Enzymes

I. Enzymes (for the most part) are proteins

• The primary means by which they catalyze reactions is by providing an alternative pathway for the reaction which involves a lower activation energy, as shown below:

• Enzymes, having a 3-dimensional structure, ensure more frequent collisions between substrate molecules in the proper orientation in space. Within each enzyme, there will be an active site into which the substrate fits.

A 3-dimensional representation of an active site -- A computer model of the active site of an enzyme called mammalian zinc metalloendopeptidase, or EP 24.15. Credit: M. J. Glucksman, appearing in:
http://www.daviddarling.info/encyclopedia/A/active_site.html

• Most enzyme names end in -ase, although a few end in -in.

Examples of enzymes with –ase suffix	Examples of enzymes with –in suffix
· Peroxidase: converts hydrogen peroxide into water and oxygen gas; · Lipase: hydrolyzes neutral fats into glycerol and 3 fatty acid molecules; · Sucrase: hydrolyzes sucrose, a disaccharide, into its constituents, one molecule of glucose and one molecule of fructose.	· Pepsin: Initiates breakdown of intact proteins into long peptide chains; · Trypsin: One of several enzymes secreted by the small intestine to continue breaking down long peptide chains into shorter ones.

• Because of the specific shape of the active site, enzymes are specific regarding the reactions they catalyze.

EXAMPLES:

The reaction H₂O₂ è H₂O + 1/2 O₂ is catalyzed by peroxidase

The conversion of the polysaccharide starch è maltose is catalyzed by amylase

• Enzymes are most effective at specific conditions pertaining to pH and to temperature.

Enzyme activity as a function of pH (Data from Fall 2008 BIOL 111 lab section):

Enzyme activity as a function of temperature (Data from Fall 2008 BIOL 111 lab section)

II. Enzymes are subject to inhibition

• Sometimes, a molecule that roughly resembles the normal substrate can fit into the active site, and thereby prevent the enzyme from acting on the substrate. This form of inhibition is called competitive inhibition:

 In non-competitive inhibition, the inhibitor molecule binds to a site other than the active site of the enzyme, and thereby changes the 3-dimensional structure sufficiently to prevent the normal subsrate from fitting into the active site. This is sometimes called allosteric inhibition.