Enzymes

Enzymes

Grade 9

Biological catalysts

Many chemical reactions can be speeded up by substances called catalysts. A catalyst alters the rate of a chemical reaction, without being changed itself.

Digestive enzymes

• For example, inside the alimentary canal, large molecules are broken down to smaller ones in the process of digestion. These reactions are speeded up by enzymes. A different enzyme is needed for each kind of food. For example, starch is digested to the sugar maltose by an enzyme called amylase. Protein is digested to amino acids by protease.

These enzymes are also found in plants - for example, in germinating seeds, where they digest the food stores for the growing seedling. Many seeds contain stores of starch. As the seed soaks up water, the amylase is activated and breaks down the starch to maltose. The maltose is soluble, and is transported to the embryo in the seed. The embryo uses it to provide energy for growth, and also to provide glucose molecules that can be strung together to make cellulose molecules, for the cell walls of the new cells produced as it grows.

Another enzyme which speeds up the breakdown of a substance is catalase. Catalase works inside the cells of living organisms - both animals and plants - for example, in liver cells or potato cells. It breaks down hydrogen peroxide to water and oxygen. This is necessary because hydrogen peroxide is produced by many of the chemical reactions which take place inside cells. Hydrogen peroxide is a very dangerous substance, and must be broken down immediately.

Not all enzymes help to break things. Many enzymes help to make large molecules from small ones. One example of this kind of enzyme is starch phosphorylase, which builds starch molecules from glucose molecules inside plant cells.

Naming enzymes

Enzymes are named according to the reaction that they catalyse. For example, enzymes which catalyse the breakdown of carbohydrates are called carbohydrases.

If they break down proteins, they are proteases. If they break down fats (lipids) they are lipases.

Sometimes, they are given more specific names than this. For example, we have seen that the carbohydrase that breaks down starch is called amylase. One that breaks down maltose is called maltase. One that breaks down sucrose is called sucrase.

The lock and key mechanism

An enzyme works by allowing the molecule of the substance on which it is acting to fit into it. The fit has to be perfect. The enzyme is like a lock, into which another molecule fits like a key. We say that the shape of the enzyme and the shape of the substrate are complementary to one another. 

The active site

The substance which is present at the beginning of the reaction is called the substrate. The substance which is made by the reaction is called the product.

For example, in saliva there is an enzyme called amylase. It catalyses the breakdown of starch to the complex sugar maltose. In this reaction, starch is the substrate, and maltose is the product.

amylase

starch ------------- ► maltose

Properties of enzymes

• 1. All enzymes are proteins. This may seem rather odd, because some enzymes actually digest proteins.

• 2. Enzymes are made inactive by high temperature. This is because they are protein molecules, which are damaged by heat.

• 3. Enzymes work best at a particular temperature. Enzymes which are found in the human body usually work best at about 37 °

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• 4. Enzymes work best at a particular pH Some enzymes work best in acid conditions (low pH). Others work best in neutral or alkaline conditions (high pH) 

• 5. Enzymes are catalysts They are not changed in the chemical reactions which they control. They can be used over and over again, so a small amount of enzyme can change a lot of substrate into product.

• 6. Enzymes are specific This means that each kind of enzyme will only catalyse one kind of chemical reaction.

Temperature and enzyme activity

Enzymes are damaged by high temperatures. For most human enzymes, this begins to happen from about 40 °C upwards. As the temperature increases beyond this, the enzyme molecules start to lose their shape. The active site no longer fits perfectly with the substrate. The enzyme is said to be denatured. It can no longer catalyse the reaction.

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The temperature at which an enzyme works fastest is called its optimum temperature. Different enzymes have different optimum temperatures. For example, enzymes from the human digestive system generally have an optimum of around 37 °C. Enzymes from plants often have optimums around 28 °C to 30 °C. Enzymes from bacteria that live in hot springs may have optimums as high as 75 °C.

pH and enzyme activity

Most enzymes are their correct shape at a pH of about 7 - that is, neutral. If the pH becomes very acidic or very alkaline, then they are denatured. Some enzymes have an optimum pH that is not neutral. For example, there is a protease enzyme in the human stomach that has an optimum pH of about 2.

This is because we have hydrochloric acid in our stomachs. This protease must be able to work well in these very acidic conditions.

Multiple Choice

Letter C...

• active site
• enzymes
• substrate
• products

Multiple Choice

Which of the following conclusions can be drawn from this graph?

• The optimum pH of the enzyme is 6.6.
  
• The optimum pH of the enzyme is 5.8
  
• The enzyme’s activity increases as pH increases 5.0 to 9.0
  
• The enzyme’s activity is greater around pH of 8.0 .
  

Multiple Choice

What happens when enzymes are heated to a high temperature?

• The enzymes die.
• The shapes of the enzyme are altered denature
• The enzymes' amino acid sequence changes.
• The enzymes remain the same

Multiple Choice

What is a substance called if it speeds up a chemical reaction?

• reusable

• catalyst

• specific

• fragile

Multiple Choice

Where do substrates bind to an enzyme?

• calcium channels

• active site

• receptor molecules

• voltage-gated channels

Multiple Choice

Enzymes are...

• nucleic acids
• proteins
• lipids
• carbohydrates

Poll

How well did you understand the enzymes?

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