Biology » Cell Metabolism » Enzymes

Summary and Main Ideas

Summary of lessons so far


Enzymes are chemical catalysts that accelerate chemical reactions at physiological temperatures by lowering their activation energy. Basically, enzymes are usually proteins consisting of one or more polypeptide chains. In general, enzymes have an active site that provides a unique chemical environment, made up of certain amino acid R groups (residues). This unique environment is perfectly suited to convert particular chemical reactants for that enzyme, called substrates, into unstable intermediates called transition states.

Enzymes and substrates are thought to bind with an induced fit. This means that enzymes undergo slight conformational adjustments upon substrate contact, leading to full, optimal binding. In fact, enzymes bind to substrates and catalyze reactions in four different ways: bringing substrates together in an optimal orientation, compromising the bond structures of substrates so that bonds can be more easily broken, providing optimal environmental conditions for a reaction to occur, or participating directly in their chemical reaction by forming transient covalent bonds with the substrates.

Enzyme Regulation

Enzyme action must be regulated so that in a given cell at a given time, the desired reactions are being catalyzed and the undesired reactions are not. Enzymes are regulated by cellular conditions, such as temperature and pH. They are also regulated through their location within a cell, sometimes being compartmentalized so that they can only catalyze reactions under certain circumstances.

Inhibition and activation of enzymes via other molecules are other important ways that enzymes are regulated. Inhibitors can act competitively, noncompetitively, or allosterically; noncompetitive inhibitors are usually allosteric. Activators can also enhance the function of enzymes allosterically. The most common method by which cells regulate the enzymes in metabolic pathways is through feedback inhibition. During feedback inhibition, the products of a metabolic pathway serve as inhibitors (usually allosteric) of one or more of the enzymes (usually the first committed enzyme of the pathway) involved in the pathway that produces them.

Glossary of Words

Active site

specific region of the enzyme to which the substrate binds

Allosteric inhibition

inhibition by a binding event at a site different from the active site, which induces a conformational change and reduces the affinity of the enzyme for its substrate


small organic molecule, such as a vitamin or its derivative, which is required to enhance the activity of an enzyme


inorganic ion, such as iron and magnesium ions, required for optimal regulation of enzyme activity

Competitive inhibition

type of inhibition in which the inhibitor competes with the substrate molecule by binding to the active site of the enzyme


process that changes the natural properties of a substance

Feedback inhibition

effect of a product of a reaction sequence to decrease its further production by inhibiting the activity of the first enzyme in the pathway that produces it

Induced fit

dynamic fit between the enzyme and its substrate, in which both components modify their structures to allow for ideal binding


molecule on which the enzyme acts

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