Component of a microsomal membrane bound long-chain fatty acid elongation system, which produces the 20-26-carbon very long-chain fatty acids (VLCFA) from long-chain fatty acid precursors and is involved ceramide and inositol sphingolipid biosynthesis. Component of elongase III, which synthesizes 20-26-carbon fatty acids from 18-carbon-fatty acyl-CoA primers such as stearoyl-CoA by incorporation of malonyl-CoA (PubMed:9211877, PubMed:12684876). The enzymes active site faces the cytosol, whereas VLCFA length is determined by a lysine near the luminal end of transmembrane helix 6 (PubMed:17719544). Plays an important role in lipotoxic cell death induced by oleic acid through maintaining a balanced fatty acid composition in thr plasma membrane (PubMed:29458843). Affects plasma membrane H(+)-ATPase activity. May act on a glucose-signaling pathway that controls the expression of several genes that are transcriptionally regulated by glucose such as PMA1, HXT3 and SNF3 (PubMed:8027068). a very-long-chain acyl-CoA + H(+) + malonyl-CoA = a very-long-chain 3-oxoacyl-CoA + CO2 + CoA H(+) + malonyl-CoA + octadecanoyl-CoA = 3-oxoeicosanoyl-CoA + CO2 + CoA H(+) + hexadecanoyl-CoA + malonyl-CoA = 3-oxooctadecanoyl-CoA + CO2 + CoA eicosanoyl-CoA + H(+) + malonyl-CoA = 3-oxodocosanoyl-CoA + CO2 + CoA docosanoyl-CoA + H(+) + malonyl-CoA = 3-oxotetracosanoyl-CoA + CO2 + CoA The HxxHH motif is essential for ELOp function in vivo and 3-keto acyl-CoA synthase activity in vitro. The C-terminal di-lysine motifs may confer endoplasmic reticulum localization. Belongs to the ELO family.