Anaerobic respiration
Overview
Anaerobic respiration (anaerobiosis) refers to the oxidation of molecules in the absence of oxygen to produce energy, in opposition to aerobic respiration which does use oxygen. Anaerobic respiration processes require another electron acceptor to replace oxygen. Anaerobic respiration is often used interchangeably with fermentation, especially when the glycolytic pathway is used for energy production in the cell. They are not synonymous terms, however, since certain anaerobic prokaryotes can generate all of their ATP using an electron transport system and ATP synthase. Definition of anaerobic respiration: the breakdown of food substances in the absence of oxygen with a small amount of energy. General word and symbol equations for the anaerobic respiration of glucose can be shown as
glucose <math>\to</math> lactic acid + energy (ATP);
C6H12O6 <math>\to</math> 2C3H6O3 + 2 ATP.
The energy released is about 120 kJ per mole glucose.
Obligate anaerobes
In some organisms called obligate (strict) anaerobes (ex: Clostridium tetani (causes tetanus), Clostridium perfringens (causes gangrene)), the presence of oxygen is lethal. This is because the presence of oxygen is processed by the organisms into the extremely toxic molecules of singlet oxygen (1O2), superoxide ion (O2-), hydrogen peroxide (H2O2), hydroxyl ion (OH-), and other toxic molecules. .
Facultative anaerobes and obligate aerobes
Facultative anaerobic organisms can survive in either oxygenated or deoxygenated environments and can switch between cellular respiration or fermentation, respectively) and obligate (strict) aerobes (organisms that can survive only with oxygen) have special enzymes (superoxide dismutase and catalase) that can safely handle these products and transform them into harmless water and diatomic oxygen in the following reactions:
2O2- + 2H+ –superoxide dismutase–> H2O2 (hydrogen peroxide) + O2.
The hydrogen peroxide produced is then transferred to a second reaction:
2H2O2 –catalase–> 2H2O + O2.
The oxidative powers of the superoxide ion have now been neutralized. Only facultative anaerobes and obligate aerobes possess the two enzymes necessary to reduce the superoxide.
In organisms which use glycolysis, the absence of oxygen prevents pyruvate from being metabolised to CO2 and water via the citric acid cycle and the electron transport chain (which relies on O2) does not function. Fermentation does not yield more energy than that already obtained from glycolysis (2 ATPs) but serves to regenerate NAD+ so glycolysis can continue. Various end products can also be created, such as lactate or ethanol.
Fermentation in animals is essential to human life.
In lactic acid fermentation, the following reaction occurs:
1. Glycolysis
C6H12O6 (glucose) + 2 NAD+ <math>\to</math> 2 C3H4O3 (pyruvic acid) + 2 NADH
2. Lactic acid creation
2 C3H4O3 (pyruvic acid) + 2 NADH <math>\to</math> 2 C3H6O3 (lactic acid) + 2 NAD+
Net reaction:
C6H12O6 (glucose) <math>\to</math> 2 C3H6O3 (lactic acid)
Fermentation in other organisms
In some plant cells and yeasts, fermentation produces CO2 and ethanol. The conversion of pyruvate to acetaldehyde generates CO2 and the conversion of acetaldehyde to ethanol regenerates NAD+.
Anaerobic respiration in prokaryotes
In the field of prokaryotic metabolism, anaerobic respiration has a more specific meaning. In this case, anaerobic respiration is defined as a membrane-bound biological process coupling the oxidation of electron donating substrates (e.g. sugars and other organic compounds, but also inorganic molecules like hydrogen, sulfide/sulfur, ammonia, metals or metal ions) to the reduction of suitable external electron acceptors other than molecular oxygen. In contrast, in fermentation the oxidation of molecules is coupled to the reduction of an internally-generated electron acceptor, usually pyruvate. Hence, scientists who study prokaryotic physiology view anaerobic respiration and fermentation as distinct processes and therefore do not use the terms interchangeably.
In anaerobic respiration, as the electrons from the electron donor are transported down the electron transport chain to the terminal electron acceptor, protons are translocated over the cell membrane from "inside" to "outside", establishing a concentration gradient across the membrane which temporarily stores the energy released in the chemical reactions. This potential energy is then converted into ATP by the same enzyme used during aerobic respiration, ATP synthase. Possible electron acceptors for anaerobic respiration are nitrate, nitrite, nitrous oxide, oxidised amines and nitro-compounds, fumarate, oxidised metal ions, sulfate, sulfur, sulfoxo-compounds, halogenated organic compounds, selenate, arsenate, bicarbonate or carbon dioxide (in acetogenesis and methanogenesis). All these types of anaerobic respiration are restricted to prokaryotic organisms.
Examples of anaerobic respiration
glucose + 3NO3- + 3H2O <math>\to</math> 6HCO3- + 3NH4+, ΔG0' = -1796 kJ
glucose + 3SO42- + 3H+ <math>\to</math> 6HCO3- + 3SH-, ΔG0' = -453 kJ
glucose + 12S + 12H2O <math>\to</math> 6HCO3- + 12HS- + 18H+, ΔG0' = -333 kJ
All of these terminal electron acceptors are further upstream in the electron transport chain, compared to O2. Consequently, anaerobic respiration is less effective than aerobic respiration. The ΔG0' of aerobic respiration is -2844 kJ.