The terpenoids, sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals derived from terpenes. Most are multicyclic structures with oxygen-containing functional groups. About 60% of known natural products are terpenoids.
Terpenoids (or isoprenoids), a subclass of the prenyllipids (terpenes, prenylquinones, and sterols), represent the oldest group of small molecular products synthesized by plants and are probably the most widespread group of natural products.
Terpenoids can be described as modified terpenes, where methyl groups are moved or removed, or oxygen atoms added. Inversely, some authors use the term "terpenes" more broadly, to include the terpenoids.
During the 19th century, chemical works on turpentine led to name "terpene" the hydrocarbons with the general formula C10H16 found in that complex plant product. These terpenes are frequently found in plant essential oils which contain the "Quinta essentia", the plant fragrance.
They are universally present in small amounts in living organisms, where they play numerous vital roles in plant physiology as well as important functions in all cellular membranes. The various functions of terpene natural products in the natural world have been reviewed. On the other hand, they are also accumulated in many cases, and it is shown that the extraordinary variety they then display can be due to ecological factors playing an evolutionary role.
More than 50,000 terpenoids have been isolated from both terrestrial and marine plants, and fungi. In contrast, only some compounds have been identified in prokaryotes. The first study of bacterial terpenes grew out of an investigation of the characteristic odor of freshly plowed soil reported in 1891 by the famous French chemist M. Berthelot. They noted that a volatile substance apparently responsible for the typical earthy odor of soil could be extracted from soil by steam distillation but they could not assign a structure to the odor constituent.
Some 75 years later M.N. Gerber studied the characteristic odor of cultures of Actinomycetales microorganisms, which are widely distributed in soil, and determined the structure of a C12 degraded sesquiterpene alcohol, the geosmin (means earth odor). Later, numerous volatile terpenes have been detected in streptomycetes. The three most commonly detected are geosmin, 2-methylisoborneol and the tricyclic a,b-unsaturated ketone albaflavenone. The two terpene alcohols are the most frequently found secondary metabolites in actinomycetes, filamentous Cyanobacteria, and Myxobacteria, and also in a small number of fungi. Enzymatic studies have suggested that genes encoding terpene synthases are widely distributed in bacteria and that these genes represent a fertile source for discovery of new natural products.