TMAO is an organic osmolyte that has the useful biological function of protecting proteins against denaturing stresses such as high concentration of urea. Various bacteria grow anaerobically using TMAO as an alternative electron transport chain, allowing for growth on non-fermentable carbon sources such as glycerol. Bacteria capable of reducing TMAO to TMA are found throughout three different ecological niches. TMAO-reducing, to date, has been observed in marine bacteria, photosynthetic bacteria living in shallow ponds, and in enterobacteria.
TMAO reductases have been studied in several organisms, and a common conserved feature is the presence of a molybdenum cofactor in all the known terminal enzymes.
Based on their substrate specificity, these enzymes can be divided into two groups:
* TMAO reductases which have high substrate specificity
* DMSO/TMAO reductases which can reduce a broad range of N and S-oxide substrates.
The first group consists of species such as Escherichia coli, Shewanella putrefaciens, and Roseobacter denitrificans while the second group consists of species such as Proteus vulgaris, Rhodobacter capsulatus, and Rhodobacter sphaeroides.
The TMAO respiratory system has been mostly widely studied at the molecular level in E. coli and Rhodobacter species.