Fermentation and Aerobic Respiration Notes

Fermentation

• Fermentation is an enzyme-catalyzed, metabolic process that produces chemical changes in organic substrates.

• It's the primary way microorganisms produce adenosine triphosphate (ATP) anaerobically (without oxygen) by breaking down carbohydrates.

• The study of fermentation and its practical uses is called zymology, which started in 1856 when Louis Pasteur showed that yeast caused fermentation.

• Fermentation happens in the cytosol of cells.

Examples of Fermentation

• Mammalian muscle cells use fermentation during intense exercise when oxygen is scarce, producing lactic acid.

• Cells recycle nicotinamide adenine dinucleotide (NAD+) from NADH through aerobic respiration, but when oxygen is limited, fermentation regenerates NAD+ for glycolysis, which releases energy as ATP.

• In the presence of oxygen, aerobic respiration yields up to 38 ATP molecules per glucose molecule using NADH and pyruvate.

• In anaerobic conditions, fermentation produces a net of 2 ATP per glucose molecule, so it's not often used when oxygen is available.

• However, it occurs in:

  • Obligate anaerobes (bacteria and fungi requiring an oxygen-free environment).

  • Facultative anaerobes (organisms like yeast that use aerobic respiration when oxygen is present but can switch to fermentation when it's absent).

  • Muscle cells.

Nicotinamide Adenine Dinucleotide (NAD)

• Nicotinamide adenine dinucleotide (NAD) is a coenzyme with two forms: oxidized (NAD+) and reduced (NADH).

• During fermentation, an organic electron acceptor (like pyruvate or acetaldehyde) reacts with NADH to form NAD+ and generates products like carbon dioxide, hydrogen, ethanol, or lactic acid.

• Fermentation is valuable in the food and beverage industries:

  • Sugars are converted into ethanol for alcoholic beverages.

  • CO2 is released by yeast for leavening bread.

  • Organic acids are produced to preserve and flavor vegetables and dairy products.

• Fermentation is also used in wastewater treatment, where aerobic bacteria ferment solid organic matter into carbon dioxide, water, and mineral salts in the activated sludge process.

Alcoholic Fermentation

• Ethanol fermentation, or alcoholic fermentation, is a biological process that transforms sugars like glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products.

• Yeasts perform this conversion without oxygen, making it an anaerobic process.

Uses:

• Preparation of beers, wines, and other alcoholic beverages, ethanol fuel production, and bakery.

• The purpose of fermentation in yeast is the same as that in muscle and bacteria, to replenish the supply of NAD+ for glycolysis.

Process (Two Steps):

1. Pyruvate, produced via glycolysis, is converted into acetaldehyde by the enzyme pyruvate decarboxylase, releasing $CO_2$.

2. Acetaldehyde is reduced by NADH to ethanol using alcohol dehydrogenase, regenerating NAD+ for glycolysis.

• Overall, glycolysis converts one glucose molecule into two pyruvates, which are then converted into two molecules of carbon dioxide and two molecules of ethanol in alcoholic fermentation.

• $C<em>6H</em>{12}O<em>6 \rightarrow 2C</em>2H<em>5OH + 2CO</em>2$

Lactic Acid Fermentation

• Lactic acid fermentation is an anaerobic reaction that converts pyruvate to lactate and regenerates NAD+, allowing glycolysis to continue making ATP in low-oxygen conditions.

• Since the supply of NAD+ is limited in a cell, it must be restored to maintain ATP production.

• NADH donates its extra electrons to pyruvate molecules, regenerating NAD+, and lactic acid is formed by the reduction of pyruvate.

• Lactate dehydrogenase catalyzes the interconversion of pyruvate to lactate with concomitant generation of $NAD^+$ from NADH.

• $C<em>3H</em>3O<em>3 \text{ (pyruvate)} + NADH \rightarrow C</em>3H<em>5O</em>3 \text{ (lactate)} + NAD^+$

• Sometimes, even with oxygen present, if pyruvate builds up faster than it can be metabolized, fermentation will happen anyway.

Types:

1. Homolactic fermentation: NADH reduces pyruvate directly to form lactate. This process does not release gas and converts one molecule of glucose into two molecules of lactate.

2. Heterolactic fermentation: Some lactate is further metabolized, yielding ethanol and carbon dioxide in addition to lactic acid, in a process called the phosphoketolase pathway.

• Lactic acid fermentation is primarily performed by certain types of bacteria and fungi.

• However, this type of fermentation also occurs in muscle cells to produce ATP when the oxygen supply has been depleted during strenuous exercise and aerobic respiration is not feasible.

Acetic Acid Fermentation

• Hydrogen gas is produced in many types of fermentation (a process called fermentative hydrogen production), as a way to recycle NAD+ from NADH.

• This is a two-step process where ultimately glucose is transformed into acetate along with evolution of hydrogen.

• Vinegar is prepared following this procedure.

Steps:

1. Formation of ethyl alcohol from sugar anaerobically using yeast.

2. Oxidation of ethanol to acetic acid using acetobacter bacteria under aerobic condition.

• For example, Clostridium pasteurianum ferments glucose to butyrate, acetate, carbon dioxide, and hydrogen gas.

• The reaction leading to acetate is:

• $C<em>6H</em>{12}O<em>6 + 4H</em>2O \rightarrow 2CH<em>3CO</em>2^- + 2HCO<em>3^- + 4H^+ + 4H</em>2$

Aerobic Fermentation

• Fermentation does not necessarily have to be carried out in an anaerobic environment.

• For instance, even in the presence of abundant oxygen, few strains of yeast greatly prefer fermentation over oxidative phosphorylation, provided sugars are readily available for consumption.

• This phenomenon where yeast forms ethanol (alcohol) in aerobic conditions at high external glucose concentrations rather than producing biomass, is known as the Crabtree effect.

• This is observed in most species of the Saccharomyces, Schizosaccharomyces, Debaryomyces, Brettanomyces, Torulopsis, Nematospora, and Nadsonia genera of yeast.

• On increasing the glucose concentrations glycolysis is accelerated, and that leads to a substantial amount of ATP.

• This reduces the need of oxidative phosphorylation done by the TCA cycle and hence, diminishes oxygen consumption.

• The phenomenon is believed to have evolved as a competitive mechanism because of the antiseptic nature of ethanol.

• In the presence of oxygen, the metabolic process through which cells metabolize sugars via fermentation by repression of normal respiratory metabolism, is called aerobic fermentation.

• In general, the process is shorter and more intense than anaerobic fermentation.

• Aerobic fermentation does not produce ATP in high yield, instead, it allows proliferating cells to convert nutrients like glucose and glutamine more efficiently into biomass by avoiding unnecessary catabolic oxidation of such nutrients into $CO_2$, preserving carbon-carbon bonds and promoting anabolism.

Aerobic Respiration

• Aerobic respiration is the process by which organisms use oxygen to turn fuel, such as, fats and carbohydrates, into chemical energy.

• The products of this process are carbon dioxide and water, and the energy transferred is utilized to cleave bonds in ADP to add a third phosphate group to form ATP.

• $C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + 38 \text{ ATP}$

• The complete process of aerobic respiration occurs in four different stages:

(i) Glycolysis:

• It is the primary step of aerobic respiration, occurring within the cytosol of the cell under oxygen- free conditions.

• During glycolysis, glucose undergoes a series of enzymatic transformations to get converted into two molecules of pyruvate, alongside the production of two ATP.

• $C<em>6H</em>{12}O<em>6 + 2ADP + 2Pi + 2NAD^+ \rightarrow 2\text{Pyruvate} + 2ATP + 2NADH + 2H^+ + 2H</em>2O$

(ii) Pyruvate Oxidation / Oxidative Decarboxylation of Pyruvate:

• Pyruvate oxidation is sometimes referred to as the transition reaction because the process links glycolysis to the citric acid cycle.

• Each pyruvate from glycolysis is transferred to the mitochondrial matrix via a protein known as pyruvate translocase.

• There, the pyruvate is combined with Coenzyme A to release a carbon dioxide molecule and forms Acetyl-CoA, that, further fuels the citric acid cycle.

• $\text{Pyruvate} + \text{Coenzyme A} + NAD^+ \rightarrow \text{Acetyl CoA} + CO_2 + NADH$

(iii) Citric Acid Cycle (CAC) / Tricarboxylic Acid Cycle (TCA Cycle) / Krebs Cycle:

• The Krebs cycle, is a series of redox reactions utilized by all aerobic organisms to release stored energy through the oxidation of Acetyl-CoA.

• These reactions take place in the matrix of the mitochondria of eukaryotic cells and in the cytoplasm for prokaryotic cells.

• The overall reaction is as follows:

• $\text{Acetyl CoA} + 3NAD^+ + FAD + ADP + Pi \rightarrow CO<em>2 + 3NADH + FADH</em>2 + ATP + H^+ + \text{Coenzyme A}$

• The reaction occurs twice for each molecule of glucose, as there are two pyruvates and hence two molecules of Acetyl CoA generated to enter the citric acid cycle.

• Both NADH and FADH2, another carrier of electrons for the electron transport chain, are formed in this process.

(iv) Oxidative Phosphorylation/Electron Transport-Linked Phosphorylation/Terminal Oxidation:

• Oxidative phosphorylation is the primary energy providing stage of aerobic respiration. It uses the folded membranes within the cell’s mitochondria to produce huge amounts of ATP.

• Almost all aerobic organisms carry out oxidative phosphorylation.

• $ADP + Pi + NADH + 1/2 O<em>2 + H^+ \rightarrow ATP + NAD^+ + H</em>2O$

• In this process, NADH and FADH2 donate the electrons they obtained from glucose during the previous steps of cellular respiration to the electron transport chain in the inner mitochondrial membrane.

• The electron transport chain consists of a number of protein complexes (Complex I‒IV, Q, and cytochrome C) that are embedded in the mitochondrial membrane and they serve to pass electrons from higher to lower energy levels.

• As the electrons move down the chain, energy is released and used to pump protons out of the matrix. This store of energy is tapped when protons flow back across the membrane and down the potential energy gradient, through an enzyme called ATP synthase in a process called chemiosmosis.

• The ATP synthase uses the energy to transform ADP into ATP, in a phosphorylation reaction. This process is why mitochondria are referred to as “the powerhouses of the cell.” The mitochondria’s electron transport chain makes nearly 90% of all the ATP produced by the cell from breaking down food.

• It is often stated that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and ~ 34 from the electron transport system). However, due to leaky membranes, energy expenses to transfer pyruvate and ADP into the mitochondrial matrix, this maximum yield is never quite reached.

• Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging and senescence.