biochemistry, a substrate is a moleculeupon which an enzymeacts. Enzymes catalyze chemical reactionsinvolving the substrate(s). The substrate binds with the enzyme's active site, and an enzyme-substrate complexis 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 renninin milk, causing milk's coagulation, the substrate is milk and the enzyme is rennin. Another example is the reaction of the enzyme catalasein the decompositionof 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
substrateconcentration, the rate of reaction will increase due to the likelihood that the number of enzyme-substrate complexes will increase; this occurs until the enzymebecomes 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 anandamideat 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. [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.] 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. [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.]
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