In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate(s). The substrate binds with the enzyme's active site, and an enzyme-substrate complex is formed. The substrate is broken down into a product and is released from the active site. The active site is now free to accept another substrate molecule. For example, in the reaction that occurs upon adding the enzyme rennin in milk, causing milk's coagulation, the substrate is milk and the enzyme is rennin. Another example is the reaction of the enzyme catalase in the decomposition of hydrogen peroxide, in which the enzyme is left the same and the substrate is changed.

2 H₂O₂ ? 2 H₂O + O₂.

A general equation is as follows:

E + S ? ES ? EP ? E + P

where E = enzyme, S = substrate(s), P = product(s) Note that only the middle step is irreversible.

By increasing the substrate concentration, the rate of reaction will increase due to the likelihood that the number of enzyme-substrate complexes will increase; this occurs until the enzyme becomes the limiting factor.

Importantly, the substrates that a given enzyme can turn over in vitro may not necessarily reflect the physiological, endogenous substrates of the enzyme in vivo. For example, while fatty acid amide hydrolase (FAAH) can hydrolyze the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide at comparable rates in vitro, genetic or pharmacological disruption of FAAH elevates anandamide but not 2-AG, suggesting that 2-AG is not an endogenous, in vivo substrate for FAAH.^[1] In another example, the N-acyl taurines (NATs) are observed to increase dramatically in FAAH-disrupted animals, but are actually poor in vitro FAAH substrates.^[2]

References

See also

• Enzyme kinetics
• Enzyme assay
• Enzyme catalysis
• Pseudosubstrate

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