Catalyzes the first and rate-limiting reaction of the four reactions that constitute the long-chain fatty acids elongation cycle. Uses malonyl-CoA to add 2 carbons per cycle to the chain of long-chain fatty acids. Condensing enzyme required for the formation of isoheptadecanoate (C17iso), which plays critical roles in animal development and growth. H(+) + isopentadecanoyl-CoA + malonyl-CoA = 3-oxoisoheptadecanoyl-CoA + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Expressed in the gut, neurons, pharynx and muscles of the vulva. Belongs to the ELO family.