• Enzymes have an?active site?where?specific substrates bind?forming an?enzyme-substrate complex
• The?active site?of an enzyme has a?specific shape?to fit a?specific substrate
• Extremes of?heat?or?pH?can?change the shape?of the active site,?preventing?substrate binding – this is called?denaturation?(the enzyme is said to be?denatured)
• Substrates?collide?with the enzymes active site and this must happen at the?correct orientation and speed?in order for a reaction to occur

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

Enzyme specificity

• The?specificity?of an enzyme is a result of the?complementary nature?between the?shape?of the?active site?on the enzyme and its?substrate(s)
• The shape of the active site (and therefore the specificity of the enzyme) is determined by the?complex tertiary structure?of the?protein?that makes up the enzyme:
  • Proteins are formed from chains of amino acids held together by peptide bonds
  • The order of amino acids determines the shape of an enzyme
  • If the order is altered, the resulting three-dimensional shape changes

   

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

The enzyme-substrate complex

• An?enzyme-substrate complex?forms when an enzyme and its substrate join together
• The enzyme-substrate complex is only formed?temporarily?before the enzyme catalyses the reaction and the?product(s)?are?released

The temporary formation of an enzyme-substrate complex

The lock-and-key hypothesis

• Enzymes are?globular proteins
• This means their?shape?(as well as the shape of the?active site?of an enzyme) is determined by the?complex tertiary structure?of the protein that makes up the enzyme and is therefore?highly specific
• In the 1890’s the first model of enzyme activity was described by Emil Fischer:
  • He suggested that both enzymes and substrates were?rigid structures?that?locked?into each other very?precisely, much like a key going into a lock
  • This is known as the ‘lock-and-key hypothesis’

   

The lock-and-key hypothesis

The induced-fit hypothesis

• The lock-and-key model was later?modified?and adapted to our current understanding of enzyme activity, permitted by advances in techniques in the molecular sciences
• The?modified?model?of enzyme activity (first proposed in 1959) is known as the ‘induced-fit hypothesis’
• Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate?interact?with each other:
  • The enzyme and its active site (and sometimes the substrate) can?change shape?slightly as the substrate molecule enters the enzyme
  • These changes in shape are known as?conformational changes
  • The conformational changes ensure an?ideal binding arrangement?between the enzyme and substrate is achieved
  • This?maximises the ability of the enzyme to catalyse the reaction

   

The induced-fit hypothesis

Enzymes and the lowering of activation energy

• All chemical reactions are associated with?energy changes
• For a reaction to proceed there must be enough?activation energy
• Activation energy is the?amount of energy needed?by the substrate to become just?unstable?enough for a?reaction to occur?and for?products to be formed
  • Enzymes speed up chemical reactions because they reduce the?stability?of?bonds?in the reactants
  • The?destabilisation of bonds?in the substrate makes it?more reactive

   

• Rather than lowering the overall energy change of the reaction, enzymes work by?providing an?alternative energy pathway?with a?lower activation energy
• Without enzymes,?extremely high temperatures?or?pressures?would be needed to reach the activation energy for many biological reactions
  • Enzymes avoid the need for these?extreme conditions?(that would otherwise?kill cells)

   

The activation energy of a chemical reaction is lowered by the presence of a catalyst (i.e. an enzyme)