GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.3. CATABOLISM OF HIGHER $n$-ALKANES AND FATTY ACIDS

$n$-Alkanes are paraffin Hydrocarbons containing 10-18 carbon atoms. Growth on paraffins is quite widespread among microorganisms (Bacteria such as Acinetobacter calcoaceticus, Pseudomonas fluorescens, Representatives of the genera Corynebacterium, Mycobacterium, Nocardia, Arthrobacter, Yeasts such as Candida lipolytica, Torulopsis colliculosa, and Molds such as Cephalosporium roseum). Hydrocarbons are insoluble compounds, making their uptake a complex process. Very often, hydrocarbon-degrading microorganisms synthesize extracellular emulsifiers or Surfactants that are capable of emulsifying hydrocarbons and facilitating their transport into The Cell. An example is emulsan, an extracellular and cell-associated lipopolysaccharide synthesized by Acinetobacter calcoaceticus. Emulsan contains up to 19% Fatty acids, making it an excellent emulsifier.

The oxidation of hydrocarbons is typically catalyzed by Monooxygenases, yielding the corresponding alcohols. Molecular oxygen and a reductant also participate in this reaction. The general equation for reactions of this type is as follows:

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where R is an alkyl radical.

Monooxygenases are multi-enzyme systems comprising a hydroxylase component (which directly catalyzes hydrocarbon oxidation) as well as electron carriers that transfer electrons from NADH and NADPH (or other potential reductants) to the hydroxylase component.

The products of hydrocarbon monooxygenation (alcohols) are oxidized first to aldehydes and subsequently to the corresponding fatty acids with the participation of dehydrogenases.

Higher Fatty Acids are degraded via β-oxidation. Initially, the fatty acid is converted into the corresponding coenzyme A thioester by the enzyme acyl-CoA synthetase. Next, the CoA-thioester undergoes oxidation at the β-position (catalyzed by acyl-CoA dehydrogenase) and is cleaved to yield acetyl-CoA and a fatty acid CoA-thioester shortened by two carbon atoms. During this process, one molecule of NAD and one molecule of FAD are reduced. The β-oxidation cycle then repeats until the fatty acid is completely degraded.

The breakdown of fatty acids with an even number of carbon atoms yields exclusively acetyl-CoA. Consequently, microorganisms growing on such substrates require The Glyoxylate cycle as an anaplerotic pathway, along with Gluconeogenesis, to synthesize CARBOHYDRATES for anabolic METABOLISM.

In the case of fatty acids with an odd number of carbon atoms, the final cycle of β-oxidation produces both acetyl-CoA and propionyl-CoA. Propionyl-CoA can be channeled into further metabolism via two pathways:

1) Conversion to succinyl-CoA, consisting of three reactions. This pathway of propionyl-CoA metabolism has been identified in various animal Tissues, rhizobia, and Paracoccus denitrificans;

2) Conversion of propionyl-CoA to Pyruvate via acrylyl-CoA and lactyl-CoA. This pathway is characteristic of E. coli,