Hemiterpenoids represent the simplest class of terpenoids, consisting of single C5 isoprene units, and serve primarily as volatile signaling molecules in plant stress responses and microbial chemical ecology. They function as immediate precursors and overflow products from isoprenoid metabolism.
Chemical Properties
Hemiterpenoids derive from one isoprene unit (C5H8), including isoprene (2-methyl-1,3-butadiene, bp 34°C), prenol (3-methyl-2-buten-1-ol), isovaleric acid ((CH₃)₂CHCH₂COOH), and hemiterpene glycosides (HTGs). Isoprene emits rapidly from leaves under heat stress (light-dependent, 1-20 μg g⁻¹ h⁻¹); prenol exhibits allylic alcohol reactivity for glycosylation. DMAPP (dimethylallyl pyrophosphate) and HMBPP (4-hydroxy-3-methyl-but-2-enyl pyrophosphate) represent phosphorylated precursors from the MEP pathway.
Biosynthetic Origins
Plants shunt MEP pathway intermediates: dephosphorylation of DMAPP → isoprene (isoprene synthase, EC 4.2.3.27); HMBPP hydrolysis → diol → glycosides under nitrate limitation (carbon flux overflow). Cytosolic MVA pathway contributes minor prenol/isovalerate via isopentenyl diphosphate. Arabidopsis accumulates 0.1-1 μmol g⁻¹ HTGs in rosettes, upregulated 3-5x by N-starvation.
