MadSci Network: Biochemistry

Re: The effect of temperature on the reaction between pectinase and apple?

Date: Tue Feb 19 08:09:54 2002
Posted By: Thiam Hock, Tan, Secondary School Teacher, Science, Dunman High School
Area of science: Biochemistry
ID: 1012841125.Bc

Dear Hannah:

I understand how you feel. When I teach this lesson, my students often 
have trouble with this topic. However, it can be overcome. 

Enzymes are organic (i.e. carbon-containing) molecules, that are large and 
complex. They are usually proteins (i.e. they are made up of long chains 
of smaller molecules called amino acids, linked together by so-called 
peptide bonds, to form either large bundles or sheets), and catalyse 
(speed up the rate of) certain reactions, without getting affected by the 
reaction themselves. I.e. they help shove the reaction along but don't 
participate in it themselves. Enzymes are very specific, and a certain 
enzyme can only cataylse a certain type of reaction, and would be useless 
with another set of reagents. 

In this case, you are using pectinase. Pectin is another organic substance 
that is present in the cell walls of the apple, and along with cellulose, 
help give it rigidity and structure. When you make jams, perserves, and 
certain kinds of jelly (jell-o) by boiling fruit and cooling the 
concentrated juice, the gelatinous texture is actualy due to the pectin. 
What pectinase does is to speed up the break-down this pectin into simpler 

Now, there are many factors affecting the rate of reaction of an enzyme. 
To ease your pain quickly, I'll start with temperature, but if you wish 
you could read on about the rest:

1. Temperature
As you know, you can speed up most chemical reactions by increasing the 
amount of energy available to the reactants, especially by heating them. 
This is because as the temperature increases, the motion of the atoms and 
molecules increases too, and they collide with each other more frequently 
and with more force, hence increasing the rate of reaction. The same with 
enzymes. At low temperatures, they do not function well, and are 
'sluggish'. However, if you gradually increase the temperature, the rate 
of reaction increases too, by about twice for every ten degrees celsius 
(centigrade). However, there reaches a maximum point where the rate of 
reaction which the enyzme catalyses is at its maximum. This is known as 
the optimal temperature. Beyond this point, the rate of reaction falls 
sharply. Why? Because the enzyme is getting denatured. Denaturation is 
when the enzyme loses its structure and starts to fall apart because the 
temperature is too high. An example of denaturation is the cooking of egg 
white. The proteins present in the egg white are denatured when heated and 
cooked, and hence undergo many changes (in texture, colour, and even 
taste). Hence, if you draw a graph of the rate of reaction (or enzyme 
activity) versus the temperature, it will gradually rise and reach a peak 
before falling off quite rapidly. The enzymes in our body work best at 
body temperature, and hence if you cool the body too much, or heat it too 
much, there will be unpleasant consequences when the enzymes vital to our 
body's function do not work as well.

2. pH
Some enzymes require high pH (alkaline conditions), while some require low 
pH to function (acidic conditions). For example, the enzymes in your 
stomach require the presence of hydrochloric acid (HCl) to be converted 
into the right form to actually be able to do its work. (e.g. pepsinogen 
is converted into pepsin by HCl. Pepsinogen doesn't do anything, while 
pepsin breaks down the proteins in the food you eat) Like temperature, 
enzymes have an optimum pH at which they function best. At pH levels above 
and below this, they do not work as well. Hence, the graph of rate of 
reaction against pH will also be like a camel's hump, and the tip of the 
hump is pH where the rate of reaction is the highest. Too high or too low 
a pH will also cause a denaturation of the enzyme. An example would be how 
milk curdles when mixed with acidic substances (like sour juices or 
vinegar), and the proteins in the milk denature and change their 
properties and appearance.

3. Concentration
Concentration that affects the rate of reaction is both the concentration 
of the enzyme and the substrate (the substance the enzyme works on). By 
increasing the concentration of either of them in the reaction, you 
increase the chances that (a) molecule(s) of substrate will bump into a 
molecule of enzyme and the reaction will take place. Hence, by increasing 
concentration, the rate of reaction increases too. However, it does not 
shoot off to the stratosphere. There are limits too. There reaches a point 
after which no increases in concentration will do any good. This is the 
'saturation point'. This is when all the molecules of enzyme are occupied 
with their business (or the other way round), and there is simply too much 
molecules or substrate or enzyme and adding more won't make any difference 
to the reaction. Hence, the graph of rate of reaction against 
concentration of substrate(s) or enzyme will be like a slope going up 
before hitting a plateau, and just flattening out.

4. Co-enzymes
Co-enzymes are substances that are not enzymes themselves, but work 
together with enzymes to cataylse a reaction. Examples of these include 
certain vitamins like B-complex vitamins. Not all enzymes require co-
enzymes, though.

I hope that this has been enough information for you to digest. Good luck 
with your work! Feel free to ask the Mad Scientists if you need any more 

Tan "Amylase" Thiam Hock

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