Metabolites – Primary and Secondary
A metabolite is any substance that is produced during metabolism* or that takes part in metabolism in the plant . Metabolites are small molecules and are generally classified into two broad classes – primary or secondary. While there exists considerable overlap in the two in terms of chemical structure, one can draw on differences based on some of the functions they perform, as Dr. Philipp Zerbe, associate professor at the University of California, Davis, highlights.
Primary/ general metabolites are essential for the survival, development, and reproduction; they are absolutely critical for the survival of the plant. Examples of primary metabolites include alcohols such as ethanol, lactic acid, and certain amino acids and sugars. While certain secondary metabolites also perform similar functions, they are essentially chemicals that plants utilise to perform specific functions . Secondary metabolites are not essential as primary metabolites as these are not directly involved in growth, development and reproduction of organisms. They have no apparent function in a plant’s primary metabolism, but often have an ecological role – as pollinator attractants, to represent chemical adaptations to environmental stresses, or serve as chemical defences against microorganisms, insects and higher predators. They are also frequently accumulated by plants in smaller quantities than primary metabolites .
* Metabolism is the total amount of biochemical reactions involved in maintaining the living condition of the cells of an organism. In other words, it refers to the chemical processes that occur within a living organism in order to maintain life.
Among secondary metabolites, the largest and most diverse group is that of the terpenes, sometimes also known as terpenoids. While the two terms are often used interchangeably, terpenoids are a modified class of terpenes, with different functional groups such as oxygen . Terpenes accumulate mostly in plants and fungi; common plant sources of terpenes are tea, thyme, cannabis, Spanish sage, and citrus fruits .
What makes the occurrence of terpenes fascinating is that all of them (over 100,000 chemical structures) typically originate from only two five-carbon hydrocarbon precursors – plants then take these hydrocarbons, act on them using enzymes, and modify their properties and bioactivities. This results in a fascinating world of plant diversity and evolution. The enzyme modification process is preceded by the creation of cyclic and acyclic structures of the five-carbon precursors. These structures are then modified by various enzymes that attach functional groups to them – hydroxyl (OH-), acetyl, methyl (CH3-), and so on. The addition of these functional groups is, essentially, what leads to diversity in function of the terpenes .
Dr. Philipp Zerbe encourages students to view the process using the “lego block analogy” – one starts out with the same block, but can add different blocks further to change the structure completely. This potential chemical diversity presents lucrative opportunities for crop modification and crop yield optimization, as well as for the formulation of medicines.
Dr. Philipp Zerbe & The Strawberry Project
One of the many projects that Dr. Zerbe and his team undertake is The Strawberry Project. The common variety of strawberries found in markets, while larger in size and tastier, lacks the sweet, pungent aroma that is characteristic of wild strawberries. This aroma, which comes largely from terpenes, is lost in the domestication process. In a collaboration with the strawberry farmers of California, Dr. Zerbe and his team work towards restoring this characteristic scent while also retaining the benefits of domestication (such as larger size and better taste). The team looks at identifying the specific terpenes that contribute to this aroma, and also studies the genes that drive their production. Using this information, it can potentially suggest specific focal genes to farmers for “selective breeding” – choosing parent plants with those particular genes to produce a genetically modified offspring.
If you’d like to hear more about the wonderful world of terpenes and one of its researchers, visit us on Spotify to listen to our ChemTalk podcast with Dr. Philipp Zerbe, associate professor and chemist at the University of California, Davis, to learn more about the Strawberry Project, what his advice is for college students, and how he promotes inclusivity and diversity through student exchange programs.
Find the ChemTalk podcast here: https://open.spotify.com/episode/4tEBTXjOCnXImQvy8jLIIT
 “What are Metabolites? – Primary and Secondary”. Geeks for Geeks. 5 July 2022. https://www.geeksforgeeks.org/what-are-metabolites-primary-and-secondary/
 Primary and Secondary Metabolites. (2021, January 4). https://bio.libretexts.org/@go/page/12413
 Seigler, D.S. (1998). Introduction to Terpenes. In: Plant Secondary Metabolism. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-4913-0_18
 Zerbe, Philipp. Personal Interview. Conducted by Roxanne Salkeld. 23 September 2022.