Secondary metabolite

Secondary metabolites, also known as specialized metabolites or natural products, are organic compounds produced by lifeforms. These include bacteria, archaea, fungi, animals, and plants. Unlike primary metabolites, these compounds are not directly required for an organism's normal growth, development, or reproduction. Instead, they serve as tools to manage ecological interactions. These interactions can provide a selective advantage by increasing an organism's ability to survive or reproduce.

These compounds work by mediating various relationships in nature. They often manage antagonistic interactions, such as competition between species or predation. They also support mutualistic interactions, such as pollination or resource sharing. For example, plants use secondary metabolites to defend against herbivory, which is when animals eat them. They also provide defense against pathogens through antibiotic, antifungal, and antiviral properties. Some, like phenylpropanoids, protect plants from ultraviolet (UV) damage from the sun.

Ecological relationships with these chemicals can be quite complex. In one forest, four different species of arboreal marsupial folivores reacted differently to a secondary metabolite in eucalypts. This suggests that different metabolites can create different ecological niches for herbivores. Some species even evolve to use these chemicals for their own benefit. Monarch butterflies, for instance, eat milkweed plants containing toxic cardiac glycosides. The butterflies are resistant to these toxins and actually sequester them in their bodies. This process helps deter their own predators.

Scientists have studied these substances for many years. The term "secondary metabolite" was first coined by Albrecht Kossel. He was a Nobel Prize laureate in medicine and physiology in 1910. About thirty years later, a Polish botanist named Friedrich Czapek provided more detail. He described secondary metabolites as the end products of nitrogen metabolism. While many of these compounds are restricted to specific lineages or species, horizontal transfer of entire pathways can occur in bacteria and fungi.

Plants organize their secondary metabolites into four major chemical classes. The first class is terpenes, which are composed of isoprene units. Terpenes are hydrocarbons, while terpenoids are oxygenated hydrocarbons. They are classified by the number of isoprene units in their structure. For example, a monoterpene has 10 carbon atoms, while a diterpene has 20.

The second class is phenolics, which are the most abundant secondary metabolites in plants. These contain an aromatic ring structure with one or more hydroxyl groups. Examples range from simple phenolic acids to complex tannins.

The third class is alkaloids, which are nitrogen-containing basic compounds. These are often derived from plant sources and can affect the central nervous system of animals. The fourth class includes glucosinolates, which contain both nitrogen and sulfur.

Humanity has used these natural products for medicine since ancient times. The herb Artemisia annua contains artemisinin, which was used in Chinese traditional medicine over 2,000 years ago. Scientist Tu Youyou was awarded the Nobel Prize in 2015 for her work with this antimalarial compound. Another vital medicine is paclitaxel, the active compound in the drug Taxol. Taxol was first isolated in 1973 from the bark of the Pacific Yew tree. It is used in chemotherapy to treat various cancers, including ovarian and breast cancer.

Alkaloids from the opium poppy also provide significant medical benefits. Friedrich Sertürner discovered morphine, the first active alkaloid extracted from the poppy, in 1804. It is used as a strong analgesic to manage pain and shortness of breath. In 1832, Pierre Jean Robiquet isolated codeine, which is used for mild pain and coughing. While morphine is highly addictive and strictly controlled, codeine is considered safer because it has only 0.1 to 0.15 of the strength of morphine. Finally, the alkaloid atropine, used since the fourth century B.C., is used today as an antidote to organophosphate poisoning.