Enzymes


An enzyme is a biological catalyst. Without them, processes in the body would occur too slowly to support life.

Enzyme Basics

Specification Reference

"Know the structure of enzymes as globular proteins."
"Understand that enzymes are catalysts that reduce activation energy."
An enzyme is a biological catalyst. A catalyst is something that changes the rate of reaction without being used up and is reformed at the end of the reaction.
  • Enzymes do this by destabilising bonds in a substrate molecule, lowering the activation energy and therefore allowing the reaction to occur more quickly.
  • Enzymes are globular proteins which have a very specifically shaped active site.
  • This means that one enzyme will only catalyse one type of reaction.
  • They show great specificity.

Different Enzymes

Specification Reference

"Know that enzymes catalyse a wide range of intracellular reactions as well as extracellular ones."
Intracellular enzymes act within the boundaries of the cell membrane such as DNA polymerase. Extracellular enzymes act outside the confines of the cell membrane. For example, amylase.
  • Anabolic reactions build up new chemicals
  • Catabolic reactions breakdown chemicals
Enzymes work by lowering the activation energy of a reaction and it is therefore important to be able to show this concept on a graph. Enzymes are also soluble in water and it is also worth knowing that the active site only makes up a small part of the enzyme.

The Induced Fit Hypothesis

Specification Reference

"Understand the concepts of specificity and the induced fit hypothesis."
Enzymes are very specific and will only bind to a particular substrate. Over the years a number of models have attempted to describe how an enzyme works. The lock and key model and the induced fit model are two examples of this.

The Lock and Key Model
  1. The substrate molecule is drawn into the enzymes active site.
  2. The active site does not change shape.
  3. An enzyme-substrate complex is formed.
  4. A reaction takes place and the enzyme-product complex is formed.
  5. The products are released and the enzyme is free for use again.
However, this model fails to account for the flexibility found within enzymes.

The Induced Fit Model
  1. A substrate molecule is drawn into the active site with a very similar shape.
  2. The substrate induces a change upon the enzyme so that there is a perfect fit and an enzyme-substrate complex is formed.
  3. The bonds in the substrate are strained and become weaker resulting in an enzyme-product complex.
  4. The products are released and the enzymes returns to its original shape.
This model accounts for the flexibility found in enzymes and explains why some enzymes are effective on more than one substrate. Because this the induced fit model is the currently accepted model.

Rate of Enzyme Activity

Specification Reference

"Understand how temperature, pH, substrate and enzyme concentration affect the rate of enzyme activity."
"Understand how the initial rate of enzyme activity can be measured and why this is important."
To work out the level of enzyme activity present, scientists measure the initial rate of reaction. This is done by drawing a tangent that passes through 0 on a rate graph and then calculating the gradient of this tangent to work out the initial rate.

Temperature & Enzyme Activity

  • As temperature increases so does kinetic energy
  • Reactants now move faster increasing the frequency of collisions between substrates and enzymes
  • This increases the rate of reaction
However, if the temperature is increased too much, bonds in the enzymes active site will break, causing the enzymes to denature. The enzyme will no longer work effectively meaning an increase in temperature will only increase the rate of reaction to a certain point.

ph & Enzyme Activity

  • Enzymes are sensitive to pH
  • Amino acids and proteins behave differently in different levels of pH
  • This means that outside of a narrow range enzymes will denature

Substrate Concentration & Enzyme Activity

  • As the substrate concentration increases, so does the rate of reaction
  • This is because there are more successful collisions
  • This relationship is directly proportional until all of the active sites are in use and therefore no more substrate can be catalysed

Enzyme Concentration & Enzyme Activity

  • As long as the substrate is in excess, an increase in enzyme concentration will increase the rate of reaction
  • However, if the substrate is limited the rate of reaction will only increase up to a certain point as there will eventually be more active sites than substrate molecules

Enzyme Inhibitors

Specification Reference

"Understand how enzymes can be affected by competitive, non-competitive and end-product inhibition."
This is any substance which changes the rate of an enzyme catalysed reaction.
  • Competitive Inhibitors - compete with the substrate for the enzymes active site
  • Non-competitive Inhibitors - bind to an allosteric site, altering the shape of the active site
With a fixed amount of competitive inhibitor, the rate of reaction increases more slowly but is eventually overwhelmed.

With a fixed amount of non-competitive inhibitor, the maximum rate is much lower since an increase in substrate concentration has no effect on the inhibitors ability to bind to an allosteric site.

End-product Inhibition

This processes is an important part in the regulation of metabolic pathways. When the amount of end product is high, the product may bind to the enzyme non-competitively. This changes the shape of the active site preventing further production of itself. If product levels drop, the enzyme will release the inhibitor allowing production to begin again.