Learning objectives You should understand that: r Enzymes have an area on their molecule, known as the active site, to which specific substrates bind. r During enzyme-catalysed reactions, molecules move about, and substrate molecules collide with the active sites on enzyme molecules. r The rate of enzyme activity is influenced by temperature, pH and substrate concentration. r Enzyme molecules can be denatured. r Immobilised enzymes are used in many industrial processes.
Enzyme a globular protein that functions as a biological catalyst of chemical reactions Denaturation irreversible changes to the structure of an enzyme or other protein so that it can no longer function Active site region on the surface of an enzyme molecule where a substrate molecule binds and which catalyses a reaction involving the substrate
2.5 Enzymes Enzymes and active sites An enzyme is a biological catalyst. Like all catalysts, enzymes speed up biochemical reactions, such as digestion and respiration, but they remain unchanged at the end of the process. All enzymes are proteins with long polypeptide chains that are folded into 3D shapes. The arrangement of these shapes is very precise and gives each enzyme the ability to catalyse one specific reaction. If the 3D shape of an enzyme is destroyed or damaged, it can no longer carry out its job and is said to be denatured. Extremes of temperature, heavy metals and, in some cases, pH changes can cause permanent changes in an enzyme. The three-dimensional shape of an enzyme is crucial to the way it works. In the structure of every enzyme is a specially shaped region known as an active site (Figure 2.15). It is here that the substrate and enzyme bind together. The substrates are the chemicals involved in the reaction catalysed by the enzyme. The shapes of the enzyme and substrate are complementary, so that they fit together perfectly like a key fits into a lock. The ‘lock-andkey hypothesis’ is a way of explaining how each enzyme can be so specific. To unlock a door requires just one special key. To catalyse a reaction requires one special enzyme. Just as only one key fits perfectly into the lock, only one substrate fits perfectly into the active site of an enzyme. substrate
enzyme–substrate complex
products
active site
enzyme a An enzyme has a cleft in its surface called the active site. The substrate molecule has a complementary shape.
enzyme
substrate
b Random movement of enzyme and substrate brings the substrate into the active site. An enzyme–substrate complex is temporarily formed. The R groups of the amino acids in the active site interact with the substrate.
c The interaction of the substrate with the active site breaks the substrate apart. The two product molecules leave the active site, leaving the enzyme molecule unchanged and ready to bind with another substrate molecule. enzyme
enzyme–substrate complex
products
active site +
Figure 2.15 How an enzyme catalyses the breakdown of a substrate molecule into two product molecules.
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