What is a Catalyst?

A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction; hence a catalyst can be recovered chemically unchanged at the end of the reaction it has been used to speed up, or catalyze.

What are enzymes?

Enzymes are biological catalysts.

  • Enzymes are special compounds produced by living cells in a body of an organism that allow chemical reactions to take place at a faster rate.
  • They act as catalysts.
  • They are made of protein.
  • They alter the rate of chemical reactions without themselves being chemically changed.
  • Each enzyme has a unique shape.

Why enzymes are important?

They are vital for life and serve a wide range of important functions in the body, such as helping in digestion and metabolism.

Enzymes can either:

    • break down complex substances into simpler substances  ( catabolism ); OR
    • build up complex substances from simpler substances  ( anabolism ).


Digestion is a catabolic activity.

Photosynthesis, which builds sugars out of smaller molecules, is a “building up,” or anabolic, pathway.

Cellular respiration breaks sugar down into smaller molecules and is a “breaking down,” or catabolic, pathway.

What Do Enzymes Do?

Enzymes catalyze chemical reactions. This generally means that they speed up the rate of reactions by lowering the reaction’s activation energy—the energy required to get the reaction done.

As the image above shows that presence of an enzyme has actually lower the activation energy but it is important to note that all of the reactions would still happen even if the enzymes were not there. But they would just happen much more slowly and require a lot of energy.

So, Enzymes are vital to our health and change the rate at which chemical reactions happen, but without any external energy source added or by being changed themselves.

Classification of enzymes

There are two forms of enzymes,

  1.  Intracellular enzymes.
  2.  Extracellular enzymes.

Intracellular enzymes are those which work within the protoplasm of the cell in which they are made.eg respiratory enzymes (decarboxylase)

Extracellular enzymes are the enzymes which are secreted outside the cells in which they work. The majority of the digestive enzymes are extracellular.


Enzyme Nomenclature:

The Conventional Way of   naming enzymes is,

By adding the suffix “ase” at the end of the food acted upon.

For Example:

Maltose is acted upon by maltase.

Sucrose is acted upon by sucrase.

A lipid is acted upon by lipase.

Cellulose is acted upon by cellulase.

Examples of different Enzymes:

  • Amylase is found in seeds. When the seed begins to germinate, the amylase is activated and catalyses the breakdown of insoluble starch to soluble maltose in the seed. The maltose is used by the growing embryo as an energy source and to make cellulose for new cell walls.
  • Biological washing powders contain enzymes, often obtained from microorganisms such as bacteria or fungi. The enzymes break down proteins or fats on the fabric, forming water soluble substances that can be washed away.
  • Pectinase is used to break down cell walls in fruits, making it easier to extract juice from them.
  • The antibiotic penicillin is made by cultivating the fungus Penicillium in a fermenter. The fermenter is kept at the correct pH and temperature for the enzymes of the fungus to work well.

Properties of Enzymes:

  • Enzymes are specific in the reactions they catalyze. A given enzyme catalyses and controls a particular reaction.
  • They are produced in the cells of living organisms.
  • They are proteins in nature.
  • Only a small amount of enzyme is needed to produce a large amount of chemical change.
  • Enzymes are not used up in the reactions they catalyse. They remain the same after the reaction.
  • Enzymes can cause reactions to go in both directions. The direction to which the direction proceeds normally depends on concentration of reactants and products. For an example Starch can be broken to maltose and then maltose can form starch as well.
  •  Enzymes work but at specific (pH) degree of activity or alkalinity. Those working best   in acidic conditions may not work in alkaline condition.
  • They are inactivated by chemical reactions eg cyanide (poison) such chemicals are called inhibitors.
  • They are denatured by heat.

How does an enzyme work?

Key Terms

  • Substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.
  • 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, it forms an enzyme-substrate complex.

Process: (lock and key hypothesis)

Lock is an enzyme molecule that contain an active site.

Key is a substrate molecule that has to fit in active site.

Enzymes contain specific active site of particular or complementary in shape to the substrate molecule, where special substrate molecule exactly fits and enzyme-substrate complex is made. Enzyme temporarily holds substrate molecules in the active site, until some substrate molecules split or breakdown to form products as in catabolic reactions.

In anabolic reaction, substrates join and develop new bonds to make products. When reaction is completed, the product molecules are released and enzyme with its active site remains unchanged.

Effects of temperature and pH on enzyme activity:

Effect of Temperature:

The activity of the enzyme and the rate of reaction is highest at the optimum temperature.

At low temperatures, enzymes are inactive.

As temperature rises, the rate of reaction increases because heating leads to an increase in kinetic energy of molecules. This leads to more successful collisions, and more formation of enzyme-substrate complexes.

Beyond the optimum temperature, the enzyme activity decreases as the enzyme is denatured.

Enzymes are protein in nature that are denatured at high temperatures and denaturation is irreversible.

Why does denaturation of enzymes occur?

The 3D shape of the active site is determined by weak hydrogen bonds that hold the chains of amino acids in place.

Due to  increase in temperature, more kinetic energy is imparted to molecules so vibrations in the atoms of the enzyme becomes faster, causing the hydrogen bonds that are chemical bonds in their structure are broken and shape of active site changed and overall shape of enzyme is destroyed. Now the structure of protein is damaged and cannot be restored.

The active site loses its shape and is no longer complementary to the substrate.

The enzyme can no longer act as a catalyst.

Effect of pH:

Enzymes are affected by the acidity or alkalinity of solutions. Some work best in acidic environment e.g. pepsin.

Some work best in alkaline environment e.g., intestinal enzymes e.g. amylase

Extreme changes in pH can denature them.

Enzymes are made of proteins.

The 3D shape of the active site is determined by weak hydrogen bonds that hold the chains of amino acids in place.

Interference of the H+ or OH ions with the hydrogen bonds causes them to be damaged.

The active site loses its specific shape and is no longer complementary to the substrate.

Thus the enzyme become denatured.

What is a limiting factor?

A limiting factor is a variable of a system that causes a noticeable change in output or another measure of a type of system.

For example, when the amount of available substrate exceeds the amount of enzymes, then no more substrate can be broken down. Then the enzyme concentration is the limiting factor slowing the reaction.

Substrate Enzyme Concentration

In the presence of a given amount of enzyme, the rate of enzymatic reaction increases as the substrate concentration increases until a limiting factor is reached, after which further increase in the substrate concentration produces no significant change in the reaction rate. At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them.

In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released (or been released without reacting).


Leave a Reply

Subscribe to our Newsletter