Enzymes

Enzymes are biological catalysts. They speed up reactions but are unchanged at the end of the reaction

'Biological' - Enzymes are protiens and found in living things but they are not alive, therefore they cannot be killed

'Catalysts' - A molecule that speeds up a reaction but is unchanged at the end of the reaction, therefore only a small amount of each enzyme is made as it is used again and again

Enzymes are specific, each one only catalyses one reaction

How do enzymes catalyse reaction?

• Enzymes reduce the activation energy of a reaction, often making them happen at lower temperatures
• This speeds up the rate of reaction

Enzymes are protein, so must be made inside living cells but may act either;

• Intracellular (act within a cell)
• Extracellular (act outside a cell)

Enzymes are large molecules formed from globular proteins. They have specific tertiary structures, maintained by hydrogen bonds, ionic bonds and disulphide bonds

They are soluble in water due to the many hydrophilic R groups on their constituent amino groups

How enzymes work

• Enzymes have a region called the active site
• The substrate enters the active site and an enzyme-substrate complex forms
• Products are then formed and released from the active site
• The enzyme is unchanged and is available to catalyse another reaction
• In order for a product to be formed, a successful collision between substrate molecules and active site of the enzyme has to occur

There are two theories on how a substrate binds with the active site of an enzyme;

• The lock and key method
• The induced fit hypothesis

The lock and key method

• Only a substrate molecule with a shape that is fully complementary to the active site of the enzyme will bind, forming an enzyme-substrate complex
• The activation energy will then be lowered, the reaction will occur and the product will leave the active sitewhich will be free to be used again

The induced fit hypothesis

• This hypothesis states that the enzymes active site is not fully complementary to the substrate
• As the substrate binds to the enzyme the active site changes shape slightly to complete the fit. This locks the substrate even more tightly to the enzyme
• Example - Lysosome = a digestive enzyme

Any factor that increases the number of successful collisions between active sites and substrates per unit time will increase the rate of reaction

Temperature

• As the temperature increases, the enzymes and substrate gain kinetic energy. This increases the rate of movement of the molecules within the solution, increasing the chance of successful collisions between active sites and substrates
• This leads to the formation of more products per unit time. Therefore rate of reaction increases
• The rate of reaction continues to increase until the enzymes optimum temperature is reached. At the optimum temperature, the rate of reaction is at it's highest
• Above the optimum temperature the rate of reaction decreases rapidly. This is because the enzymes active sites become denatured
• As the molecules gain kinetic energy, they vibrate. Above the optimum temperature, the vibrations become so great that they cause the hydrogen bonds holding in place the enzymes tertiary structure break . This changes the shape of the active site and the enzyme is denatured. The enzyme and substrate no longer fit together so no enzyme substrate complexes are formed and no products are formed

• As the temperature continues to rise, more active sites are denatured. The rate of reaction continues to fall until all of the active sites have denatured and the reaction stops
• Enzymes have different optimum temperatures . Most human enzymes have an optimum temperature just below body temperature

pH

• All enzymes have an optimum pH value
• Most work best at neutral pH7, but there are exceptions e.g. pepsin works best at pH2 
• The rate of reaction is highest at the optimum pH. Slight changes above and below this optimum cause the rate of reaction to fall
• A small change from the optimum can cause the enzyme to become temporarily inactivated. This is due to changes in the charge of the active site, causing the active site to be repelled from the active site, preventing the formation of an enzyme-substrate complex. This is only a temporary change, so if the enzyme is returned to it's optimum pH it will work again
• A large change from the optimum will cause the enzymes tertiary structure to change, altering the shape of the active site and resulting in the enzyme becoming permanently denatured
• Different enzymes have different pH optima
• As enzymes are sensitive to changes in pH, a pH buffer is used during experiments to maintain a constant pH

Concentration of Substrate

• As the subsrate concentration increases, the rate of reaction will increase. This is because the presence of more substrate molecules increases the chance of successful collisions between active sites and the substrate
• The rate of reaction contiunes to increase as substrate concentration increases until a maximum rate of reaction is reached, after which furthur increases in substrate concnetration no longer increase the rate of reaction
• A maximum rate of reaction is reached because there is a fixed number of enzymes, and therefore active sites
• At this point, all of the active sites of the enzymes are full all of the time, so the maximum number of successful collisions is occuring
• This means there can be no increase in product per unit time
• As it is the enzymes preventing the rate of reaction increasing furthur, we can say that the enzyme concentration is the limiting factor
• The maximum rate of reaction can only be increased if the enzyme concentration is increased

Concentration of an enzyme

• As the enzyme concentration increases, the rate of reaction also increases
• There is always enough substrate available to fill all of the available active sites. This ensures the substrate concentration is not a limiting factor for the rate of reaction
• However, if the substrate is not in excess, then the maximum rate of reaction will be reached and the graph will level off

Enzyme activity can be prevented or slowed down by enzyme inhibitors. Inhibition can be competitive or non-competitive

Competitive Inhibition

• Competitive inhibitors are molecules which have a similar shape to the substrate 
• They compete with the substrate to bond to the active site, but no reaction follows
• Instead they block the active site, forming an enzme-inhibitor complex so no substrate can fit in it
• e.g. Malonate - Competes with a substrate involved in respiration
• How much inhibitions happens depends on the relative concentrations of the inhibitor and substrate. If there is a lot of inhibitor, it will take up all the active sites ad stop any substrate getting to the enzyme

Non-Competitive Inhibitors

• These molecules do not have a similar shape to the substrate molecule
• They bind to another site on the enzyme called the allosteric site and this causes the active site to change shape

• They don't compete with the substrate for the active site, because even if there is a substrate in the active site, the inhibitor can still bind with the enzyme
• E.g. Potassium Cyanide - A non-reversible inhibitor of an enzyme involved in respiration which prevents the production of ATP

The effect of a competitive inhibitor can be overcome by increasing the substrate concentration - As the substrate concentration increases, the chance of successful collisions between the substrate and active site is greater than the chance of collision between the inhibitor and the active site

So as the substrate concentration increases, the effect of the inhibitor decreases until eventually the substrate concentration is high enough that the competitive inhibitor will no longer be able to effect the rate of reaction

In the presence of a non-competitive inhibitor, increasing the substrate concentration will also overcome the effect of the inhibitor and result in the rate of reaction increasing

However, the rate of reaction will never reach the maximum, no inhibitor present because the competitive inhibitor reduces the number of active sites available in the reaction

So no matter how much substrate concentration is increased, the rate of reaction will never reach it's maximum

Competitive and Non-Conpetitive Inhibitors

• Can be reversible or non-reversible depending on the strength of the bond between the enzyme and the inhibitor
• If it is a strong covalent bond, then the inhibitor cannot be easily removed and the inhibition is irreversible
• If it is a weaker hydrogen bond or a weaker ionic bond then the inhibitior can be removed and inhibition is reversible