GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.5. CATABOLISM OF AROMATIC COMPOUNDS

Plants synthesize numerous compounds containing aromatic rings. Quantitatively, Lignin predominates among them, accounting for up to 20% of wood by mass. The ability to degrade such compounds via aromatic ring Cleavage is characteristic of many Bacteria and Fungi. Molecular oxygen is required for The breakdown of Aromatic Compounds.

Polycyclic compounds (naphthalene, phenanthrene, anthracene) are initially degraded into aromatic compounds (Fig. 12.8). Most naturally occurring aromatic compounds are first cleaved to pyrocatechol or protocatechuic acid, which then undergo dioxygenation (double oxygenation).

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Fig. 12.8. Pathways of degradation of certain aromatic Amino Acids

Prior to aromatic ring cleavage, monooxygenation takes place; it is catalyzed by Monooxygenases (hydroxylases) utilizing reducing power and molecular oxygen.

Potential substituents of the aromatic ring are removed before its cleavage. The ring cleavage is catalyzed by oxygenases and proceeds via either ortho- or meta-cleavage pathways (Fig. 12.9).

During ortho-cleavage, pyrocatechol is converted into cis,cis-muconic acid, whereas protocatechuic acid is converted into 3-carboxy-cis,cis-muconic acid.

During meta-cleavage, pyrocatechol is transformed into 2-hydroxymuconic semialdehyde, and protocatechuic acid into 4-carboxy-2-hydroxymuconic semialdehyde.

Gentisic acid serves as another intermediate in the degradation of aromatic compounds. Aromatic ring cleavage yields maleylpyruvic acid (see Fig. 12.9).

Subsequently, maleylpyruvic acid is cleaved into fumarate, malate, and Pyruvate. 2-hydroxymuconic semialdehyde degrades into formate, acetaldehyde, and pyruvate, while 4-carboxy-2-hydroxymuconic semialdehyde yields two molecules of pyruvate and one molecule of formate. Cis,cis-muconic and 3-carboxy-cis,cis-muconic acids are transformed into acetyl-CoA and succinate.

Summary data on the METABOLIC ACTIVITY OF aerobic heterotrophs are presented in the table