BIO - Enzymes [LESSON]

Enzymes

Enzymes are proteins that catalyze (speed up) biochemical reactions. Most proteins end in “-ase” (like catalase or sucrase). These enzymes are essential for chemical processes like digestion and cellular metabolism. Without enzymes, most physiological processes would proceed so slowly that life could not exist.

Because structure determines function, each enzyme is specific to its substrate(s). The substrate is the reactant that undergoes the chemical reaction catalyzed by the enzyme. The location where substrates bind to or interact with the enzyme is known as the active site. Enzymes and substrates must collide in the correct orientation for a reaction to occur. For many years, scientists thought that enzyme-substrate binding took place in a simple "lock-and-key" fashion. This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However, current research supports a more refined view called induced fit. As the enzyme and substrate come together, their interaction causes a mild shift in the enzyme's structure that confirms an ideal binding arrangement between the enzyme and the substrate. This dynamic binding maximizes the enzyme's ability to catalyze its reaction by lowering the activation energy required for the reaction. This is achieved by stabilizing the transition state.  We will learn much more about the energetics of this in later units.

Enzymes (and all other proteins) have another binding site called the allosteric site. The allosteric site is used to regulate the enzyme (turn it on or off). It is important for cells to be able to control the reactions taking place in order to conserve energy. Sometimes, the enzyme is on by default and an inhibitor can turn it off. Likewise, sometimes an enzyme is off by default and requires an activator to turn it on. The image below summarizes the difference.

In competitive inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to the enzyme's active site to stop it from binding to the substrate. It "competes" with the substrate to bind to the enzyme.

In noncompetitive inhibition, an inhibitor molecule binds to the enzyme at the allosteric site. The substrate can still bind to the enzyme, but the inhibitor changes the shape of the enzyme’s active site so it is no longer in an optimal position to bind substrate and catalyze the reaction. To regulate this process, it is important for this binding to be reversible so that the enzyme can be turned back on. Deadly poisons are typically irreversible inhibitors of key enzymes. For example, cyanide (CN-) is an irreversible inhibitor of cytochrome c oxidase, a crucial enzyme in the cellular respiration pathway. 

Feedback inhibition is when a reaction product is used to regulate its own further production. Cells have evolved to use feedback inhibition to regulate enzyme activity in metabolism, by using the products of the enzymatic reactions to inhibit further enzyme activity. In order to maintain chemical equilibrium and meet the needs of the cell, some metabolic products inhibit the enzymes in the chemical pathway while some reactants activate them.  We’ll dive deeper into this later in the semester.

Many enzymes only work if bound to non-protein helper molecules called cofactors (helper molecules that activate enzymes) and coenzymes (organic molecules that transfer chemical groups during an enzyme reaction). An enzyme binding to these molecules promotes optimal conformation and function for their respective enzymes.

Use the Enzymes Self-Assessment practice questions below to check your knowledge of enzyme structure and function.

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