The document discusses the fate of pyruvate under anaerobic conditions, detailing processes like lactate and alcohol fermentation, where NAD+ is regenerated without oxygen. It highlights the roles of various microorganisms in fermentation, their effects on food production, and the thermodynamics of these processes. Additionally, it emphasizes that fermentation is energy-extracting without altering the concentrations of NAD+ or NADH.

1. Fate of Pyruvate under
anaerobic conditions
MSC. Semester I(Roll No.1)
Presented by:-
Tabsherahkausher M ansari
Guidance by :-
Prof. Dr. Gayatri Mam
Prof. Dr. Riddhi Mam
College Name:-
Gujarat Science college Ahmedabad

2. 1. Introduction.
2. Fermentation pathway.
3. Production of some other foods and industrial
chemical by use of fermentation.
4. Energetics of Fermentation.
5. Summary.
Fate of Pyruvate under
anaerobic conditions

3. 1. Introduction
Definitions:
 Oxidation:- An increase in Positive valance.
 Reduction:- An increase in Negative valance.
 Respiration:- An energy yielding process in which the
energy substrate is oxidized using an exogenous or
externally derived Electron acceptor.
 Fermentation:- An energy yielding process in which an
organic molecule is oxidized without an exogenous
Electron donor.
 Bioenergetics:- The study of the flow and the
transformations of energy that occur in living organisms.

5. Glycolysis pathway

7. Diverse Fates of Pyruvate

8. Anaerobic vs Aerobic Pathway

9. Fates of Pyruvate under Aerobic &
Anaerobic conditions :-
 Pyruvate, the product of glycolysis, represents an
important junction point in carbohydrate catabolism.
Under Aerobic conditions:-
 Pyruvate is oxidized to acetate, which enters the Citric
acid and oxidized to CO2 and H2O.
 The NADH formed by the dehydrogenation of
glyceraldehyde 3- phosphate is reoxidized to NAD+ by
passage of its electrons to oxygen in the process of
mitochondrial respiration.

10. Under Anaerobic conditions:-
 As in very active skeletal muscles,in submerged plants or in
lactic acid bacteria NADH generated by glycolysis cannot be
reoxidized by Oxygen.
 Failure of regenerate NAD+ would leave cell with no electron
acceptor for the oxidation of glyceraldehyde 3- phosphate and
yielding reactions of glycolysis would stop.
 So NAD+ must therefore be regenerated by some other
reactions.

11.  The earliest cells to arise during evolution lived in an
atmosphere almost devoid of Oxygen and had to develop
strategies for carrying out glycolysis under anaerobic
conditions.
 Most modern organisms have retained the ability to
continually regenerate NAD+ during anaerobic glycolysis by
transferring electrons from NADH to form a reduced end
product such as Lactate or Ethanol

12. 2. Fermentation Pathway
In the fermentation pathway there is a mainly two
types of fermentation:-
A. Lactate fermentation.
B. Alcohol fermentation.

13. Fermentation
Pathway
A. Lactate fermentation B. Alcohol fermentation

14. Lactic acid
Fermentation
Homolactate
fermentation
Heterolactate
fermentation
A. Lactate fermentation:-

15. (I) Homolactate fermentation:-
 Examples:- Human being & other mammals, certain
Lactobacilli & Streptococci.
 Pyruvate is the Terminal electron acceptor in Lactic acid
Fermentation.
 Homolactate fermentation produces only end product
lactate.

16.  In muscle, particularly during vigorous activity when the
demand for ATP is high and Oxygen has been depleted,
special kind of enzyme catalyses the oxidation of NADH by
pyruvate to yield NAD+ and Lactate within the generation
of ATP. Example Athletes

17.  Some tissues and cell types ( such as erythrocytes, which
have no mitochondria and thus can not oxidize pyruvate
to CO2) produce lactate from glucose even under aerobic
conditions.
 The reduction of pyruvate in this pathway is catalyzed by
Lactate dehydrogenase (LDH), which forms the L- isomer
of lactate at pH 7.

18.  The overall equilibrium of the reaction strongly favors
lactate formation, as shown by the large negative
standard free- energy change.
 In glycolysis, dehydrogenation of the two molecules of
glyceraldehyde 3- phosphate derived from each molecule
of glucose converts two molecules of NAD+ to two
molecules of NADH.
 Because the reduction of two molecules of pyruvate to
two molecules of lactate regenerates two molecules of
NAD+, there is no net change in NAD+ or NADH.

19.  Much of the lactate, the end product of anaerobic glycolysis,
is exported From the muscle via blood to the liver where it is
reconverted to glucose.
 Although Conversion of glucose to lactate includes two
oxidation-reduction steps, there is no net change in the
oxidation state of carbon; in glucose (C6H12O6) and lactic acid
(C3H6O3), the H:C ratio is the same.

20.  Nevertheless, some of the energy of the glucose molecule
has been extracted by its conversion to lactate enough to
give a net yield of two molecule consumed.
 The overall process of anaerobic glycolysis in muscle can
be represented:
Glucose + 2ADP + 2Pi
2lactate + 2ATP + 2H2O + 2H+

21. (II) Heterolactic fermentation:-
 Examples:- Leuconostoc species & various
Lactobacillus species.
 Some microorganisms carry out a heterolactate
fermentation, using the Pentose phosphate pathway
rather than the Embden-Meyerhof pathway of
glycolysis.
 In Heterolactate fermentation, ethanol and CO2 are
produced in addition to lactate.

23. Glucose + ADP + Pi
lactate + ethanol + CO2+ ATP
The overall reaction can be expressed as follows:-
 The heterolactate fermentation pathway produces only
one molecule of ATP per molecule of glucose substrate
metabolized.

24. B. Alcohol fermentation (Ethanol
fermentation):-

25.  Under anaerobic conditions in yeast , NAD+ is
regenerated in manner:- conversation of pyruvate to
Ethanol and CO2.

26.  Yeast and other microorganisms ferment glucose to ethanol
and CO2, rather than to lactate.
 Ethanol is the reduced product in Alcohol Fermentation.
 Yeast employes Ethanol as a kind of antibiotic to eliminate
competing organisms.
 This is because yeast can grow in Ethanol concentrations
>12%, whereas few other organisms can survive in 5%
Ethanol.
 Ethanol is widely used antiseptic.

27.  Fermentation of Ethanol is of two step process:-

28.  In first step:-
 The X- ray structure of Pyruvate DeCarboxylase (PDC) in
complex with TPP, was determined by William furey and
Martin sax.
 Pyruvate undergoes decarboxylation in the irreversible
reaction catalyzed by Pyruvate DeCarboxylase (PDC- not
present in animal) and produce acetaldehyde and CO2.
 This reaction is a simple decarboxylaion reaction and does
not involve the net oxidation of pyruvate.
 Pyruvate decarboxylase require Mg2+ and TPP coenzyme
Which it binds tightly but noncovalentaly.
 TPP = Thiamine Pyrophoshate ( Also called Thiamine
DiPhosphate (ThDP).

29. Mechanism of Pyruvate DeCarboxylase
(PDC):-
 TPP is a essential Co- factor of pyruvate decarboxylase.
 A coenzyme derived from vitamin B1.
 The absence of vitamin B1 in Human diet leads to the
condition known as Beriberi, characterized by an
accumulation of body fluids (swelling), pain, paralysis, and
ultimately death.

30. • The reactive carbon atom in the thiazolium ring is
shown in purplish pink.

31. • This “active acetaldehyde” Group (in pink) is
subsequently released as acetaldehyde.

32. Figure- Thiamine pyrophosphate mechanism

33. The mechanism of Pyruvate DeCarboxylase
(PDC) catalysis is as follows:-
 TPP plays an important role in the cleavage of bonds nearer to
a carbonyl group, such as the decarboxylation of alpha- keto
acids (Pyruvate), and in chemical rearrangements in which an
activated acetaldehyde group is transferred from one carbon
atom to another(Table-14-1).
 Step-I :- The functional part of TPP, the thiazollium ring, has a
relatively acidic proton at C-2.
 Step-II :- Loss of this proton produces a carbanion that is the
active species in TPP-dependent reactions.

34.  Step-III :- The carbanion readily adds to carbonyl groups,
and the thiazolium ring is thereby positioned to act as an
“electron sink” that greatly facilitates reactions such as
the decarboxylation catalyzed by Pyruvate decarboxylase
and it will generate a resonance stabilization of
carbanion.
 Step-IV :- Protonation of carbanion and will generate
hydroxyethyl TPP.
 Step-V :- Elimination of the Thiazolium cation to form
acetaldehyde and regenerate the active enzyme.
 This mechanism has been confirmed by the isolation of
the hydroxyethyl pyrophosphate intermediate.

36.  Pyruvate decarboxylase is present in brewer’s and baker’s
yeast (Saccharomyces cerevisiae) and in all other
organisms that ferment glucose to ethanol, including some
plants.
 The CO2 produced by pyruvate decarboxylation in
brewer’s yeast is responsible for the characteristic
carbonation of champagne.
 The ancient art of brewing beer involves several
enzymatic processes in addition to the reactions of
ethanol fermentation.
 In baking, CO2 released by pyruvate decarboxylase when
yeast is mixed with a fermentable sugar causes dough to
rise.
 The enzyme is absent in vertebrate tissue and in other
organisms that carry out lactic acid fermentation.

37. In second step:-
 Acetaldehyde is reduced to ethanol through the action of
alcohol dehydrogenase, with the reducing power
furnished by NADH derived from the dehydrogenation of
glyceraldehyde 3- phosphate.
 Here, Alcohol dehydrogenase contains Zn2+ which polarises
the carbonyl oxygen of acetaldehyde, will allow transfer
of a hydride ion from NADH and then by the way of
protonation yield ethanol form and NAD+ will generate.

38.  This reaction is a well studied as under :-
Figure:- Alcohol dehydrogenase mechanism

39.  As in the lactic acid fermentation, there is no net
change in the ratio of hydrogen to carbon atoms when
glucose (H:C ratio = 12/6 = 2) is fermented to two
ethanol and two CO2 (combined H:C ratio = 12/6 = 2).
 In all fermentations, the H:C ratio of the reactants and
products remains the same.

40. 2ethanol + 2CO2 + 2ATP + 2H2O
Glucose + 2ADP + 2Pi
 Ethanol and CO2 are thus the end products of ethanol
fermentation, and the overall equation is:-

41.  Alcohol dehydrogenase is present in many organisms that
metabolize ethanol, including humans.
 In liver it catalyzes the oxidation of ethanol, either
ingested or produced by intestinal microorganisms, with
the concomitant reduction of NAD+ to NADH.
 In this case, the reaction proceeds in the direction
opposite to that involved in the production of ethanol by
fermentation.

42. 3. Production of some other foods and
industrial chemical by use of fermentation:-
 Million years ago use of fermentation in the production
and preservation of foods is started.
 Certain microorganisms present in raw food products
ferment the carbohydrates and yield metabolic products
that give the foods their characteristic forms, textures,
and tastes.
 Yougurt, is produced when the bacterium Lactobacillus
bulgaricus ferments the carbohydrate in milk, producing
lactic acid; the resulting drop in pH causes the milk
proteins to precipitate, producing the thick texture and
sour taste of unsweetened yougurt.

43.  Another bacterium, Propionibacterium freudenreichii,
ferments milk to produce propionic acid and CO2; the
propionic acid precipitates milk proteins, and bubbles of
CO2 cause the holes characteristic of Swiss cheese.
 Many other food products are the result of fermentations:
pickles, sauerkraut, sausage, soy sauce, and a variety of
national favorites, such as kimchi (Korea), tempoyak
(Indonesia), kefir (Russia), dahi (India), pozol (Mexico).
 The drop in pH associated with fermentation also helps to
preserve foods, because most of the microorganisms that
cause food spoilage cannot grow at low pH.
 In agriculture, plant byproducts such as corn stalks are
preserved for use as animal feed by packing them into a
large container with limited access air; microbial
fermentation produces acids that lower the pH.

44.  The silage that results from this fermentation process can
be kept as animal feed for long periods without spoilage.
 In 1910 Chaim Weizmann discovered that the bacterium
Clostridium acetobutyricum ferments starch to butanol
and acetone.
 This discovery opened the field of industrial
fermentations, in which some readily available material
rich in carbohydrate ( corn starch or molasses, for
example) is supplied to a pure culture of a specific
microorganisms, which ferments it into a product of a
greater commercial value.

45.  The ethanol used to make “ gasohol” is produced by
microbial fermentation, as are formic, acetic, propionic,
butyric, and succinic acids, and glycerol, methanol,
isopropanol, butanol, and butanediol.
 These fermentations are generally carried out in huge
closed vats in which temperature and access to air are
controlled to favor the multiplication of the desired
microorganism and to exclude contaminating organisms.

47. 4.Energetics of Fermentation
 Thermodynamics permits us to dissect the process of
fermentation into its component parts and to account for
the free energy changes that occur.This enables us to
calculate the efficiency with which the free energy of
degradation of glucose is utilized in the synthesis of ATP.
 The overall reaction of homolactic fermentation is:-
Glucose 2 lactae + 2H+
∆G°’ = -196 kJ • mol-1 of glucose
 For alcoholic fermentation, the overall reaction is:
Glucose 2 CO2 + 2 ethanol
∆G°’ = -235 kJ • mol-1 of glucose

48.  Each of these reactions is coupled to the net formation of
two ATPs, which requires ∆G°’ = +61 kJ • mol-1 of glucose
consumed. Dividing the ∆G°’ of ATP formation by that of
lactate formation indicates that homolactic fermentation is
31% “efficient”; that is, 31% of the free energy released by
this process under standard biochemical conditions is
sequestered in the form of ATP. The rest is dissipated as
heat, thereby making the process irreversible
 Likewise, alcoholic fermentation is 26% efficient under
biochemical standard state conditions. Actually, under
physiological conditions, where the concentrations of
reactants and products differ from those of the standard
state, these reactions have free energy efficiencies of
>50%.

49. 5. Summary:-
1. The NADH formed in glycolysis must be recycled to
regenerate NAD+, which is required as an electron acceptor
glyceraldehyde 3- phosphate. Under aerobic conditions
electrons pass from NADH to Oxygen in mitochondrial
respiration.
2. Under anaerobic or hypoxic conditions, many organisms
regenerate NAD+ by transferring electrons from NADH to
pyruvate forming lactate. Other organisms, such as Yeast,
regenerate NAD+ by reducing pyruvate to ethanol and CO2. In
these anaerobic processes (fermentations), there is no net
oxidation Or reduction of the carbons of the glucose.

50. 3. A variety of microorganisms can ferment sugar in fresh
foods, resulting in changes in pH, taste, texture and
preserving food from spoilage. Fermentations are used in
industry to produce a Wide variety of commercially valuable
organic compounds from inexpensive starting materials.

51. • Fermentation is the general term for such
processes, which extract energy ( as ATP) But
do not consume oxygen or change the
concentrations of NAD+ or NADH.
• Fermentations are carried out by a wide
range of organisms, many of which occupy
anaerobic niches, and they yield a variety of
end products, some of which find commercial
uses.

52. REFERENCES
 Lehninger, A.L., Nelson, D.L., & Cox,
M.M.(2008). Lehninger principles of biochemistry.(5th
Ed.). New York: Worth Publishers.
 Voet, D., & Voet, J. G. (1995). Biochemistry. New York: J.
Wiley & Sons.
 Ronald M. Atlas. (1997). Principles of microbiology.(2nd
Ed.). New York: McGraw-Hill Higher Education.
 Themedicalbiochemistrypage.org
 M.sparknotes.com

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