Sesquiterpene synthase catalyzing the production of (E,E)-alpha-farnesene, the predominant terpene produced during storage of fruits. Produces all six isomers (E,E)-alpha-farnesene, (Z,E)-alpha-farnesene, (E,Z)-alpha-farnesene, (Z,Z)-alpha-farnesene, (E)-beta-farnesene and (Z)-beta-farnesene from a mix of isomeric forms of the farnesyl diphosphate precursor. Able to convert geranyl diphosphate to the monoterpenes (E)-beta-ocimene, linalool and beta-myrcene. Has also a prenyltransferase activity producing alpha-farnesene directly from geranyl diphosphate and isoprenyl diphosphate. (2E,6E)-farnesyl diphosphate = (3E,6E)-alpha-farnesene + diphosphate activity enhanced 5-fold on addition of 30-50 mM potassium. Optimum pH is 7.0-8.5. Monomer. Up-regulated by ethylene. Inhibited by 1-methylcyclopropene (1-MCP). The activity enhancement by potassium is associated only with sesquiterpene synthase activity and not monoterpene or prenyltransferase activities. Oxidation products of (E,E)-alpha-farnesene are the causal agents of superficial scald. Belongs to the terpene synthase family. Tpsb subfamily.