Propionic acid fermentation

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fermentation
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In the propionic acid fermentation , also referred to as Propionatgärung, be bacterial carbohydrates or lactic acid ( lactate ) to propionic acid (propionate), acetic acid (acetate) and carbon dioxide (CO ₂ ) fermented . It serves the bacteria as an energy source. If the substrate is lactic acid, which was formed as a fermentation product by other microorganisms, one speaks of secondary fermentation. The fermentation product from the first fermentation is further fermented.

Occurrence

Microorganisms that ferment propionic acid were originally discovered in Swiss Emmentaler cheese. They are bacteria of the genus Propionibacterium (e.g. P. freudenreichii ), gram-positive anaerobes . In addition, this fermentation is carried out by some representatives of the genera Selenomonas , Veillonella , Clostridium and Peptostreptococcus .

biochemistry

Glucose , other hexoses or lactic acid are used as starting materials for propionic acid fermentation . In any case, pyruvate is formed from these substances , either from the sugars in the course of glycolysis or from lactic acid through oxidation using lactate dehydrogenase . In both cases, one molecule of NAD ^{+ is} reduced to NADH for each molecule of pyruvate formed .

Propionic acid fermentation is divided into two branches: In the oxidative branch of fermentation, pyruvate is oxidized to acetyl-CoA with the involvement of coenzyme A (CoA) , whereby another molecule of NAD ^{+ is} reduced to NADH. Acetyl-CoA is transesterified to acetyl phosphate by means of inorganic phosphate , whereby CoA is released again; this reaction is catalyzed by a phosphotransacetylase. Through substrate chain phosphorylation using an acetate kinase, acetate is finally formed and a molecule of adenosine triphosphate (ATP) is obtained (transfer of the phosphate residue from acetyl phosphate to adenosine diphosphate (ADP)).

${\displaystyle \mathrm {Pyruvat+NAD^{+}+ADP+P_{i}{\xrightarrow {oxidativerAst}}Acetat+CO_{2}+NADH+H^{+}+ATP} }$

In the reductive branch of fermentation, the NADH, which functions as a hydride ion carrier and which was formed in the oxidative branch by reduction from NAD, is reoxidized, with pyruvate being reduced to propionate. In the reductive branch of propionic acid fermentation, two different routes of chemical conversion are known: the methylmalonyl-coenzyme A route and the acrylyl-coenzyme A route.

Methylmalonyl-CoA route

Propionic acid fermentation using the methylmalonyl-CoA route. The illustration shows the fermentation of three molecules of lactate into two molecules of propionate and one molecule of acetate.

Propionibacteria use the methylmalonyl-CoA route in propionic acid fermentation. The C4 compound oxaloacetate is formed from the C3 compound pyruvate through carboxylation , which is catalyzed by a transcarboxylase , the methylmalonyl coenzyme A carboxyl transferase. This is a biotin- containing enzyme characteristic for this metabolic pathway and transfers a CO ₂ group from methylmalonyl-CoA, which is formed in the later course of the reaction, to pyruvate.

Oxaloacetate is converted into succinate via malate and fumarate in further reaction steps such as in the reductive citric acid cycle . Here, two molecules of NADH are oxidized to NAD ⁺ . When reducing fumarate, a proton gradient is also built up. Succinate is converted to succinyl-CoA by a coenzyme A transferase . This enzyme transfers the coenzyme A from propionyl-CoA, which is formed in the later course of the reaction, to succinate. Methylmalonyl-coenzyme A mutase, an adenosylcobalamin- containing enzyme, isomerizes succinyl-CoA to methylmalonyl-CoA, which is then decarboxylated by the above-mentioned transcarboxylase to produce propionyl-CoA. Finally, propionyl-CoA is converted into propionate in the last step, with coenzyme A transferase transferring coenzyme A to succinate.

The metabolic pathway enables the "recycling" of coenzyme A and CO ₂ . As a result, no energy has to be used in the carboxylation of pyruvate and the binding of succinate to CoA, which without this “recycling” would consume energy.

The bottom line for implementation is:

${\displaystyle \mathrm {Pyruvat{\xrightarrow[{Methylmalonyl-CoA-Weg}]{reduktiverAst}}Propionat+H_{2}O+\Delta \Psi (H^{+})} }$

Acrylyl CoA route

The bacteria Clostridium propionicum and Megasphaera elsdenii take an easier route in propionic acid fermentation. This is called the acrylyl-CoA route . Lactate is activated to lactyl-CoA, which catalyzes a CoA transferase (A, see picture below). Lactyl-CoA is converted to the eponymous acrylyl-CoA after dehydration by a lactyl-CoA dehydratase (B). Acrylyl-CoA is reduced to propionyl-CoA with oxidation of NADH, which catalyzes an acrylyl-CoA reductase (C). The coenzyme A bound to propionate is transferred to lactate by a CoA transferase (A), propionyl-CoA thus serves as a CoA donor for another molecule of lactate.

${\displaystyle \mathrm {Lactat+NADH+H^{+}{\xrightarrow[{Acrylyl-CoA-Weg}]{reduktiverAst}}Propionat+NAD^{+}} }$

Propionic acid fermentation using the acrylyl-CoA route. The lower part of the figure shows the reductive branch of fermentation. The oxidative branch corresponds to that in the figure above.

Balance sheets

If propionic acid fermentation is based on three molecules of lactate , one third of the lactate is converted into acetate and carbon dioxide with reduction of NAD ⁺ to NADH and two thirds to propionate by the methylmalonyl-CoA route with reoxidation of the NADH. In total, 1 mol of ATP can be formed. In addition, a proton gradient is built up from which ATP is also generated. Propionic acid bacteria thus gain energy from the waste product of lactic acid fermentation with the help of the reaction sequences shown: The change in free enthalpy under standard conditions (pH = 7) ΔG ₀ 'for fermentation based on lactate is −162 kJ / mol.

${\displaystyle \mathrm {3\ Lactat+ADP+P_{i}\longrightarrow 2\ Propionat+Acetat+CO_{2}+H_{2}O+ATP+\Delta \Psi (H^{+})} }$

During the fermentation of glucose in the propionic acid fermentation on the methylmalonyl-CoA pathway, the bacteria can form 4 mol of ATP from "1.5" moles of glucose (≙ three C ₃ molecules) by phosphorylation of ADP as a short-term energy store and carrier. In direct comparison to homofermentative lactic acid fermentation , slightly more ATP is formed.

${\displaystyle \mathrm {1,5\ Glucose+4\ ADP+4\ P_{i}\longrightarrow 2\ Propionat+Acetat+CO_{2}+H_{2}O+4\ ATP+\Delta \Psi (H^{+})} }$

Importance of propionic acid fermentation

Propionic acid fermentation is used, among other things, for the maturation of hard cheese , especially Emmentaler. Propionic acid bacteria with a mixture of homofermentative streptococci and lactic acid bacteria are used here. The latter ferment lactose into lactic acid and quark is formed . This is poured off, whereupon the propionic acid bacteria further convert the lactic acid into acetic acid and propionic acid, which are important components of the cheese flavor. The CO _{2 that} is also formed in the process leads to the formation of holes in the cheese.

In propionibacteria, a cobalamin enzyme is involved in the energy metabolism, so it is contained in the organisms in greater concentration than in other organisms in which cobalamin enzymes are only required for certain reactions in the building metabolism. Cobalamins (vitamin B ₁₂ ) are therefore obtained from propionibacteria (e.g. from Propionibacterium shermanii ) .

The biotin-CO ₂ compound is unstable and breaks down at low CO ₂ concentrations, whereby the propionic acid fermentation and thus the energy production of the propionibacteria come to a standstill. Propionibacteria are therefore dependent on a higher CO ₂ concentration, which must be taken into account in their culture. This CO ₂ dependency led to the discovery of the anaplerotic carboxylation reactions by Harland G. Wood and Chester Werkman .

Individual evidence

1. ↑ Katharina Munk (ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 568.

literature

• Georg Fuchs (Ed.), Hans Günter Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 372f.
• Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 3-8274-1107-6 ; P. 240ff.
• Katharina Munk (Ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 381ff.
• Michael T. Madigan, John M. Martinko, Jack Parker, and Thomas D. Brock: Microbiology . Spectrum Academic Publishing House; ISBN 3-8274-0566-1 ; P. 573f.

See also

• fermentation