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19. Enzymes - How Temperature Affects Enzyme Activity

In this A-Level Biology Lesson "Enzymes: How temperature affects Enzyme Activity"

Here we'll learn How enzymes Lower Activation Energy

The effects of temperature on enzyme activity (introduction)

As Temperature increases... The Enzyme as Denatured...

Enzymes have optimal temperatures.

How to Analyse the graph...

Enzymes have optimum temperatures at which they work best (fastest). Mammalian enzymes have an optimum temperature of about 40°C. Remember though, you're leaning biology - and that includes all life... Thus, there are enzymes that work best at vastly different temperatures than ours, e.g. enzymes from the arctic snow flea have an optimal temperature of about -10°C, and enzymes from thermophilic bacteria such as Thermus aqaticus work have optimal temperatures ranging from 72°C - 95°C (some even higher!)

Taking a look at the graph showing how temperature affects enzymes, you'll see that up to the optimum temperature the rate increases geometrically with temperature (i.e. it's a curve, not a straight line). The rate increases because the enzyme and substrate molecules both have more kinetic energy so collide more often, and because these molecules have sufficient energy to overcome the energy barrier which of course due to the enzyme is greatly reduced.

The increase of enzyme activity rate as temperature increases can be quantified as a Q10, which is the relative increase for a 10°C rise in temperature.

Q10 is usually 2 - 3 for enzyme catalysed reactions (meaning the rate of reaction doubles wit every 10°C increase in temperature). Q10 is typically less than 2 for non-enzyme reactions.

It is important that you understand also, that the rate of enzyme activity is not zero at 0°C, meaning enzymes still work at 0°C (and lower, but slower) e.g. food in the fridge still goes 'off', it's just that the enzymes breaking down the food work much more slowly. Enzymes can even work in ice, albeit the rate of reaction is extremely slow due to the extremely slow diffusion of enzyme and substrate molecules through the ice lattice.

Above the optimum temperature the rate of reaction decreases because more and more of the enzymes denature. The heat energy breaks the hydrogen bonds holding the secondary and tertiary structure of the enzyme together, so the enzyme (and especially the active site) loses its shape to become a random 'coil'. The substrate can no longer bind, and the reaction is no longer catalysed.

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