Enzymes and Metabolism

Key players in biochemical reactions.

Enzymes and Metabolism

Overview of Enzymes

Enzymes are proteins in the body that function as a catalyst for biological reactions. A catalyst lowers the activation energy of a reaction and increases the reaction rate. Enzymes act on substrates, which are the reactants in the biological reaction.

The location where the enzyme binds to the substrate is called the active site. Enzymes only work with a specific group of substrates – a concept known as enzyme specificity.

In other words, enzymes make it easier for reactions to occur by changing other molecules, which are known as substrates. The enzymes are otherwise not involved in the reaction. The enzymes are not changed, nor are they consumed.

Reaction Rates

The reaction rates of enzymes are affected by:

  1. Concentration. As the concentration of the substrate increase, so does the reaction rate. On the other hand, enzyme concentration has little effect on the reaction rate.
  2. Temperature. As the temperature increases, so does the reaction rate. However, keep in mind that most enzymes function best at body temperature. Furthermore, enzymes are composed of proteins. When proteins are exposed to higher temperatures, they tend to degrade and become ineffective.
  3. pH. Enzymes typically can only be active within a certain pH range. In general, an increase in pH within this range will cause reaction rates to increase. Keep in mind that although many enzymes operate best at a neutral pH of around 7.0, some enzymes, such as pepsin, require acidic pH levels. Other enzymes require pH levels that are more alkaline.
  4. Cofactors; Coenzymes. Cofactors are molecules, such as vitamins and minerals, that assist enzymes and thereby increase reaction rates. Coenzymes are enzymes that help other enzymes by acting as group-transfer reagents. Coenzymes typically transfer protons, electrons, or other molecular substances.

The following image provides a graphical representation depicting how different environmental factors effect the reaction rate governed by enzyme activity.

Enzyme Reaction Rates

Enzyme Inhibition

Enzyme activity can stopped by a number of inhibitors – a concept known as enzyme inhibition. Inhibitors are classified into three categories:

  1. Irrevesible Inhibitors. Inhibitors that prevent enzymes from functioning through the use of covalent bonds with an enzyme.
  2. Competitive Inhibitors. Inhibitors that prevent enzymes from functioning through the use of noncovalent bonds to the active site of the enzyme.
  3. Noncompetitive Inhibitors. Inhibitors that prevent enzymes from functioning through the use of noncovalent bonds at a location on the enzyme other than the active site. These inhibitors change the conformation of the enzyme to render the enzyme non-functional.

Feedback Inhibition

You should expect to see questions about feedback inhibition, or negative feedback,  on the MCAT. The concept of feedback inhibition is the control of a series of reactions by one of the products. The ultimate product of a series of biological reactions is either an activator or inhibitor of an earlier reaction.

In the figure below, E1 and E2 are enzymes that catalyze the two reactions. “F” is the product of the reactions and can be an allosteric activator or inhibitor of the first step in the reaction. This same concept could be applied to the second step in the reaction instead of the first step.

The Role of Enzymes in Feedback Inhibition

A common analogy for feedback inhibition involves describing a home air conditioning system. When the temperature in your home goes above an acceptable range, the thermostat causes the A/C unit to turn on. Likewise, when the temperature gets below an acceptable range, the thermostat causes the A/C unit to turn off. In the end, it’s the product (the cold air) that causes the reaction (the running A/C unit) to discontinue.

Enzyme Classification

You will definitely encounter questions regarding enzymes on the MCAT. You can recognize an enzyme when you see the suffix ‘-ase’. There are six categories of enzymes:

  1. hydrolases
  2. isomerases
  3. ligases
  4. lyases
  5. transferases
  6. oxidoreductases

Enzyme Regulation

Enzymes are regulated by:

  1. Proteolytic cleavage (irreversible covalent modification)
  2. Reversible covalent modification
  3. Control proteins
  4. Allosteric interactions



Enzymes are regulated in an effort to reduce the amount of energy that is used or lost in the reaction. There are two primary mechanisms by which enzymes are regulated: covalent modification and allosteric regulation. Many enzymes are regulated by both methods of regulation.

Covalent modification includes small molecule modifications and large molecule modifications. Small molecule modifications are those such as phosphorylation, acetylation and glycosylation. Large molecule modifications include those such as ADP-reibosulation, glutathionylation, and ubiqiutination.

Allosteric regulation, or non-covalent regulation, is a mechanism where allosterically regulated enzymes have at least one attachment site in addition to the enzyme’s active site. This additional site in where an allosteric activator or an allosteric inhibitor can bind. The binding of the allosteric molecule alters the shape, and therefore the function, of the enzyme.

Next: DNA and Protein Synthesis.



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