acetoacetyl-coa acetyl-coa biochemistry cas registry number chemical formula cholesterol dimethylallyl pyrophosphate dimethylallyltranstransferase ergosterol farnesyl pyrophosphate geraniol geranylgeranyl pyrophosphate hmg-coa isopentenyl pyrophosphate lanosterol medical subject headings metabolic intermediate mevalonic acid molar mass phosphomevalonic acid pubchem simplified molecular input line entry specification squalene terpene terpenoid