Prosolanapyrone-III cycloisomerase (, Sol5, SPS, solanapyrone synthase (bifunctional enzyme: prosolanapyrone II oxidase/prosolanapyrone III cyclosiomerase)) is an enzyme with systematic name prosolanapyrone-III:(-)-solanapyrone A isomerase. This enzyme catalyses the following chemical reaction
The enzyme is involved in the biosynthesis of the phytotoxin solanapyrone in some fungi.