Molecule of the Month: Isoprene Synthase

You might be surprised to hear that plants emit a billion metric tons of organic gases into the atmosphere every year. About half of this is isoprene, a volatile organic compound produced by tall trees such as oak, aspen, poplar, and eucalyptus, found in forests around the globe. The amount of isoprene emitted by plants is so significant that it filters sunlight, forming the beautiful blue haze that intrigued Leonardo da Vinci and is seen today in scenic locations such as the Blue Ridge Mountains of the United States and the Blue Mountains of Australia.

In plants, the enzyme isoprene synthase (shown here from PDB entry 3n0g) makes isoprene. Isoprene synthase breaks down dimethylallyl diphosphate (DMAPP), a building block used to construct many “isoprenoid” molecules such as fatty acids or steroid hormones. Isoprene synthase uses metal ions to remove the phosphates from DMAPP, thereby releasing isoprene. This isoprene plays important roles in the chemistry of our atmosphere. For instance, researchers have found that it reacts with hydroxyl radicals and nitrogen oxides in the air, ultimately increasing ozone concentrations. Our understanding of isoprene synthase is also allowing researchers to engineer new forms of the enzyme, to make isoprene for industrial production of synthetic polymers and rubber.

Leaves exposed to sunlight experience rapid changes in their surface temperature, so isoprene is thought to protect leaves from reactive molecules formed by heat. Several lines of evidence support this beneficial function: synthesis of isoprene synthase in plants is linked to the seasons and leaves at the top of trees produce more isoprene since they are exposed to more sunlight. Although at lower levels, plants also emit isoprene in the absence of light, so isoprene is also thought to act as a defense mechanism to fight herbivores or reduce stress caused by drought.

The isoprene precursor DMAPP and the similar molecule isopentenyl diphosphate (IPP) are used to build many important molecules in all kinds of living things, ranging from steroid hormones to electron carriers such as ubiquinone. Depending on which of these two molecules is needed, DMAPP and IPP are interconverted by isomerase enzymes. Two forms of these enzymes are found in nature: type 1 isomerases use a metal ion to catalyze the interconversion (PDB entry 1i9a) and type 2 enzymes use a nucleotide cofactor to do the same thing (PDB entry 1p0n).