23 Enzymes

Key Idea: Enzymes are biological catalysts. The active site is critical to this functional role.

` Most enzymes are globular proteins. Enzymes are biological catalysts because they speed up biochemical reactions, but the enzyme itself remains unchanged. The substrate in a reaction binds to a region of the enzyme called the active site, which is formed by the folding of the enzyme's amino acid chain (its tertiary structure). ` Enzymes control metabolic pathways. One enzyme will act on a substance to produce the next reactant in a pathway, which will be acted on by a different enzyme. ` Enzymes are often named after their substrate and/or the type of reaction they catalyse together with the suffix -ase. For example, lipase breaks downs lipid molecules, glucose oxidase catalyses the oxidation of glucose.

The active site

Enzymes have an active site to which specific substrates bind. The shape and chemistry of the active site is specific to an enzyme, and is a function of the polypeptide's complex tertiary structure.

The substrate is the chemical that an enzyme acts on. A specific enzyme acts on a specific substrate. Extremes of temperature or pH can alter the enzyme's active site and lead to loss of function. This is called denaturation. Enzymes can be defined based on where they are produced relative to where they are active. An intracellular enzyme is an enzyme that performs its function within the cell that produces it. Most enzymes are intracellular enzymes, e.g. respiratory enzymes. Example: Catalase. Many metabolic processes produce hydrogen peroxide, which is harmful to cells. Catalase converts hydrogen peroxide into water and oxygen (below) to prevent damage to cells and tissues.

An extracellular enzyme is an enzyme that functions outside the cell from which it originates (i.e. it is produced in one location but active in another). Examples: Amylase and trypsin. Amylase is a digestive enzyme produced in the salivary glands and pancreas in humans. However, it acts in the mouth and small intestine respectively to hydrolyse starch into sugars. Trypsin is a protein-digesting enzyme. It is produced in an inactive form (called trypsinogen) and secreted by the pancreas into the lumen of the small intestine (where it will work). It is activated in the intestine by the enzyme enteropeptidase to form trypsin. Active trypsin can convert more trypsinogen to trypsin.

2H2O2 2H2O + O2

Substrates collide with an enzyme's active site Enzymes can be intracellular or extracellular

For a reaction to occur, the reactants must collide with sufficient speed and with the correct orientation. Enzymes enhance reaction rates by providing a site for reactants to come together in such a way that a reaction will occur. They do this by orientating the reactants so that the reactive regions are brought together. They may also destabilise the bonds within the reactants making it easier for a reaction to occur. Catalase

Incorrect reactant orientation = no reaction

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Enzyme orientates the reactants making the reaction more likely.

1. What is an enzyme's active site and how it is it formed?

2. How do substrate molecules come into contact with an enzyme's active site?

3. Why would a protein digesting enzyme such as trypsin need to be activated extracellularly (outside the cell)?