2.4.2 Enzyme Action

Question 1

Mechanism of Enzyme Action

Answer 1

Enzymes have an active site where specific substrates bing 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

Question 2

Enzyme Specificity

Answer 2

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 tertiary structure of thye 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

Question 3

The Enzyme-Substrate Complex

Answer 3

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

Question 4

The Lock-and-Key Hypothesis

Answer 4

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 structure of the protein that makes up the enzyme and therefore is highly specific
In the 1890’s, the first model of enzyme activity was described by Emil Fisher
- 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’

Question 5

The Induced-Fit Hypothesis

Answer 5

The lock-and-key hypothesis 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 are known as the conformational changes
- the conformational changes ensure an ideal binding arrangement between the enzyme and substrate in achieved
- This maximises the ability of the enzyme to catalyse the reaction

Question 6

Enzymes and the Lowering of Activation Energy

Answer 6

All chemical reactions are associated withy energy changes
For a reaction to proceed there muster 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 for be formed
- enzymes speed up chemical reactions because they reduce the stability of bond in the reactants
- the destabilisation of bonds in the substrate make 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 and these extreme conditions (that would otherwise kill cells)