Catalysis of the reaction: a very-long-chain acyl-CoA + H+ + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA. This reaction is the first (condensation) step of the four-step fatty acid elongation cycle in the endoplasmic reticulum that extends fatty acids of C-16 or longer with an additional 2-C unit.
elongation of very long chain fatty acids activity