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

oute1: In aerobic organisms or tissues under aerobic condition

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).