Understand the Mechanisms of Glycolysis
Biological Oxidation-reduction
Reactions
The
transfer of phosphoryl groups is a the main characteristics of metabolism.
Equally important is another kind of transfer, electron transfer in
oxidation-reduction reactions. The flow of electrons in oxidation-reduction
reactions is responsible directly or indirectly for all work done by living
organisms. In non-photosynthetic organisms, the source of electrons are reduced
compounds (foods), in photosynthetic organisms, the initial electron donor is a
chemical species excited by the absorption of light. The path of electron flow
in metabolism is complex. Electrons move from various metabolic intermediates
to specialized electron carriers in enzyme-catalyzed reactions. The carriers in
turn donate electrons to acceptors with high electron affinities, with the
release of energy.
The Flow of Electrons
can do Biological Work
We
use motors, or an electric heater, gasoline in a car engine; we actually use
the flow of electrons to accomplish work. If you look at an electrical circuit
that powers a motor, you can see the source of electrons as a battery that contains
two chemical species differing in their electron affinities. Electrical wires
provide a pathway for electron flow from the chemical species at one pole of
the battery, through the motor, to the chemical species at the other pole of
the battery. Since the two chemical species differ in the affinity for
electrons, electrons flow through the circuit, driven by a force proportional
to the difference in electron affinity, the electromotive force (emf). The emf
(typically a few volts) can accomplish work of an appropriate energy transducer
(a transducer is any device that turns one form of energy into another.
Example, a speaker turns electrical energy into sound). The transducer here is
a motor in the circuit. The motor can be coupled to a variety of mechanical
devices to accomplish useful work.
ELECTRICAL CIRCUIT:
Battery = Source of electrons
Motor = Electrical transducer
BIOLOGICAL CIRCUIT:
Glucose (-ve) + Oxygen (+ve) = Source of electrons
Motor = ATP synthetase
Emf = Proton motive force
Device = Cellular organelles
Living cells have an analogous biological circuit with a relatively reduced compound such as “glucose,” as the source of electrons. As glucose is enzymatically oxidized, the electrons released flow spontaneously through a series of electron-carrier intermediates to another chemical species such as O₂. This electron flow is exergonic because O₂ has a higher affinity for electrons than do the electron-carrier intermediates. The resulting electromotive force (emf) provides energy to a variety of molecular energy transducers (enzymes and other proteins) that do biological work.
In the mitochondria, for example, membrane-bound enzymes couple electron flow to the production of a transmembrane pH difference, accomplishing osmotic and electrical work. The proton gradient thus formed has potential energy, sometimes called proton-motive force by analogy with electromotive force. Another enzyme ATP synthase in the inner mitochondrial membrane uses the proton motive force to do chemical work.
An
overview of the different pathways in the biological system for producing
energy:
Cellular
Oxidation of Glucose to Carbon Dioxide Requires Specialized Electron Carriers
The oxidation of
glucose supplies energy for the production of ATP. The complete oxidation of
glucose has a standard free energy change of: -2,840 KJ/mol
C₆H₁₂O₆ +
6O₂ ---------------> 6O₂ + 6H₂O
Cells do not convert glucose to CO₂
in a single, high-energy releasing reaction rather in a series of controlled
reactions, some of which are oxidations. The free energy released in these
oxidation steps is of the same order of magnitude as that required for ATP
synthesis from ADP, with some energy to spare. Electrons removed in these
oxidation steps are transferred to coenzymes specialized for carrying
electrons, such as NAD⁺,
NADP⁺, FMN, and FAD, besides others.
These coenzymes are water soluble and can undergo reversible oxidation and reduction
in many of the electron transfer reactions of metabolism. The nucleotides NAD⁺
and NADP⁺ more readily form one enzyme to
another.
Note: NAD⁺
generally functions in oxidations – usually as part of a catabolic reaction;
NADPH is the usual coenzyme in reductions – nearly as part of an anabolic
reaction.
NAD⁺ + 2e⁻ + 2H⁺ ------------> NADH + H⁺
NADP⁺ + 2e⁻ + 2H⁺ ------------> NADPH +
H⁺
Glycolysis
D-glucose is the major fuel of most
organisms and occupies a central position in metabolism. It is relatively rich
in potential energy. The complete oxidation of glucose to carbon dioxide and
water proceed a standard free energy change of -2,840 KJ/mol
Glycolysis is almost universal central
pathway of glucose metabolism, the pathway with the largest flux of carbon in
most cells. Glycolysis occurs in all living organisms (prokaryotes and eukaryotes).
The principal reactions associated with the classic glycolytic and fermentative
pathways in plants are almost identical with those of animal cells. However,
plant glycolysis has unique regulatory features, as well as a parallel partial
glycolytic pathway in plastids and alternative enzymatic routes for several
cytosolic steps. It involves a series of reactions, carried out by a group of
soluble enzymes located in both the cytosol and the plastid (chloroplasts or
amyloplasts), with reactions in the different compartments catalysed by
separate isoenzymes.
The
two phases of glycolysis:
(a) Preparatory Phase: In this phase
phosphorylation of glucose and its
conversion to glyceraldehyde 3-P
(b) Payoff Phase: Oxidative conversion of
glyceraldehyde 3-P to pyruvate and the coupled formation of ATP and NADH
The net yield of glycolysis = 2
molecules of ATP
Energy is also conserved in the payoff
phase in the form of two molecules of NADH/ molecule of glucose used.
FATES OF
PYRUVATE
FERMENTATION: In
the absence of oxygen, some cells can convert pyruvic acid into other compounds
following additional biochemical pathways that occur in the cytosol. The
combined reactions of glycolysis plus these additional pathways, which
regenerate NAD⁺
is known as fermentation. Only glycolysis pathway will produce ATP in the
fermentation reactions. There are several fermentation pathways that differ
significantly in terms of the enzymes that are used and the chemical compounds
that are formed from pyruvic acid. Two common pathways linked with fermentation
are lactic acid and ethyl alcohol.
Route2:
Lactic Acid Fermentation (Under low-oxygen conditions = Hypoxia):
Since NADH cannot be reoxidized to NAD⁺,
no ETS cycle will occur and therefore, no oxidation of pyruvate under these
conditions.
In addition to
animal muscles, lactic acid fermentation also occurs in some microorganisms
under anaerobic condition.
Short
Notes on Lactic Acid Fermentation:
In this type of fermentation, an enzyme lactate dehydrogenase converts pyruvic
acid to lactic acid. The process involves the transfer of one hydrogen atom
from NADH and the addition of one H⁺
to pyruvic acid. As a result, NADH is oxidized to NAD⁺.
The resulting NAD⁺ is used up again in glycolysis.
Lactic acid fermentation plays a significant
role in the manufacturing of many kinds of dairy products. Milk if not
refrigerated will ferment naturally. Such fermentation of milk is called
“spoiling.” However, scientists have discovered the microorganisms that cause
this process in a controlled manner to produce cheese, yoghurt, and many other
cultured dairy products. Only harmless active microorganisms are used for the
fermentation process.
Route3:
Fermentation of Alcohol (or ethanol): Alcohol fermentation
In some plant tissues and in certain
invertebrates, protists and microorganisms such as brewer’s yeast –alcohol
fermentation occurs.
Short
Notes on Alcohol Fermentation: Some plant cells and
unicellular organisms such as yeast use a process called alcohol fermentation.
In this fermentation process, pyruvic acid is converted into ethyl alcohol.
After the glycolysis step is completed, this fermentation process begins and
requires two steps. In the first process, pyruvate is converted into
acetaldehyde in the presence of an enzyme called pyruvate dehydrogenase.
Acetaldehyde is then converted into ethyl alcohol in the presence an enzyme
called alcohol dehydrogenase.
Alcoholic fermentation by yeast cells is
the basis of the wine and beer industry. Bread making also depends on alcoholic
fermentation by yeast cells. In this process, the CO₂
produced, makes the bread rise by forming bubbles inside the dough and ethyl
alcohol evaporates during baking.
Note: Under standard
conditions, and in the cell, glycolysis is essentially an irreversible process.
The efficiency of anaerobic respiration including glycolysis is only 5.2%
(146/2840)*100 of the total energy of glucose released by complete oxidation.
The two molecules of pyruvate formed by
glycolysis still contain most of the chemical potential energy of the glucose
molecule, energy that can be extracted by oxidative reactions in the citric
acid cycle and oxidative phosphorylation (ETS).
Because of the low efficiency of energy
conservation under fermentation, an increased rate of glycolysis is needed to
sustain the ATP production necessary for cell survival. This is called the Pasteur effect after the French
microbiologist Louis Pasteur, who first noted it when yeast switched from
aerobic respiration to anaerobic alcoholic fermentation. The higher rates of
glycolysis result from changes in glycolytic metabolite levels, as well as from
increased expression of genes encoding enzymes of glycolysis and fermentation
(Sachs et al. 1996).
No comments:
Post a Comment