Enzyme function depends on protein 3D structure

Enzymes are globular proteins that act as biological catalysts. They speed up chemical reactions. The precise 3D shape adopted by an enzyme includes a depression on the surface called the active site. 

Lock-and-key theory
Either a single molecule with a complementary shape, or more than one molecule that together have a complementary shape, can fit into the active site. These substrate molecules form temporary bonds with the amino acids of the active site to produce an enzyme-substrate complex. The enzyme holds the substrate molecule in such a way that they react more easily. When the reaction has taken place the products are released leaving the enzyme unchanged. The substrate is often likened to a 'key' which fits into the enzyme's 'lock', so this is known as the lock-and-key theory of enzyme action. Each enzyme will only catalyse one specific reaction because only one shape of substrate will fit into its precisely-shaped active site. 

Induced fit theory
It has been found that the active site is often flexible. When the substrate enters the active site, the enzyme molecule changes shape slightly, fitting more closely around the substrate. Its like when a person puts on a wetsuit; the wetsuit shape changes to fit the body but returns to its original shape when taken off. This is known as the induced fit theory of enzyme action. Only a specific substrate will induce the change in shape of an enzyme's active site. 

Activation energy
To convert substrates into products, bonds must change within and between molecules. Breaking chemical bonds require energy whilst energy is released when bonds form. The energy needed to break bonds and start the reaction is known as the activation energy. Without an enzyme, heating a substrate would provide this energy. The heat energy agitates atoms within the molecules, the molecules become unstable and the reaction can then proceed. In cells, enzymes reduce the amount of energy needed to bring about a reaction; this allows reactions to occur without raising the temperature of the cell.

How do enzymes reduce the activation energy?
The specific shape of the enzyme's active site and of its complementary substrates is such that electrically charged groups on their surfaces interact. The attraction of oppositely charged groups may distort the shape of the substrates and assist in the breaking of the bonds or in formation of new bonds. In some cases, the active site may contain amino acids with acidic side chains; the acidic environment created within the active site may provide conditions favourable for the reaction. 
Enzymes are present in all organisms and catalyse a huge range of reactions. They speed up reactions by at least a million times and most biological reactions wouldn't happen at all without enzymes.  

Enzymes:

• globular proteins
• have an active site that allows binding with a specific substrate
• catalyse reactions
• reduce the activation energy required for a chemical reaction to take place
• do not alter the end-product or nature of a reaction
• remain unchanged at the end of a reaction, able to bind with another substrate molecule 

How do enzyme substrate concentrations affect the rate of reactions?
The 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 that are available to form enzyme-substrate complexes. The increase in rate will continue in this linear fashion assuming that there is an excess of substrate. At high substrate concentrations it is the enzyme concentration that limits the rate of reaction.Every active site is occupied and substrate molecules cannot enter an active site until one becomes free again.