Figure 6

Two hypothetical biosynthetic pathways for the formation of a C ₂₀ [5]-ladderane fatty acid. Locus in K. stuttgartiensis (and D. psychrophila) identified as potentially responsible for catalysing steps in the pathway have been indicated. (a) the precursor PUFA molecule is synthesised from a C_20:0 fatty acid using a desaturase to insert double bonds. The PUFA is then folded into a ladderane using a radical cascade mechanism, analogous to a previously proposed polycyclisation mechanism [48]. (b) Alternatively, the ladderane moiety could be synthesised via an as yet unknown pathway and brought into FASII via an unusual paralogue of FabF, attached to an acyl chain and released from FASII. The post-processing step is thought to be the same, irrespective of the pathway the ladderanes are synthesised. The last step shows the ester linkage of the ladderane fatty acyl group to L-glycrol 3-phosphate, during the synthesis of ladderane glycerophospholipids.