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In this lesson, you will discover what enzymes are, explore how they work, and learn why they’re needed for your cells’ day-to-day functions. The lesson concludes with a quiz to test your knowledge.

What Are Enzymes?

Enzymes are proteins that act as catalysts within living cells. Catalysts increase the rate at which chemical reactions occur without being consumed or permanently altered themselves. A chemical reaction is a process that converts one or more substances (known as reagents, reactants, or substrates) to another type of substance (the product). As a catalyst, an enzyme can facilitate the same chemical reaction over and over again.

Structure and Function

Like all proteins, enzymes are composed of one or more long chains of interconnected amino acids. Each enzyme possesses a unique sequence of amino acids that causes it to fold into a characteristic shape. An enzyme’s amino acid sequence is determined by a specific gene in the cell’s nucleus. This ensures that each copy of the enzyme is the same as all others.On the surface of each enzyme is a special cleft called the active site, which provides a place where reagents can ‘meet’ and interact. Much like a lock and its key, an enzyme’s active site will only accommodate certain reagents, and only one type of chemical reaction can be catalyzed by a given enzyme.For example, during the manufacture of hemoglobin (the oxygen-carrying pigment in your red blood cells), a single atom of iron must be inserted into the center of the molecule to make it functional.

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An enzyme called ferrochelatase brings the reagents (iron and the empty molecule) together, catalyzes their union, and releases an iron-containing molecule. This is the only reaction catalyzed by ferrochelatase. Keep in mind that enzymes can combine reagents (as in the synthesis of hemoglobin), they can split a single reagent into multiple products, or they can simply transform a single reagent into a single product that looks different from the original reagent.

Enzyme Active Site

When reagents enter an enzyme’s active site, the enzyme undergoes a temporary change in shape that encourages interaction between the reagents. Upon completion of the chemical reaction, a specific product is released from the active site, the enzyme resumes its original conformation, and the reaction can begin again with new reagents.Many enzymes are incorporated into metabolic pathways.

A metabolic pathway is a series of chemical reactions that transform one or more reagents into an end-product that’s needed by the cell. The enzymes in a metabolic pathway–much like people passing a pail of water along a bucket brigade–move a reagent along until the end-product is produced. A metabolic pathway can be quite short, or it can have many steps and multiple enzymes. The metabolic pathway that converts tryptophan (an amino acid found in dietary protein) to serotonin (a chemical that’s necessary for normal brain function) is only two steps long.In order to function, many enzymes require the help of cofactors or coenzymes. Cofactors are often metal ions, such as zinc, copper, iron, or magnesium.

Magnesium, one of the most common cofactors, activates hundreds of enzymes, including those that manufacture DNA and many that help metabolize carbohydrates.Many coenzymes are derived from vitamins. In fact, one of the main reasons you need vitamins in your diet is to supply the raw material for essential coenzymes. For example, vitamin C is needed by the enzyme that produces collagen and builds healthy skin, a coenzyme derived from vitamin B12 is necessary for synthesizing the insulation around your nerve cells, and a vitamin B6-based coenzyme is vital for producing serotonin.Coenzymes and cofactors bind to the active sites of enzymes, and they participate in catalysis, but they are not generally considered reagents, nor do they become part of the product(s) of the reaction. In many cases, cofactors and coenzymes function as intermediate carriers of electrons, specific atoms, or functional groups that are transferred during the overall reaction.

Why Do We Need Enzymes?

One of the immutable laws of nature is that energy is required to initiate a chemical reaction. After all, if you want to change something, you have to put some work into it. For every chemical reaction, the energy of activation–a specific amount of energy–must be applied to the reagents before the reaction will proceed. The energy of activation is like a hill between the reagents and the product, and the reagents must be pushed over this hill before the reaction can continue.

Energy of Activation

That’s where enzymes come in: they lower the energy ‘hill’ so chemical reactions can get started more easily and proceed in conditions that are ‘cell friendly.

‘ Enzymes along a metabolic pathway can also divide one high-energy chemical reaction into many steps, each requiring only a fraction of the energy needed for the whole process. Finally, enzymes can accelerate the rate of a reaction millions of times over, which is critical to cells that often need large quantities of a particular product within just a few seconds’ time.

Regulation of Enzymatic Activity

Although your cells couldn’t function without enzymes, their catalytic activities can’t simply be allowed to proceed unchecked. This would lead to unnecessary consumption of raw materials and would siphon energy from other critical processes.

Conversely, your cells must be able to quickly produce more copies of a given enzyme when metabolic needs increase. To address these situations, your cells turn enzymes off and on by regulating their activity (feedback inhibition) or by increasing or decreasing their availability (genetic control).Feedback inhibition slows an enzyme’s activity when its product (or an end-product of a metabolic pathway) begins accumulating. Feedback inhibition can be accomplished either competitively or non-competitively.

  • In competitive inhibition, an enzyme’s activity is slowed or stopped when the end-product of its metabolic pathway attaches to the enzyme’s active site and prevents it from interacting with its usual reagents. In other words, the inhibitor directly competes for the active site by ‘elbowing’ the reagents out of the way.
  • In noncompetitive inhibition, the inhibitor binds to a second site on the enzyme called the allosteric site.

    This changes the shape of the enzyme’s active site and prevents it from interacting with its reagents.

Genetic control is a complex process that is guided from within a cell’s nucleus. When more of a specific enzyme’s products are needed, regulatory proteins bind to the cell’s DNA and ‘switch on’ the genes that produce that enzyme. Conversely, when a particular enzyme or metabolic pathway begins churning out too much product, different regulatory proteins stop enzyme production by attaching to the cell’s DNA and turning those same genes off.


Scientists are constantly discovering new enzymes. Newly discovered enzymes are named according to the reagent they act upon, followed by the action they perform. The suffix -ase is added to the name to identify it as an enzyme.

Any time you see -ase at the end of a long chemical name, you can bet you’re dealing with an enzyme. Thus, sucrase is a digestive enzyme that breaks down sucrose (table sugar), and alcohol dehydrogenase is an enzyme that detoxifies alcohol by removing hydrogen atoms from it.

Lesson Summary

Put simply, enzymes are proteins that act as catalysts within living cells. Enzymes provide the energy that we need to function. Every living cell contains hundreds or even thousands of enzymes, all interacting in a coordinated dance that keeps the cell functioning efficiently and, in the case of multicellular organisms like humans, working together for the good of the whole.

Notes on Enzymes

  • Enzymes are proteins that serve as catalysts and reduce the need for specific chemical reactions within a cell
  • Enzymes are often part of metabolic pathways
  • Coenzymes or cofactors are sometimes needed for an enzyme to function
  • Enzymes are controlled by competitive or non-competitive feedback inhibition

Learning Outcomes

If you see this lesson through to the end, you should then be able to fulfill the goals that follow:

  • Explain the role of enzymes in cells
  • Recall what inputs enzymes need to assist in reactions
  • Describe how an enzyme’s actions are checked or encouraged

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