#18. Enzymes - active site, activation energy, enzyme specificity

Enzymes are globular proteins that serve as biological catalysts. 

They speed up or slow down metabolic reaction, but remain unchanged.

They may facilitate the breaking of an existing bond or the formation of a new bond.

Substrates = the molecules that bind to the enzyme
Products   = new substances formed.

1. Active sites

Active site = area in enzyme's molecule where the substrate bind to enzyme --> enzyme-substrate complex. 

The R groups of amino acids at the active site form temporary bonds with the substrate molecule. This pulls the substrate slightly out of shape, causing it to react and form products.

2. Activation energy

• Activation energy = energy the substrates need for changing themselves into products. Heating provides activation energy. 
• Enzymes reduce activation energy needed ---> reaction take place at low t^o. They do this by distort the shape of the substrate when it binds at the enzyme's active site.

3. Enzyme specificity

Lock and Key hypothesis (Emil Fisher, 1894)

• The shape of the active site of the enzyme and the substrate molecules are complementary. 
• They possess specific 3-D shapes that fit exactly into one another. 
• Like a key into a lock, only the correct size and shape of the substrate (the key) would fit into the active site of the enzyme (the lock). 
• This shows the high specificity of enzymes, however it is too rigid. 

The active site has a complementary shape
 to the substrate. 

Induced fit hypothesis (Koshland, 1958)

• The shape of the active site of the enzyme and the substrate molecules are NOT complementary. 
• In the presence of the substrate, the active site continually reshapes by its interactions with the substrate, until the substrate is completely fit into it. 
• The enzyme is flexible and molds to fit the substrate molecule like gloves fitting one’s hand or clothing on a person. 
• This hypothesis is more acceptable. 

The active site forms a complementary shape
to the substrate only after binding. 

Syllabus 2015 (a) explain that enzymes are globular proteins that catalyse metabolic reactions; (b) explain the mode of action of enzymes in terms of an active site, enzyme-substrate complex, lowering of activation energy and enzyme specificity (the lock and key hypothesis and the induced fit hypothesis should be included);

Syllabus 2016  - 2018 Mode of action of enzymes There are many  different enzymes, each  one specific to a particular reaction. This specificity  is the key to understanding the efficient functioning of cells and living organisms. a)   explain that  enzymes are globular proteins that  catalyse metabolic reactions b)   state that  enzymes function  inside cells (intracellular enzymes) and outside cells (extracellular  enzymes) c)   explain the mode of action of enzymes in terms of an active site,  enzyme/substrate complex, lowering  of activation  energy and enzyme specificity  (the lock and key hypothesis and the induced fit hypothesis should  be included)