Fatty acid biosynthesis pathway uses acetyl-CoA as a starting molecule [114]. Acetyl-CoA is converted into malonyl-CoA by the addition of a carboxyl group using acetyl-CoA carboxylase as a catalyst. The acetyl and malonyl groups on acetyl-CoA and malonyl-CoA are transferred to a small protein called acyl carrier protein (ACP), which has 77 amino acid residues with a phosphopantothene group specifically attached to a serine residue. Acetyl-ACP and malonyl-ACP are con­densed to generate acetoacyl-ACP. This molecule, then goes through reduction, dehydration, and another reduction step to form a 2,3,4-saturated fatty acyl-ACP. The fatty acids synthesized have a long carbon chain backbone, which stores a large amount of energy. To transform fatty acids into combustible fuels, pathways leading to biodiesels and long-chain alkanes/alkenes have been proposed. The fatty acyl-CoA can be reduced to the corresponding fatty aldehydes, which are in turn decarboxylated to long-chain alkanes or further reduced to fatty alcohols that can also be esterified to biodiesel with acetyl-CoA by an alcohol acyltransferase or ester synthase [1, 148]. Biodiesel as a possible substitute for petroleum-based diesel fuel is made from plant oils through transesterification of triacylglycerols with methanol or ethanol. Large-scale application of biodiesel seems difficult because of the seasonal restrictions and the costliness of the transesterification procedure [89]. To overcome these drawbacks, E. coli was engineered to produce fatty acid ethyl esters, where the traditional pathway of ethanol consisting of pyruvate decarboxylase (PDC) and alcohol dehydrogenase was introduced to supply ethanol as building units. The metabolically engineered E. coli was reported to have capability to produce fatty acid ethyl esters at a titer of 1.28 g/l, by using glucose and oleic acid as substrates.

9.4.3.1 Isoprenoid Pathway

Isoprenoids are natural hydrocarbons biosynthesized for a wide variety of func­tions. The isoprenoid pathway has been engineered in heterologous hosts to pro­duce nutraceuticals or pharmaceuticals [155]. Despite this, isoprenoids synthesized from isoprenyl diphosphate and dimethylallyl pyrophosphate which are either synthesized from glyceraldehydes-3-phosphate or pyruvate. Recently, two genes in Bacillus subtilis 6051 whose products can convert the prenyl diphosphate pre­cursors into corresponding isoprenoids have been reported [195].