# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 3 BVOC Biosynthetic Pathways and Their Intersections with Primary and Secondary Metabolism
### BVOCs and Secondary Metabolism
#### 3.2.3 Alkane and Alkene Metabolism in Bacteria  
CHCs play a central role in social insect communication and influence various aspects of their behavior and health. Many studies have focused on CHCs as both pheromones and signaling molecules. However, few studies have addressed their biosynthetic pathways, which therefore remain poorly characterized. Initial studies identified the biosynthetic source of CHCs in the insects' cells only, but further studies demonstrated the relevance of the microbiota metabolism as described above. Many studies report the ability of certain groups of bacteria to synthesize linear alkanes and alkenes, but even in this case, the biosynthetic pathways remain poorly characterized, with some exceptions. Most of the information comes from studies on cyanobacteria [175]. Different biosynthetic pathways leading to alkanes and/or alkenes from fatty acid precursors were proposed: (I) the Acyl-ACP reductase-aldehyde-deformylating oxygenase pathway (AAR-ADO), (II) the olefin biosynthetic pathway (Ole, encoded by the _oleABCD_ operon), (III) the terminal olefin pathway (OleTjE), (IV) the olefin synthase pathway (Ols) (Figure 6). Of all these pathways, only the first two (AAR-ADO and Ole) are well-characterized [175]. The AAR-ADO was first identified in _Synechococcus_[176] using a differential and subtractive genomic approach based on the differences between alkane/alkene producing and nonproducing strains. Evidence was provided that alkanes and alkenes biosynthesis start from an acyl-acylated acyl-carrier protein (ACP). The acyl-ACP substrate is then reduced by acyl-ACP reductase (AAR) and the resulting aldehyde is converted to terminal alkane or alkene by aldehyde-deformylating oxygenase (ADO) [177]. ADO is a unique enzyme of cyanobacteria, which converts acyl aldehyde precursors into alkane/alkene by the combined action of ferredoxin and NADPH:ferredoxin/flavodoxin oxidoreductaseates and O\({}_{2}\) as an electron acceptor [177]. Figure 6: Linear hydrocarbons biosynthesis. Microorganisms produce hydrocarbons through different biosynthetic pathways. The blue arrows indicate bacterial metabolism for a precursor of hydrocarbons (propionic acid). As shown, different pathways produce propionate including fermentation, biotransformation of succinate, and biotransformation of intermediates from amino acid metabolisms. In _Z. nevadensis_, propionate is produced by bacterial community starting from succinate and is then assembled in final cuiticular hydrocarbons by the insect. The green arrows show the direct synthesis of alkanes and alkenes in bacteria. There are four pathways: AAR/ADO, Ols, OleABCD, and  
and OleTJE. Abbreviations: SCS = succinyl-CoA synthetase, MUT = methylmalonyl-CoA mutase, MCEE = methylmalonyl-CoA/ethylmalonyl-CoA epimerase, ScpB = methylmalonyl-CoA decarboxylase, PCCA = propionyl-CoA carboxylase alpha chain, EC 2.1.3.1 = methylmalonyl-CoA carboxyltransferase, P\(\mathrm{\#ID}\) = formate C-acetyltransferase, PorA = pyruvate ferredoxin oxidoreductaseulae alpha subunit, Pta = phosphate acetyltransferase, P\(\mathrm{\#duL}\) = phosphate propanoyltransferase, AckA = acetate kinase, TdcD = propionate kinase; Pct = propionate CoA-transferase, AcdA = acetate-CoA ligase (ADP-forming) subunit alpha. The Ole pathway was characterized in _Microococcus luteus_[167]. The _M. luteus_ olefin biosynthetic operon _oleABCD_ is composed of three genes that codes for enzymes: Thiolase (OleA), bifunctional \(\alpha\)/\(\beta\) hydrolase and AMP-dependent ligase/synthetase (OleB/C), and short-chain dehydrogenase (OleD) [178]. The pathway starts with a nondecarboxylative Claisen condensation catalyzed by OleA, which generates a \(\beta\)-keto acid that is then reduced by OleD in a NADPH-dependent reaction to produce a \(\beta\)-hydroxy acid. OleC catalyzes the conversion of the \(\beta\)-hydroxy acid to an alkene using ATP. OleB is thought to perform scaffolding or regulatory function in the Ole complex. This pathway generates nonterminal long-chain alkenes by fatty acid head-to-head condensation.