Enzymes and Enzyme Characteristics

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Chemical Basics of Life

Enzymes are globular proteins that promote chemical reactions by lowering the activation energy requirements.


Enzymes are globular proteins that promote chemical reactions by lowering the activation energy requirements. Activation energy is the energy that must be overcome for a chemical reaction to occur. Therefore, they make chemical reactions possible and catalyze the reactions that sustain life. This means that enzymes are catalysts. Enzyme molecules are manufactured by cells to promote specific reac-tions. Enzymes are among the most important of all the body’s proteins. Nearly everything that occurs in the human body relies on a specific enzyme. In the body, enzymes assist in the digestion of food, drug metabolism, protein formation, and many other types of reactions. Enzymes make metabolic reac-tions possible inside cells by controlling tempera-ture conditions that otherwise would be too mild for them to occur.

Enzymes are complex molecules. When they are not used in the reactions they catalyze, they are recy-cled. Enzymatic reactions, which are reversible, can be written as:

Enzymes cannot cause a chemical reaction between molecules that would not react without them.

They increase the speed of enzymatic reactions greatly, between 100,000 to more than 1 billion times the rate of a reaction that is uncatalyzed. Otherwise, biochemical reactions would occur extremely slowly, almost to no effect. Enzymes are vital in making these reactions occur at an adequate pace.

Enzyme Characteristics

Enzymes differ in their makeup. Some are only made of proteins, whereas others have a two -part struc-ture, consisting of a protein portion (the apoenzyme) and a cofactor. Collectively, these two parts are referred to as the holoenzyme. Enzyme cofactors may be either a metal element ion (such as iron or copper) or an organic molecule that assists the reac-tion. Most organic cofactors are derived from B (or other) vitamins, in which case they are referred to as coenzymes.

Enzymes have chemical-specific actions. Some control one chemical reaction, whereas others regu-late a small group of similar reactions by binding to molecules that are only slightly different. Enzymes act on substances referred to as substrates. Certain enzymes, when present, determine which reactions are sped up and which reactions will occur. If there is no enzyme, there is no reaction. Enzymes are often named after their substrates, using the suffix -ase. For example, a lipid is catalyzed by an enzyme called a lipase. Another enzyme, called a catalase, breaks downhydrogen peroxide into water and oxygen. Hydrogen peroxide is a toxic substance that results from certain metabolic reactions.

Every cell holds hundreds of various enzymes, each of which recognizes its specific substrates. Enzyme molecules have three-dimensional shapes (conformations) that allow them to identify their substrates. The coiled and twisted polypeptide chain of each enzyme fits the shape of its substrate. The activesite of an enzyme molecule combines with portionsof its substrate molecules temporarily. This forms an enzyme–substrate complex (FIGURE 2-16).

When enzyme–substrate complexes are formed, some chemical bonds within the substrates are distorted or strained. Requiring less energy as a result, the enzyme is released as it was originally ­configured. Enzyme-catalyzed reactions can be summarized as:

These reactions are often reversible. Sometimes, the same enzyme catalyzes the reaction in both directions. The reactions occur at differing rates, based on the number of molecules of the enzyme and its substrate. Some enzymes process a few substrate ­molecules every second, whereas others can process thousands in the same length of time.

1. Define enzymes and explain their functions.

2. Define the terms cofactor, holoenzyme, and coenzymes.

3. Define substrates.

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