Enzyme substrate (biology)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate(s). In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or more products, which are then released from the active site. The active site is now free to accept another substrate molecule. In enzymes with more than one substrate, these may bind in a particular order to the active site, before reacting together to produce products.
For example, curd formation (rennet coagulation) is a reaction that occurs upon adding the enzyme rennin to milk. In this reaction the substrate is a milk protein (e.g. casein) and the enzyme is rennin. The products are two polypeptides which have been formed by the cleavage of the larger peptide substrate. Another example is the chemical decomposition of hydrogen peroxide carried out by the enzyme catalase. As enzymes are catalysts, they are not changed by the reactions they carry out. The substrate(s) however is/are converted to product(s). Here, hydrogen peroxide is converted to water and oxygen gas.
- 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
- ^ Cravatt BF, Demarest K, Patricelli MP, Bracey MH, Giang DK, Martin BR, Lichtman AH. (2001) Supersensitivity to anandamide and enhanced endogenous cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proc. Natl. Acad. Sci. USA. 98(16):9371-9376.
- ^ Saghatelian A, Trauger SA, Want EJ, Hawkins EG, Siuzdak G, and Cravatt BF. (2004) Assignment of endogenous substrates to enzymes by global metabolite profiling. Biochemistry. 43(45):14332-14339.