Enzymes

Enzymes are protein catalysts. Enzymes bind temporarily to one or more of the reactants of the reaction they catalyse. In doing so, they lower the amount of activation energy needed and thus speed up the reaction.

Examples:
Catalase
  • It catalyses the decomposition of hydrogen peroxide into water and oxygen.
  • H2O2 -> H2O + O2
  • A single molecule of catalase can break 5.6 million molecules of hydrogen peroxide each minute.
Carbonic anhydrase
  • It is found in red blood cells where it catalyses the reaction
  • CO2 + H2O <-> H2CO3
  • It enables red blood cells to transport carbon dioxide from the tissues to the lungs. A single molecule of carbonic anhydrase can process 36 million molecules of substrate each minute.

In order to work an enzyme must unite (very briefly) with the reactants. Usually the forces that hold the enzyme and its substrate are weak temporary bonds (e.g. hydrogen bonds)

Most of the interactions are weak so for successful binding the enzyme and substrate have to be able to approach each other closely over a fairly broad surface. So it's kind of like a substrate molecule binds its enzyme like a key in a lock.

This requirement for shape fitting of substrate and enzyme explains the specificity of most enzymes. Generally an enzyme is able to catalyse only one chemical reaction.

 

Competitive inhibition
The need for a good fit between enzyme and substrate explains competitive inhibition.
In the digram the usual reaction is shown in (a). In (b) an inhibitor molecule fits into the active site of the enzyme. The action of the enzyme is strongly inhibited. This is because the inhibitor can bind to the same site on the enzyme but there is no reaction so no quick release of products. The inhibition is called competitive because if you increase the ratio of substrate to inhibitor in the mixture, you restore the rate of catalysis.

 

Factors Affecting Enzyme Action

The activity of enzymes is strongly affected by changes in pH and temperature. Each enzyme works best at a certain pH (left graph) and temperature (right graph), its activity decreasing at values above and below that point. This is because of the importance of tertiary structure (i.e. shape) in enzyme function and forces, e.g., ionic interactions and hydrogen bonds, in determining that shape.
  • Changes in pH alter the state of ionisation of charged amino acids that may play a crucial role in substrate binding and/or the catalytic action itself.
  • Hydrogen bonds are easily disrupted by increasing temperature. This, in turn, may disrupt the shape of the enzyme so that its affinity for its substrate diminishes. The ascending portion of the temperature curve reflects the general effect of increasing temperature on the rate of chemical reactions. The descending portion of the curve reflects the loss of catalytic activity as the enzyme molecules become denatured at high temperatures.

 

Feedback Inhibition.  
If the product of a series of enzymatic reactions, e.g., an amino acid, begins to accumulate within the cell, it may specifically inhibit the action of the first enzyme involved in its synthesis (bar). Thus further production of the enzyme is halted.

In the case if feedback inhibition, the activity of the enzyme is being regulated by a molecule which is not its substrate. In these cases, the regulator molecule binds to the enzyme at a different site than the one to which the substrate binds (active site). When the regulator binds to its site, it alters the shape of the enzyme so that its activity is changed. This is called an allosteric effect. The allosteric effect lowers the affinity of the enzyme for its substrate.

 

Lysozyme: a model of enzyme action

A number of lysozymes are found in nature e.g. in human tears and saliva. The enzyme is antibacterial because it degrades the polysaccharide that is found in the cell walls of many bacteria. It does this by catalysing the insertion of a water molecule (hydrolysis) at the position indicated by the red arrow. This hydrolysis breaks the chain at that point.

The bacterial polysaccharide consists of long chains of monosaccharides.

Lysozyme is a globular protein with a deep cleft across part of its surface. Six saccharide units of the substrate fit into this cleft. With so many oxygen atoms in sugars, up to 14 hydrogen bonds form between the substrate and enzyme. Plus hydrophobic interactions help hold the substrate in position.

When in the enzyme substrate complex the fourth saccharide in the chain becomes twisted out of its normal position. This imposes a strain on the glycosidic bond and the polysaccharide is broken at this point. A molecule of water is inserted between these two hexoses (hydrolysis), which breaks the chain. Here, then, is a structural view of what it means to lower activation energy. The energy needed to break this covalent bond is lower now that the atoms connected by the bond have been distorted from their normal position.

As for lysozyme itself, binding of the substrate induces a small movement of certain amino acid residues so the cleft closes slightly over its substrate. So the "lock" as well as the "key" changes shape as the two are brought together (This is sometimes called "induced fit"). The reaction is now complete. The chain is broken, the two fragments separate from the enzyme, and the enzyme is free to attach to a new location of the bacterial cell wall and continue its work of digesting it.

 back

 home