction Catalyzed by
Pyruvate Kinase (PK) • Reaction requires both monovalent (K+) and divalent ions (Mg+)
Step 1: Nucleophilic attack of phosphorus atom of PEP by β-phosphoryl oxygen of ADP, thus displacing enolpyruvate and forming ATP; the reaction is endergonic
Step 2: Enolpyruvate converts to pyruvate, and this is a very exergonic reaction
• Overall reaction of PEP to pyruvate is exergonic
ERT 211/1 - Chapter 3
Summary on Glycolysis
• The overall reaction of glycolysis is:
Glucose + 2NAD+ + 2ADP + Pi → 2NADH + 2Pyruvate + 2ATP + 2H2O + 4H+
• The reaction occurs in 10 enzymatically catalysed reactions
• The mechanisms of the 10 glycolytic enzymes have been elucidated through chemical and kinetic measurements combined with X-ray structural studies
• 3 of the 10 reactions are non-equilibrium, which ensure the pathway go forward:
Reaction 1: Glucose to G6P by HK
Reaction 3: F6P to FBP by PFK
Reaction 10: PEP to pyruvate by PK
ERT 211/1 - Chapter 3
The Three Products of Glycolysis
1. ATP
2. NADH
3. PYRUVATE
1. ATP
• 2 ATP per molecule of glucose were invested and subsequently 4ATP were generated by substrate-level phosphorylation, giving a net yield of 2ATP per glucose
• ATP produced satisfies most of the cell’s energy needs.
2. NADH
• 2 NAD+ are reduced to 2 NADH
• Reduced NADH represent a source of free energy that can be recovered by subsequent oxidation
• Under aerobic condition, electron pass from reduced coenzymes thru’ a series of electron carriers to the final oxidizing agent, O2, in a process known as electron transport
• The free energy of electron transport drives the synthesis of ATP from ADP
• In aerobic organism, the sequence of events also serves to regenerate oxidized NAD+
• Under anaerobic conditions, NADH must be reoxidized by other means in order to keep the glycolytic pathway supplied with NAD+
ERT 211/1 - Chapter 3
The Three Products of Glycolysis (cont…)
3. PYRUVATE
• 2 pyruvate molecules are produced
• Under aerobic condition, complete oxidation of
pyruvate to CO2 and H2O via citric acid cycle and
oxidative phosphorylation, where ATP is generated
• In anaerobic metabolism, pyruvate is metabolized to
a lesser extent to regenerate NAD+, via a process
known as fermentation
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY
•Why the Pentose Phosphate Pathway (The PPP)
An alternative mode of glucose oxidation where G6P is converted to R5P or
hexoses to pentoses, and R5P and its derivatives are required for the synthesis of
RNA, DNA, etc.
The production of NADPH
•Tissues most heavily involved in lipid biosynthesis (liver, mammary gland, adipose tissue
and adrenal cortex) are rich in the PPP enzymes
•About 30% of the glucose oxidation in liver occurs via the PPP
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY (continue…)
•NADH vs NADPH
They are not metabolically interchangeable
NADH uses the free energy of metabolite oxidation to synthesise ATP (oxidative
phosphorylation), whereas NADPH uses the free energy of metabolite oxidation for
reductive biosynthesis (eg. biosynthesis of fatty acid and cholesterol require NADPH in
addition to ATP)
This differentiations is possible because the dehydrogenases involved in oxidative
and reductive metabolism are highly specific for their respective coenzymes, NADH or
NADPH
ERT 211/1 - Chapter 3
Nicotinamide nucleotides in catabolism and biosynthesis
• NAD+ is the cofactor for most enzymes that act in the direction of substrate oxidation
(dehydrogenases) whereas NADPH usually functions as a cofactor for reductases,
enzymes that catalyze substrate reduction
• NADPH is synthesized either from NADP+ in the PPP or from NADH through the action
of mitochondrial energy-linked transhydrogenase
• NADP+ is synthesized from NAD+ by an ATP-dependent kinase reaction
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY (continue…)
Overall rxn:
3 G6P + 6 NADP+ + 3 H2O 6 NADPH + 6 H+ + 3 CO2 + 2 F6P + GAP
Occurs in 3 stages:
Stage 1: Oxidative rxn (Rxn 1-3) which yield NADPH and ribulose-5-phosphate (Ru5P)
3 G6P + 6 NADP+ + 3 H2O 6 NADPH + 6 H+ + 3 CO2 + 3 Ru5P
G6P generated from hexokinase on glucose (glycolysis) or glycogen breakdown
Only Rxn 1-3 of pathway are involved in the production of NADPH
2 molecules of NADPH are generated per 1 molecule of G6P that enter pathway
Stage 2: Isomerization and epimerization rxn (Rxn 4 and 5) which transform Ru5P either
to ribose-5-phosphate (R5P) or xylose-5-phosphate (Xu5P)
3 Ru5P R5P + 2 Xu5P
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY (continue…)
Stage 3: A series of C-C bond cleavage and formation rxn (Rxn 6-8) that convert 2
molecules of Xu5P and one molecule of R5P to 2 molecules of F6P and one molecule of
GAP
transketolase
transaldolase
transketolase
2 Xu5P + 1 R5P 2 F6P 1 GAP
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY (continue…)
1. Gluco-6-phosphate
dehydrogenase (G6PD)
2. 6-phospho-glucono-
lactonase
3. 6-phospho-gluconate
dehydrogenase
4. Ribulose-5-phosphate
isomerase
5. Ribulose-5-phosphate
epimerase
6. Transketolase
7. Transaldolase
8. Transketolase
Note: Epimers are sugars that
differ only by the
configuration at one carbon
atom
ERT 211/1 - Chapter 3
THE PENTOSE PHOSPHATE PATHWAY (continue…)
Relationship between
Glycolysis and The Pentose
Phosphate Pathway
•The PPP starts with G6P produces
in Step 2 of Glycolysis
•It generates NADPH for use in
reductive biosynthesis and R5P for
nucleotide biosynthesis
•Excess R5P is converted to
glycolytic intermediates by a
sequence of reactions that can
operate in reverse to generate
additional R5P, if needed
ERT 211/1 - Chapter 3
Entner Doudoroff (ED) Pathway
Overall stoichiometry of the reaction:
Glucose + ATP + NADP + → glyceralde 3-phosphate +
pyruvic acid + ADP + NADPH + H+
2 moles of ADP were phosphorylated by reacting one
mole of GA-3P to pyruvate through the same reaction
as used for this conversion in EMP pathway
Energy yield: onemole of ATP per mole glucose
processed
ERT 211/1 - Chapter 3
Summary of Carbon Catabolism
3 Glucose molecules enter glycolysis (EMP pathway), produce 6
ATP
3 Glucose molecules through the Pentose Phosphate Pathway and
then reenter glycolysis, produce 5 ATP
3 Glucose molecules enter ED pathway, produce 3 ATP
ERT 211/1 - Chapter 3
FERMENTATION
1. Homolactic Fermentation
•In muscle, under anaerobic condition e.g., during vigorous activity when the demand for ATP is
high and O2 is in short supply:
NADH is oxidised by pyruvate to generate NAD+ and lactate by lactate dehydrogenase
(LDH)
The reaction is reversible, so pyruvate and lactate level are readily equilibrated
The reaction is also known as anaerobic glycolysis, the stage where the conversion of
pyruvate to lactate is classified as Reaction 11
The overall process of an anaerobic glycolysis in muscle can be represented as:
Glucose + 2 ADP + 2Pi 2 Lactate + 2 ATP
The lactate may be:
exported out of the cell by blood to the liver where it is used to synthesise glucose, or
converted back to pyruvate, to enter the pathway for further degradation
ERT 211/1 - Chapter 3
1. Homolactic Fermentation (continue…)
ERT 211/1 - Chapter 3
FERMENTATION (continue..)
2. Alcoholic Fermentation
• In yeast, under anaerobic condition, pyruvate is converted to ethanol and CO2 while
regenerating NAD+
• Yeast produces ethanol and CO2 via 2 consecutive reactions:
The carboxylation of pyruvate to form acetaldehyde and CO2 as catalyzed by
pyruvate decarboxylase (enzyme not present in animals), then
The reduction of acetaldehyde to ethanol by NADH as catalysed by alcohol
dehydrogenase, thereby generating NAD+
ERT 211/1 - Chapter 3
Alcoholic Fermentation (continue…..)
The two reactions of alcoholic fermentation:
Thiamin
diphosphate
(TPP)-essential
cofactor of
pyruvate
decarboylase
1. Pyruvate
decarboxylase
2. Alcohol
dehydrogenase
ERT 211/1 - Chapter 3
Fermentation (continue..)
The Energetics of Fermentation
•Overall Reaction in alcoholic fermentation (from glucose)
Glucose + 2Pi +2ADP 2 Ethanol + 2CO2 + 2ATP ∆G = -196 kJ.mol-1
•Overall Reaction in homolactic fermentation (from glucose)
Glucose + 2Pi +2ADP 2 Lactate + 2ATP ∆G = -235 kJ.mol-1
•Each of these processes is coupled to the net formation of 2ATP molecules
ERT 211/1 - Chapter 3
Fermentation (continue..)
The Energetics of Fermentation (continue…)
•They are non-oxidative process
•No net oxidation and reduction
•NAD+ and NADH do not appear in the overall net reaction
•NADH formed in the oxidation of GAP is consumed in the reduction of pyruvate
•The regeneration of NAD+ in the reduction of pyruvate to lactate or ethanol sustains the
continued operation of glycolysis under anaerobic conditions
•Only a small fraction of the energy of glucose is released in the anaerobic conversion to
lactate or ethanol (2ATP per glucose vs. 38ATP per glucose in ox. phos.)
•The rate of ATP production is higher in anaerobic than in aerobic glycolysis
•Much more energy can be extracted aerobically via citric acid cycle and oxidative
phosphorylation step
ERT 211/1 - Chapter 3
METABOLISM OF OTHER HEXOSES
• Three other hexoses apart
from glucose are:
1. Fructose
2. Galactose
3. Mannose
• After digestion monosaccharides
enter the blood stream, which
carries them to various tissues
• Fructose, galactose and mannose
are converted to glycolytic
intermediates then metabolized
in the glycolytic pathway
ERT 211/1 - Chapter 3
METABOLISM OF OTHER HEXOSES (continue..)
1. Fructose
• From fruits and sucrose (a disaccharides of fructose and glucose)
• 2 pathways for the metabolism of sucrose because of the presence of different enzymes
in different tissues
• In muscle, fructose converts to F6P which then enter the glycolytic pathway. This only
require 1 step, thus 1 enzyme,HK
• In liver, fructose converts to intermediates and finally GAP which then enter the
glycolytic pathway. This require 7 steps thus 7 enzymes
ERT 211/1 - Chapter 3
METABOLISM OF OTHER HEXOSES (continue…)
Metabolism of
fructose
1. fructokinase
2. 2 Fructose-1-
phosphate
aldolase
3. Glyceraldehyde
kinase
4. Alcohol
dehydrogenase
5. Glycerol kinase
6. Glycerol
phosphate
dehydrogenase
7. Triose phosphate
isomerase
METABOLISM OF OTHER HEXOSES (continue..)
1. Fructose (continue…)
•In muscle, after Rxn 4, there are 2 pathway leading from glyceraldehyde to GAP, before
entering the pathway, both consume ATP
•NADH is oxidized in in Rxn 4 but is being reduced again in Rxn 6
•What happens when there is an excessive fructose in blood, eg. In IV feeds?
•Fructose intolerance-a genetic diseases with deficiency in Fructose-1-phosphate aldolase
Fructose-1-phosphate may be produced faster than aldolase can cleave,
accumulation of fructose-1-phosphate depletes Pi in liver, ATP drops, glycolysis and
production of lactate is activated, high concentration of lactate in blood is deadly
ERT 211/1 - Chapter 3
Metabolism of other HEXOSES (continue..)
2. Galactose
•Obtained from the hydrolysis of lactose from dairy products
•Lactose is disaccharide of galactose and glucose
•Hexokinase only recognise glucose, fructose and mannose but not galactose
•Epimerization rxn must occur before galactose enter glycolysis
•Galactose is converted to G6P which then enter the glycolytic pathway. This require 4 steps
and 4 enzymes
ERT 211/1 - Chapter 3
Metabolism of other HEXOSES (continue..)
Metabolism of
galactose
1. Galactokinase
2. Galactose-1-
phosphate uridyl
transferase
3. UDP-galactose-4-
epimerase
4. phosphoglucomutase
ERT 211/1 - Chapter 3
Metabolism of other HEXOSES (continue..)
2. Galactose (continue…)
•Galactosemia- a genetic disease characterized by the inability to convert galactose to
glucose
•Involve the deficiency of enzyme of Rxn 2
•Symptoms:
Failure to grow well
Mental retardation
Death from liver damage
•Treatment- galactose free diet
ERT 211/1 - Chapter 3
Metabolism of other HEXOSES (continue..)
3. Mannose
•A product of digestion of polysaccharides and glycoprotein
•A C2 epimer of glucose (Epimer is sugars that differ only by the configuration at one C
atom)
•Mannose enter the glycolytic pathway after its conversion to F6P. It need 2 steps thus 2
enzymes
ERT 211/1 - Chapter 3
Metabolism of other HEXOSES (continue..)
Metabolism of mannose
1. hexokinase
2. Phosphomannose isomerase
ERT 211/1 - Chapter 3