MCAT Content / Enzymes / Mechanism


Topic: Enzymes

Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.

Enzymes bind with chemical reactants called substrates. There may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. The enzyme’s active site binds to the substrate. It is specific (only fits that certain substrate) due to the active site being composed of a unique combination of amino acid residues (side chains or R groups). The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site, including changes in local pH. A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate.

Induced Fit 

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.

Enzyme-Substrate Complex

When an enzyme binds its substrate, it forms an enzyme-substrate complex. This complex lowers the activation energy of the reaction and promotes its rapid progression by providing certain ions or chemical groups that actually form covalent bonds with molecules as a necessary step of the reaction process.

Enzymes have many different mechanisms that they work. One method is approximation when the enzyme brings the reactants into close proximity. There is covalent catalysis when one of the reactive groups becomes temporarily bonded to the substrate. Some enzymes employ acid-base catalysis using protons (H+ ions) with the reactive group. Many enzymes contain transition metals that can be used to bind substrates.

One of the important properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze. After an enzyme is done catalyzing a reaction, it releases its products (substrates).

Cofactors and Coenzymes

Many enzymes only work if bound to non-protein helper molecules called cofactors and coenzymes. Binding to these molecules promotes optimal conformation and function for their respective enzymes. These molecules bind temporarily through ionic or hydrogen bonds or permanently through stronger covalent bonds.

Cofactors are inorganic ions such as iron (Fe2+) and magnesium (Mg2+). Coenzymes are organic helper molecules with a basic atomic structure made up of carbon and hydrogen. The most common coenzymes are dietary vitamins.

Practice Questions


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MCAT Official Prep (AAMC)

Sample Test C/P Section Passage 9 Question 48

Section Bank B/B Section Passage 12 Question 94

Practice Exam 1 B/B Section Question 31

Practice Exam 1 B/B Section Passage 6 Question 33

Practice Exam 1 B/B Section Passage 6 Question 35


Key Points

• The enzyme‘s active site binds to the substrate.

• The induced-fit model states a substrate binds to an active site, and both change shape slightly, creating an ideal fit for catalysis.

• When an enzyme binds its substrate, it forms an enzyme-substrate complex.

• Enzymes promote chemical reactions by bringing substrates together in an optimal orientation, thus creating an ideal chemical environment for the reaction to occur.

• The enzyme will always return to its original state at the completion of the reaction.

• Many enzymes only work if bound to non-protein helper molecules called cofactors and coenzymes.

Key Terms

substrate: a reactant in a chemical reaction is called a substrate when acted upon by an enzyme

induced fit: proposes that the initial interaction between enzyme and substrate is relatively weak, but that these weak interactions rapidly induce conformational changes in the enzyme that strengthen the binding

active site: the active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed

enzyme-substrate complex: when an enzyme binds its substrate

activation energy: the energy required to get two chemicals to react

coenzyme: an organic molecule that is necessary for an enzyme to function

cofactor: an inorganic molecule that is necessary for an enzyme to function

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