5-hydroxy-(6E,8Z,11Z,14Z,17Z)-eicosapentaenoate + reduced [NADPH--hemoprotein reductase] + O2 => 5,20-dihydroxy-(6E,8Z,11Z,14Z,17Z)-eicosapentaenoate + oxidized [NADPH--hemoprotein reductase] + H2O + H(+)