Natural Product Function
Natural products are often divided into two main categories: primary and secondary metabolites. Primary metabolites are organic molecules that have an intrinsic function essential to the survival of the organism that produces them (i.e. the organism would die without these metabolites). Primary metabolites include, for example, the basic molecules (nucleic acids, amino acids, sugars and fatty acids) needed to make the main macromolecules (DNA, RNA, proteins, carbohydrates and fats) responsible for maintaining life. Secondary metabolites, on the other hand, are organic molecules that generally have an extrinsic function that mainly affects other organisms outside the producer. Secondary metabolites are not essential for survival, but they increase the body's competitiveness in its environment.
Natural products, particularly in the field of organic chemistry, are often defined as primary and secondary metabolites. A more restrictive definition limiting natural products to secondary metabolites is commonly used in the fields of medicinal chemistry and pharmacognosy, the study and use of natural products in medicine.
Primary metabolites
Primary metabolites are components of the basic metabolic pathways that are necessary for life. They are associated with essential cellular functions such as nutrient uptake, energy production and growth and development. They have a wide distribution of species that extend over many phyla and often more than one kingdom. Primary metabolites include the building blocks necessary to make the body's four main macromolecules: carbohydrates, fats, proteins and nucleic acids (DNA and RNA).
These are large polymers of the body that are made up of small repeating monomer units. The monomer units for the construction of nucleic acids, DNA and RNA, are the nucleotide bases, while the monomers for proteins are amino acids, carbohydrates, sugar residues and lipids, fatty acids or acetyl groups.
Primary metabolites involved in energy production include many enzymes that break down food molecules, such as carbohydrates and fats, and capture the energy released in adenosine triphosphate (ATP) molecules. Enzymes are biological catalysts that accelerate the rate of chemical reactions. Typically, they are proteins, which are composed of amino acid building blocks. The basic structure of cells and organisms is also composed of primary metabolites. These include cell membranes (e. g. phospholipids), cell walls (e. g. peptidoglycan, chitin) and cytoskeletons (proteins). The DNA and RNA that store and transmit genetic information are composed of primary nucleic acid metabolites. Primary metabolites also include molecules involved in cellular signalling, communication and transport.
Secondary metabolites
Unlike primary metabolites, secondary metabolites are available and are not absolutely necessary for survival. In addition, secondary metabolites generally have a narrow distribution of species. For example, the deadly belladonna, Atropa belladonna, produces toxic hallucinogenic compounds, such as scopolamine, but other plant species do not have this ability. To date, hundreds of thousands of secondary metabolites have been discovered!
Secondary metabolites have a wide range of functions. These include pheromones that serve as social signalling molecules with other individuals of the same species, other communication molecules that attract and activate symbiotic organisms, agents that solubilize and transport nutrients, called siderophors, and competing weapons (repellents, venoms, toxins, etc.) that are used against their competitors, preys and predators. The function of many other secondary metabolites is unknown. One hypothesis is that they give a competitive advantage to the organization that produces them. According to another view, by analogy with the immune system, these secondary metabolites do not have a specific function, but it is important to have the machines in place to produce these various chemical structures. Some secondary metabolites are therefore produced and selected according to the exposure of the organism during its lifetime.
Secondary metabolites have a variety of structures and include examples such as alkaloids, phenylpropanoids, polyketins and terpenoids. Alkaloids are secondary metabolites that contain nitrogen as a component of their organic structure and can be divided into several subclasses of compounds. Nicotine, the substance that is addictive in tobacco, is provided as an example of an alkaloid. Phenylpropanoids are a diverse family of organic compounds that are synthesized from the amino acids phenylalanine and tyrosine). Cinnamic acid is a phenylpropanoid, one of the volatile flavor molecules found in cinnamon. Polyketides are assembled from the constituent elements of acetate and malonate to form large complex structures. Alflatoxin B1, shown below, is a polyketide structure produced by fungi of the genus Aspergillus. These types of moulds generally grow in stored food crops, such as corn and peanuts, and contaminate them with aflatoxins. Aflatoxins damage DNA molecules and act as a carcinogen or carcinogen. Aflatoxin-contaminated food crops have been associated with liver cancer. Terpenoids are another major class of natural products that are manufactured from monomeric units with 5 carbon atoms called isoprenes. Natural rubber is a good example of a terpenoid-based structure. It is assembled from multiple repeating isoprene units. As we explore organic structures in more detail in the next chapters, we will continue to evaluate examples of these various classes of metabolites and how they influence our lives.
