Biotransformation of monoterpenes, bile acids, and other isoprenoids in anaerobic ecosystems

Abstract

Isoprenoic compounds play a major part in the global carbon cycle. Biosynthesis and mineralization by aerobic bacteria have been intensively studied. This review describes our knowledge on the anaerobic metabolism of isoprenoids, mainly by denitrifying and fermentative bacteria. Nitrate-reducing beta-Proteobacteria were isolated on monoterpenes as sole carbon source and electron donor. Thauera spp. were obtained on the oxygen-containing monoterpenes linalool, menthol, and eucalyptol. Several strains of Alcaligenes defragrans were isolated on unsaturated monoterpenes as growth substrates. A novel denitrifying beta-Proteobacterium, strain 72Chol, mineralizes cholesterol completely to carbon dioxide. Physiological studies showed the presence of several oxidative pathways in these microorganisms. Investigations by organic geochemists indicate possible contributions of anaerobes to early diagenetic processes. One example, the formation of p-cymene from monoterpenes, could indeed be detected in methanogenic enrichment cultures. In man, cholic acid (CA) and chenodeoxycholic acid (CDCA), are synthesized in the liver from cholesterol. During their enterohepatic circulation, bile acids are biotransformed by the intestinal microflora into a variety of metabolites. Known bacterial biotranformations of conjugated bile acids include: deconjugation, oxidation of hydroxy groups at C-3, C-7 and C-12 with formation of oxo bile acids and reduction of these oxo groups to either alpha- or beta-configuration. Quantitatively, the most important bacterial biotransformation is the 7 alpha-dehydroxylation of CA and CDCA yielding deoxycholic acid and lithocholic acid, respectively. The 7 alpha-dehydroxylation of CA occurs via a novel six-step biochemical pathway. The genes encoding several enzymes that either transport bile acids or catalyze various reactions in the 7 alpha-dehydroxylation pathway of Eubacterium sp. strain VPI 12708 have been cloned, expressed in Escherichia coli, purified, and characterized.

The Enzymes, Regulation, and Genetics of Bile Acid Synthesis

“…For example, in humans and rats, cholic acid and chenodeoxycholic acid are the primary bile acids, whereas in mice cholic acid and ␤-muricholic acid predominate. The chemical diversity of the bile acid pool is further expanded by the actions of anaerobic bacteria in the gut, which convert primary bile acids into dozens of secondary and tertiary bile acids (3). The plethora of different bile acids in the enterohepatic circulation ensures complete solubilization of hydrophobic nutrients in the small intestine.…”

Microbial monoterpene transformations—a review

“…The proposed degradation mechanism involves two initial oxidation reactions leading to menth-2-enone, followed by a hydration and an additional oxidation step. Finally, ring cleavage may occur and the molecule is attached to coenzyme A to yield 3,7-dimethyl-5-oxo-octyl-CoA (Foss and Harder, 1998; Hylemon and Harder, 1998). …”

“…Therefore, this review on the transformation of monoterpenes focusses on enzymes for which the gene and protein sequences are available in public databases as well as on microorganisms that at least have been deposited in a public culture collection and ideally are validly described (Table 1). A broad overview on microbial biotransformations is also provided by a number of older review articles (Trudgill, 1990, 1994; van der Werf et al, 1997; Hylemon and Harder, 1998; Duetz et al, 2003; Ishida, 2005; Li et al, 2006; Bicas et al, 2009; Li and Lan, 2011; Schewe et al, 2011; Tong, 2013). KEGG and MetaCyc, two widely used reference datasets of metabolic pathways (reviewed by Altman et al, 2013), include degradation pathways of limonene, pinene, geraniol, and citronellol.…”

A new family of CoA‐transferases

“…The BaiF protein (for bile acid‐induced) plays a role in cholic acid reduction to deoxycholate by an Eubacterium sp. and is encoded together with genes for other enzymes of the pathway in the bai operon [29]. The first committed step, activation of cholic acid to the CoA‐thioester, was proposed to be catalysed by a CoA ligase encoded in the operon (BaiB; Fig.…”