Reference Work Entry

Handbook of Hydrocarbon and Lipid Microbiology

pp 1011-1024

Anaerobic Degradation of Hydrocarbons: Mechanisms of C–H-Bond Activation in the Absence of Oxygen

  • M. BollAffiliated withInsitute of Biochemistry, University of Leipzig
  • , J. Heider

Abstract:

Hydrocarbons are highly abundant in nature and are formed either via geochemical or biological processes. Their high C–H bond dissociation energies are responsible for low chemical reactivities. Due to the toxicity of many hydrocarbons, their biological degradation is of environmental concern. In the presence of oxygen, the C–H-bond is activated by oxygenases involving enzyme-bound reactive oxygen species in exergonic reactions. In contrast, anaerobic hydrocarbon-degrading bacteria use a number of alternative enzymatic reactions for the mechanistically sophisticated C–H-bond activation. Some of these reactions are only known from anaerobic hydrocarbon degradation pathways, and some follow unprecedented biochemical mechanisms. The known oxygen-independent C–H-activation reactions comprise: (1) hydroxylation with water by molybdenum cofactor containing enzymes, (2) addition to fumarate by glycyl-radical enzymes, (3) reverse methanogenesis involving variants of methyl-coenzyme M reductase, (4) methylation, and (5) carboxylation catalyzed by yet-uncharacterized enzymes. The available knowledge about these enzymes varies greatly: the ethylbenzene hydroxylating molybdenum enzyme has been characterized structurally and functionally, whereas even the mode of initial activation is at issue in case of benzene degradation (methylation vs. carboxylation).