Enzymes and chemical digestion
Joanna George-Johnson, Contributor

Winston Jones High School students (from left) Monique Brooks, Kemorie Davis, Carlene Brown, Claudine Trench and Tameka Hinds finished as overall winners of the school's Paper Quiz 2005. Standing with the girls are (from left) Bobbette Barrett, head of the Language Arts Department, The Gleaner's Tasia Cargill and Shernet Reid. The awards ceremony was held in Pratville, Manchester, in April last year. - File photo

The chemical reactions which occur in the body would take place spontaneously if they were allowed to. But they would have taken a great deal of time. This is clearly not ideal as these processes, respiration, for example, are integral in the maintenance of life. Chemicals known as enzymes speed up chemical reactions in the body.

Catalysis

A catalyst is something which speeds a chemical reaction, remaining unchanged at the end of the reaction pathway. It is responsible for the hydrolysis (splitting by water) of ingested macronutrients.

Mode of action of enzymes - The lock-and-key hypothesis.

This theory simply states that all enzymes have active site which is complementary in shape to the substrate (molecule being broken down). As the name suggests, the binding is similar to how a key fits into a lock. As we know, a lock can only be opened by a particular key. In the same way the substrate (key) can only bind to one particular enzyme (lock) for the chemical reaction to occur. The active site is the place on the enzyme where the reaction takes place. The enzyme binds to the substrate forming an enzyme-substrate complex. The chemical reaction takes place forming the product of the reaction. The shape of the product is not complementary to the enzyme's active site, so the bonds linking it to the enzyme are broken.

A certain amount of energy is required for all chemical reactions to take place. This is known as the activation energy. Enzymes work by reducing the activation energy of the reaction.

The lock and key hypothesis.

Properties of enzymes:

• They are proteins.
  
  
• They have an optimum temperature. The optimum temperature is that at which the enzyme works best. In humans, this is 37ºC. As they are proteins, they are denatured by excessively high temperature.
  
  
• Enzymes are made inactive by low temperature.
  
  
• They have an optimum pH.
  
  
• Enzymes are specific. This means that a particular enzyme only catalyses the breakdown of a particular substrate. This is because for the enzyme-substrate complex to be formed, the shape of the molecule must be complementary to the shape of the active site.

Questions

1. Humans cannot digest cellulose. However, the micro-organism, trichonympha can. Suggest why this is so.

2. An examination candidate wrote, "The substrate binds to the enzyme and they are both the same shape," Explain why this is incorrect and give the correct answer.

3. Sketch a graph of the rate of an enzyme-controlled reaction versus time, and explain the shape of the graph.

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.