5. Effect of temperature - What is the optimum temperature for an enzyme catalysed reaction?

Enzymes perform best in their 'optimum' ambient conditions

When investigating the effect of temperature on enzyme activity, three factors must be kept constant, (i) the pH and the concentrations of both the (ii) substrate and (iii) enzyme must all be kept constant (see experimental methods).

General description of graphs: For any enzyme, initially, as you increase the temperature, the rate of reaction increases.

Then as the temperature increases, the reaction rate reaches a maximum at the optimum temperature and then decreases with further increase in temperature.

The optimum temperature varies from one enzyme to another, but for many warm blooded animals like us, it is often close to 37^oC - but beware of enzymes in extremophile organisms in hot springs, volcanic vents and 10 km down at the bottom of an ocean,

Temperature: The structure of the protein enzyme can depend the temperature. If the enzyme does not have the correct 'lock' structure, it cannot function efficiently.  The shape of the graph is due to two factors.

A too high a temperature can affect the shape of the active site or the ability of the substrate molecule to 'dock in' to the active site - some kind of change is promoted by a high temperature - a denaturing effect due to interfering with the bonds holding the enzyme together in its unique 3D shape - the active site is damaged and the substrate molecule can't fit in.

Explaining the left graph of enzyme reaction rate versus temperature

(a) The initial rise in rate of reaction is what you normally expect for any chemical reaction

 The increase in temperature increases the average kinetic energy (KE) of the molecules to increase the chance of the product forming from the higher KE substrate-enzyme effective 'fruitful' collisions.

In other words, at a higher temperature, more molecules have sufficient energy to overcome the activation energy to break bonds and change from reactants to products.

The minimum energy needed for the reaction change is called the activation energy, and is much lower for the enzyme catalysed reaction, than the uncatalysed reaction (the green and black profiles in the diagram above), its the size of the hump compared to the reactant potential energy baseline that is important).

There is also an increase in the rate of collision of the substrate and enzyme molecules.

For more details see the GCSE chemistry notes

Effect on rate of changing the temperature of reactants

(b) At higher temperatures the rate goes through a maximum and then slows down!

However as the temperature rises further, the increasing thermal vibration of the enzyme molecule causes its structure to break down (denature) and so the 'lock' is damaged so the enzyme is less efficient in interacting with the substrate molecule (see key-lock below). It means the substrate molecule cannot properly dock into the active site on the enzyme.

This may be due to the failure of weak intermolecular bonding forces or actual ionic or covalent bonds being broken, but the 3D molecular structure  of the enzyme is changed so that the substrate molecule cannot 'dock into' the active site on the enzyme to be changed into products.

This damage to the enzyme's active site at higher temperatures - the denaturing of the enzyme, is NOT reversible. The enzyme will not go back to its normal shape even if the reaction mixture is cooled down.

The above diagram shows the effect of high temperatures on an enzyme molecule - the crucial and effective 3D shape is destroyed when bonds in the protein molecule are disrupted.

The optimum temperature for the fastest rate of reaction is often around 30-40^oC (note our body temperature is about 37^oC, no coincidence!). Eventually at high temperatures the enzyme completely ceases to function.

Note: NOT every enzyme reaction has an optimum of ~30-40^oC!, some organisms exist and survive at very low temperatures and some at very high temperatures - e.g. extremophile organisms that live in very hot water near volcanic vents deep in the ocean on the sea bed.

See Adaptations, lots of examples explained including extremophiles

By combining points made in (a) and (b) we can completely explain the shape of the graph.

The actual graph that you obtain from experiments is effectively the result of adding two trends together,

(a) The increase in rate due to increase in temperature - 'normal chemical behaviour,

and (b), the decrease in rate as denaturing of the enzyme increases with increase in temperature.

The resulting graph then has two minimums at the lower and higher temperatures, and one maximum - the hump in the graph is the point of maximum speed of the reaction, 'highlighting' the optimum most effective temperature range.

So, the first graph diagram is typical for temperature controlled enzyme activity.

The 2nd temperature graph shows what happens to the speed of the enzyme catalysed process of photosynthesis as the temperature is increased.

As the temperature increase the rate of catalysis increases (normal effect on the speed of reaction as the average kinetic energy of the molecules increases), but at high temperatures the protein structure of the enzyme is destroyed, so the active site on the enzyme is damaged.

More extreme condition: Some enzymes in bacteria found in:

(i) hot springs have an optimum temperature of over 80^oC - at this temperature most our enzymes would be denatures,

(ii) bacteria living in cold deep ocean water have optimum temperatures as low as 0^oC - at this temperature, most of our enzymes would only function very slowly.

See a 'decay' investigation using milk and lipase  gcse biology revision notes