1. Propionyl-CoA metabolism in plants:Propionyl-CoA
is derived from the breakdown of chlorophyll, odd-chain fatty acids, or
amino acids like isoleucine and is oxidized to 3-hydroxypropionate
which is then further oxidized to acetyl-CoA (Giovanelli and Stumpf,
1958;
Lucas et al., 2007; Rendina and Coon, 1957). Some animals and algae may
also metabolize propionate via a similar route (Callely and Lloyd 1964;
Hanarnkar et al., 1985; Lloyd et al., 1968).
2. Some autotrophic CO2 fixation pathways:
In
bacteria and archaea, the reductive conversion of acetyl-CoA and CO2 to
propionyl-CoA via 3-hydroxypropionate is part of two CO2 fixation
pathways; however, different enzymes are used in either pathway to
catalyze the common steps in the conversion of acetyl-CoA and CO2 to
propionyl-CoA (Berg et al., 2007; Berg et al., 2002; Herter et al.
2001; Talarico et al., 1988). For example, the reductive conversion of
3-hydroxypropionate to propionyl-CoA is catalyzed by a fusion protein,
named propionyl-CoA synthase, in Chloroflexus aurantiacus (3-hydroxypropionate bi-cycle), whereas Metallosphaera sedula
(hydroxypropionate/4-hydroxybutyrate cycle) requires three separate
enzymes to catalyze the same reaction sequence (Alber and Fuchs, 2002;
Teufel et al., 2009).
3. Dimethylsulfonopropionate (DMSP) metabolism.
3-Hydroxypropionate
is an intermediate in the metabolism of the secondary metabolite
DMSP by microorganisms (Ansede et al., 1999; Todd et al., 2007). DMSP
is synthesized by marine algae and some land plants, and there are
currently three different mechanisms known for the initial step of DMSP
degradation: demethylation to methylmercaptopropionate (Talarico et
al., 1988, Teufel et al., 2009), cleavage by a DMSP lyase into
dimethylsulfide and acrylate (Ansede et al., 1999; Hanarnkar et al.,
1985; Wagner and Stadtman, 1962), and the cleavage of DMSP into
3-hydroxypropionate and dimethylsulfide by an unusual CoA-transferase
(Todd et al., 2007; Todd et al., 2010). Acrylate or
3-hydroxypropionate generated from the cleavage of DMSP may be further
metabolized to acetyl-CoA and CO2; in the case of acrylate, this
proceeds via 3-hydroxypropionate (Ansede, 1999; Ansede 2001; Todd et
al., 2007). However, some bacteria use DMSP solely as a sulfur source
and may therefore release 3-hydroxypropionate or acrylate as an end
product (Gonzalez et al., 1999).
4. Uracil degradation:
3-Hydroxypropionate has been identified as the end product of two different pathways for uracil degradation in bacteria like Escherichia coli as well as in the yeast Saccharomyces kluyveri (Andersen et al, 2008; Loh et al., 2006, Osterman, 2006).
5. Anaerobic metabolism of glycerol:
There
have been reports of 3-hydroxypropionate formation by the fermentation
of glycerol by lactic acid bacteria (Luo et al. 2011; Talarico et al.,
1988) and the anaerobic oxidation of glycerol by a sulfate-reducing
bacterium (Qatibi et al., 1998).