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Structure and Function of Enzymes

What types of molecules are enzymes and how do they work? Enzymes are almost always large protein molecules, folded into a particular three-dimensional configuration (see Chapter 2). Recently, some RNA molecules have been found to have enzymatic functions, but the vast majority of enzymes are proteins. The protein is folded so that a particular portion of the molecule, the active site, is accessible and forms a surface that attracts and aligns the reactant(s)


Activation energies of catalyzed and uncatalyzed reactions


In a favorable configuration. The reactant(s) are referred to as the enzyme’s substrate, and when they associate with the enzyme, the intended reaction is able to proceed efficiently at the relatively low temperature of the cell. The amino acids that comprise the active site are close together in space, but may be far apart in the primary structure of the protein. Thus factors that disturb the overall folding of the protein may decrease or totally destroy the enzyme’s ability to function.
The way the enzyme interacts with the substrate is only now becoming completely clear. Initially, the association was envisioned as a “lock and key” model, in which the shape of the active site matched the shape of the substrate exactly. This would account for their ability to come together easily. A more modern idea is the “induced fit” model, in which the active site and substrate have an affinity for each other, but the binding of the substrate may change the conformation of the active site, inducing a better fit and perhaps straining the bonds that will be broken in the substrate
Enzymes have great specificity, which means that one enzyme can catalyze only one reaction or a set of related reactions. This is of great benefit because it allows the cell to control different reactions independently, by regulating the activity or quantity of the enzyme involved. Furthermore, the enzyme is not permanently altered in any way by participating in the reaction, and so is “recyclable”, being used over and over again. Therefore, enzymes typically do not need to be manufactured in large quantities.

A model of enzyme action (induced fit)

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