A biosynthesis pathway, or biosynthetic pathway, is a description of the steps of the chemical reactions that occur when a living organism creates a new complex molecule out of simpler, smaller precursors. The word "biosynthesis" comes from two root words: "bio," which indicates that the reaction is taking place within a living organism as opposed to within a laboratory; and "synthesis," which indicates that simple starting materials are being combined to form larger products. A biosynthesis pathway is a summary of these chemical reactions, broken down by each step. To describe a pathway completely, extra relevant information is often included, such as which enzymes, coenzymes and cofactors are used in each reaction.
Not all the molecules used by a living organism need to be directly synthesized by the organism itself. Often, these necessary molecules are instead obtained from the surroundings. Humans, for instance, are unable to synthesize essential amino acids such as lysine; these nutrients instead come from protein-rich food such as beans and tree nuts. Cells generally synthesize only those molecules that are either scarce or not readily obtained from their environment.
A biosynthesis pathway often begins with a readily available precursor molecule that is similar to the product. The cell then combines this precursor with other small molecules, chemically modifying the product along the way. At each step, the substrate will progressively resemble the final product. A multi-step biosynthesis pathway can have dozens of steps along the way, undergoing constant modification by enzymes until the final compound is formed.
Studying biosynthesis can yield many practical insights into cures for human illnesses. Understanding the chemistry of the human body clearly helps when an illness results from malfunctioning biosynthesis. Sometimes, however, studying the biosynthetic pathways of other organisms can turn up valuable clues for developing new medicines as well.
Many researchers have turned their attention to studying the biosynthesis of plants, especially concerning natural products. Extracts from certain plants can have powerful pharmacological effects that can be used in creating powerful new drugs. For example, digitalis, an active compound extracted from the common foxglove, has been used to treat heart disease. By gaining an understanding of the biosynthesis pathway of a natural product, chemists can gain insight into how the drug is synthesized and potentially mimic its synthesis in the laboratory. Ultimately, biologists would like to be able to clone these genes to produce transgenic organisms, which would be engineered to produce natural products at greater concentration and purity at a fraction of the price.
Biosynthetic pathways have been elucidated for many common molecules such as fatty acids, amino acids and nucleotides. Many pathways, however, have yet to be discovered. Perhaps the medicines of the future will find their origins in the biosynthetic pathways that are being researched today.