Enzymes

Enzymes are Biological Catalysts. A Catalyst is a Substance that Speeds Up a Chemical Reaction Without Being Used in the reaction.

Enzymes Catalyse Metabolic Reactions, both within a Cell (e.g. respiration) or within the Organism as a Whole (e.g. Digestion). They can Affect Structures in an organism (i.e. The Production of Collagen which is an important protein in the connective tissues of mammals). Enzyme Action can either be Intracellular (within cells) or Extracellular (outside cells).

EXAMPLES

Catalase (Intracellular)

1. Hydrogen Peroxide (H2O2) is  a By-product of several cellular reactions. It is also Toxic, so if it is left to build up, it can Kill Cells.
2. Catalase is an Enzyme that, inside cells, speeds up the Breakdown of H2O2 to Oxygen (O2) and Water (H2O).

Amylase and Trypsin (Extracellular)

1. Both of these work outside the cells in the Human Digestive System.
2. Amylase is found in Saliva. It is Secreted into the Mouth via the Salivary Glands. It Catalyses the Hydrolysis of Starch Into Maltose in the mouth.
3. Trypsin Catalyses the Hydrolysis of Peptide Bonds - It turns Big Polypeptides into Smaller Polypeptides. The enzyme is Produced in the Pancreas and Secreted into the Small Intestine.

Enzymes are Globular Proteins. They all have an Active Site which has a Specific Shape. The active site is the part of the enzyme that the molecules that interact with it (called Substrates) bind to. The Specific Shape of an Active Site is Determined by the Enzyme's Tertiary Structure. For the enzyme to work, the Substrate Shape has to Fit into the Active Site. Otherwise, the Reaction Would Not be Catalysed. This means that Enzymes usually only work with One Substrate.

Activation Energy

In any reaction, a certain amount of Energy is Required by the Reactants Before the Reaction can Start. This quantity is called the Activation Energy. It is often Heat. 

Enzymes Reduce the amount of Activation Energy required, so reactions can happen at a Lower Temperature than they would without an enzyme. This Speeds Up the Rate of the Reaction.

When a Substance Binds to an enzyme's Active Site, an Enzyme-Substrate Complex is formed. This is what Lowers the Activation Energy. There are two reasons why this happens:

• If two Substrate molecules need to be Joined, Attaching to the Enzyme brings them Closer Together, Reducing any Repulsion between molecules so it is Easier for them to Bind.
• If an Enzyme is Catalysing a Breakdown Reaction, Fitting into the Active Site puts Strain on the Bonds in the Substrate. This means it is Easier for the Substrate to Break Up.

LOCK AND KEY MODEL

Enzymes only work with Substrates that Fit their Active Site. Early scientists came up with this theory, that the Substrate fits into the Enzyme similar to the way a Key fits into a Lock. However, this was found to be Incomplete, as Enzymes were found to Change Shape slightly to complete the fit. The original model was modified into the Induced Fit Model.

INDUCED FIT THEORY
It helps to explain why Enzymes are so Specific. Not only does the Substrate have to be the Right Shape to fit the Active Site, It has to make the Active Site Change Shape in the right way too.