What is the temperature effect on enzymes

Influencing enzyme activity

Temperature dependence

To illustrate the following considerations, we will again carry out a (hypothetical) experiment. Again we imagine a glucose decarboxylase that is able to split off carbon dioxide from glucose in such a way that the released CO2 can also measure.

By the way: The search for glucose decarboxylase on the Internet is not very successful, there are over 180 references, but in the references you will not find the word "glucose decarboxylase", but mostly "glucose dehydrogenase" (English) and then on the same page " glucuronate decarboxylase ", for example in the Budh" Polysaccarides in Food "by BLANSHARD and MITCHELL on page 9.

If you enter the search term glucose decarboxylase in your search engine in the near future, you will most likely come across this page here, as the term glucose decarboxylase is mentioned several times - now again. In fact, at the beginning of March 2016, I did another test looking for glucose decarboxylase. This page was the first to be displayed on Google!

Update April 2018: This page is still displayed first on Google!

Glucose solution with concentration c (glucose) = 0.1 mol / l is divided into ten test tubes. With the help of water baths, the test tubes are brought to different temperatures. Then we take the (hypothetical) enzyme glucose decarboxylase, which produces a CO from each of the glucose molecules2-Molecule splits off. We prepare an aqueous solution of the enzyme and add exactly 10 drops of this solution to each test tube. So we now have 10 test tubes with the same glucose concentrations, but each one different temperature.

We now measure the carbon dioxide that is released from each test tube in a certain time (bubble counting method, collecting the gas in a small syringe, etc.).

The enzyme activity can now be measured relatively easily, e.g. by collecting the carbon dioxide formed.

Expectations:

It is known from physical chemistry that the speed of a chemical reaction increases with temperature. According to the RGT rule, a temperature increase of 10 K or 10 ºC leads to a doubling, tripling or even quadrupling of the reaction rate. Most of the carbon dioxide should therefore be produced in test tube no. 10.

Observations:

The expectations are not met. The following table shows the measured values ​​of such a (hypothetical) experiment:

T in ° C V (CO2) / min
00
53
107
1512
2018
2524
3025
3522
4010
450

Here is the graphic evaluation:

Why do you see a clear optimum curve on the graph and no continuous increase in enzyme activity?

Explanation

There are two clear limits to the RGT rule in living cells. Below 0 ºC, ice crystals form in the cell, which destroys important structures and the enzymes can no longer work or no longer work properly. Of course there are some living things that have adapted to low temperatures in the course of evolution and, for example, can prevent the formation of ice crystals with the help of chemical substances ("antifreeze"), but this is the exception.

Above 40 to 50 ºC most proteins denature so that they can no longer work. Here, too, there are many organisms that have adapted to high temperatures in the course of evolution, their enzymes are still fully functional even at 60 or 70 ºC, but that too is the exception that we don't need to talk about here.

Normal enzymes therefore have a temperature range in which they work at maximum speed, and this temperature range is around 20 to 30 ºC at "normal" physiological temperatures. There the enzyme activity-temperature curve has its mathematical maximum; in biology, however, one speaks of an optimum here.

For more details see also: "Influencing the tertiary structure" in the section on proteins and "Denaturation" on my nutritional science pages.

Enzymes are temperature dependent. If the temperature is too low, the cell freezes; if the temperature is too high, the proteins denature. Therefore, every enzyme has a temperature range in which it works optimally - i.e. at maximum speed.

pH dependence

A similar experiment can now be carried out to determine the dependence of the enzyme activity on the pH value. The same concentration of glucose is added to test tubes with different pH values ​​between 3 and 12. Here, too, an optimum curve can be observed when evaluating and displaying the measurement results.

Enzymes are pH dependent. If the pH values ​​are too low or too high, enzymes no longer work optimally because they are denatured. Every enzyme has a pH value at which it works optimally, the pH optimum.

Explanation

Why this pH dependency exists is explained very nicely on the protein pages, so there is no need to repeat the explanations here. Please visit the corresponding protein pages in the cytology section of this homepage or the "Denaturation" page on the nutritional science pages.