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 responsible for the elongation of palmitate (hexadecanoate, 16:0), also involved in polyunsaturated fatty acid (PUFA) biosynthesis. H(+) + hexadecanoyl-CoA + malonyl-CoA = 3-oxooctadecanoyl-CoA + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Expressed in various tissues and parts of the body, including the ventral cord, pharyngeal muscles, uterus, and the tail, and most strongly in intestinal cells. Suppression of ELO-2 causes an accumulation of palmitate and an associated decrease in the PUFA fraction in triacylglycerides and phospholipid classes. This imbalance in the fatty acid composition results in multiple phenotypic defects such as slow growth, small body size, reproductive defects, and changes in rhythmic behavior (PubMed:12586704). Suppression of ELO-2 decreases growth rate and reduces brood size (PubMed:32961767). Belongs to the ELO family.