Enzyme Function

Enzyme Function

• Globular proteins that act as biological catalysts
• Speed up chemical reactions that would normally be slow at cell temperature
• Precise 3-D shape includes a depression surface; active site
• Amino acids involved on active site whilst other remain 3-D shape

Lock and Key theory:

• The molecule (substrate) must have a specific, complimentary shape, to fit into the active site.
• Substrate forms temporary bonds with amino acids of the active site to create an enzyme-substrate complex
• Enzyme holds the substrate so that they react more quickly
• When reaction has finished, the products are released, leaving the enzyme unchanged

Induced Fit theory:

• Active site is flexible
• When the substrate (s) enter the active site the enzyme changes shape slightly
• When reaction finishes and the enzyme returns to its normal shape
• Only a specific substrate will induce the change in shape of an enzyme’s active site

Activation Energy:

• To convert substances into products bonds must change (within and between molecules)
• Breaking chemical bonds requires energy to start the reaction – activation energy
• Heating a substrate or using an enzyme would give this energy
• Heat agitates atoms that become unstable and start the reaction
• Enzymes reduce the time this process takes

How?

• The specific shape of the enzyme and the substrate is such that electrically charged groups on their surfaces interact
• Attraction of oppositely charged groups may distort the shape of the substrate and assist in breaking/making bonds
• Sometimes the amino acids contain acidic side chains which may provide conditions favourable for the reaction
• Substrate molecules are sometimes altered, not added to or broken down, these are isomerase enzymes

Finding rates of enzyme-controlled reactions:

• Rate of reaction measured by: the quantity of substrate used or the quantity of product formed in a given time
• The reaction will firstly occur quickly however as the substrate is used up there are fewer substrate molecules to bind with the enzyme and the reaction slows down
• The rapid slope is the initial rate of reaction

Enzyme and Substrate concentration; rate of reaction:

• Initial rate of reaction is directly proportional to the enzyme concentration because the more enzyme that is present the greater the number of active sites
• It will continue in this linear fashion if there is an excess of substrate
• Reaction can be limited by the enzyme if there is increased substrate concentrations but not enough active sites