Catalyzes the first and rate-limiting reaction of the four reactions that constitute the long-chain fatty acids elongation cycle. This endoplasmic reticulum-bound enzymatic process allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids (VLCFAs) per cycle. Condensing enzyme that elongates fatty acids with 12, 14 and 16 carbons with higher activity toward C16:0 acyl-CoAs. Catalyzes the synthesis of unsaturated C16 long chain fatty acids and, to a lesser extent, C18:0 and those with low desaturation degree. May participate in the production of saturated and monounsaturated VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA H(+) + hexadecanoyl-CoA + malonyl-CoA = 3-oxooctadecanoyl-CoA + CO2 + CoA (9Z)-hexadecenoyl-CoA + H(+) + malonyl-CoA = 3-oxo-(11Z)-octadecenoyl-CoA + CO2 + CoA dodecanoyl-CoA + H(+) + malonyl-CoA = 3-oxotetradecanoyl-CoA + CO2 + CoA H(+) + malonyl-CoA + tetradecanoyl-CoA = 3-oxohexadecanoyl-CoA + CO2 + CoA (9Z)-octadecenoyl-CoA + H(+) + malonyl-CoA = (11Z)-3-oxoicosenoyl-CoA + CO2 + CoA (9Z,12Z)-octadecadienoyl-CoA + H(+) + malonyl-CoA = (11Z,14Z)-3-oxoicosa-11,14-dienoyl-CoA + CO2 + CoA (9Z,12Z,15Z)-octadecatrienoyl-CoA + H(+) + malonyl-CoA = (11Z,14Z,17Z)-3-oxoeicosatrienoyl-CoA + CO2 + CoA The reaction is stimulated by the presence of HSD17B12, the enzyme catalyzing the second step of the elongation cycle. Lipid metabolism; fatty acid biosynthesis. Expressed in liver and barely in brain. N-Glycosylated. Belongs to the ELO family. ELOVL6 subfamily.