Auxiliary enzyme in the beta-oxidation of mono- and polyunsaturated fatty acids (PubMed:26474213, PubMed:7818490). Together with the 2,4-dienoyl-CoA reductase (DECR) and the Delta(3,5)- Delta(2,4)-dienoyl-CoA isomerase (ECH1), they allow the reentrance of the enoyl-CoA into the beta-oxidation cycle (PubMed:26474213). Able to isomerize both 3-cis (3Z) and 3-trans (3E) double bonds into the 2- trans (2E) form in a range of fatty enoyl-CoA species, with a preference for (3Z)-enoyl-CoAs over (3E)-enoyl-CoAs (PubMed:26474213, PubMed:7818490). The catalytic efficiency of this enzyme is not affected by the fatty acyl chain length, and based on its established catalytic mechanism, it is predicted to act also on other fatty acids besides those tested experimentally (By similarity). {ECO:0000250|UniProtKB:P23965, ECO:0000269|PubMed:7818490, ECO:0000303|PubMed:26474213}. This is the function of ECI1 (enoyl-CoA delta isomerase 1, Ensembl gene identifier ENSG00000167969).