(5Z,8Z,11Z,14Z,17Z)-eicosapentaenoate + reduced [NADPH--hemoprotein reductase] + O2 = 17,18-epoxy-(5Z,8Z,11Z,14Z)-eicosatetraenoate + oxidized [NADPH--hemoprotein reductase] + H2O + H(+) — reaction