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A Level Biology: Enzymes 3 Ð Factor Affecting Rate
Hi! Welcome to my third video on the series about Enzymes. Today, we are going to be looking
at rates of reaction and how different conditions are going to affect enzyme-controlled reactions.
One of the first things that you could do to measure the effect of the enzymes is you
can compare an uncatalysed reaction with a catalysed one and then see how quickly it
happens with or without. We know that enzymes can increase the rate by which something happens
between 108 and 1026. There is a huge difference and if you think that in terms of the reactions
happening inside your body, 108 is essentially a hundred million. It is such a huge number.
106 is a very, very big number and so the increase in rate is enormous. None of the
reactions in your body will be happening at all without enzymes.
We got a graph showing what happens when you increase the substrate concentration and what
effect it has on the rate. So substrate concentration means the things that are being broken down
or things that are being put together. If we use an example of a protease which breaks
done proteins, the substrate in this instance would be protein. You would expect that the
more protein present, you would expect to go up in rate.
WhatÕs happening here at lower substrate concentrations, there are some active sites
still free and there are some enzymes still free to then react with some of the substrates
and therefore, thatÕs why the rate is still going up. Approaching the maximum rate of
reaction, then whatÕs happening is virtually all the enzymes are being engaged with a substrate
and therefore whatÕs happening here is you get a leveling off of this rate of reaction.
So whenever doing an enzyme-catalysed reaction, measuring the rate of reaction, this is your
independent variable, this is the one that you would change and the thing that you are
measuring is the rate. What to measure is the initial rate, by initial we are talking
in the first few moments of the reaction happening, and the reason for that is what you donÕt
want to occur is any other factor that could affect the results to kick in and to cause
an effect. If you are measuring the rate up here, letÕs say if you are altering substrate
concentration, for instance, the temperature may have an effect on that due to the reaction
giving off heat. So we always measure the initial rate of the reaction at the start
of the reaction so no other acts or variable can have an effect.
One thing that can affect enzyme-controlled reaction is temperature. What we have to measure
the effect of temperature is we have this temperature co-efficient called Q10 and it
is the rate of reaction X+10?C divided by the rate of reaction of X. So values between
0-40?C - that gives us the value of 2.
To really to understand, you need to look a temperature and rate of reaction graph.
Here, what we have is the temperature and the rate of reaction at the right. We have
a line here that shows the rate of reaction and how that differs at different zones. We
have here the optimum temperature for this reaction. If we think back to the co-efficient
that we just looked up, essentially what it is showing is the rate doubles every 10?C
that you increase by. So our formula is the rate of reaction of X+10 divided by the rate
of reaction at X. What you expect going up to 40? is the rate of reaction at X+10 to
be greater or to be doubled in effect with the rate of reaction X would be.
To demo this, we look at our values here. If this is our value for 10?C and this is
our value for 20?C, just pick some arbitrary figures for rate of reaction here. You notice
that the rate here is 2 and the rate here is 4. So corresponding back to these values,
our rate of reaction at X +10, that corresponds to the 4 and then this corresponds to our
value for X. So 4 divided by 2 gives us the value of 2. So therefore, up to 40?, you would
expect the rate or the Q10 co-efficient value to be 2.
Now once the enzyme is denatured, it canÕt catalyse the reaction properly so what you
would expect to happen to this co-efficient is not really valid past the optimum temperature.
So therefore, you are certain to get values less than 1. Therefore, that would show that
the enzyme is denatured.
There are some bacteria that have enzymes with the optimum rate of 5, so this isnÕt
usually set, there isnÕt a set temperature that all enzymes were best at, but you can
expect the temperature of which they work best that usually in humans to be the body
temperature of 37?.
Another thing that can affect how well enzymes work is the pH. We have three different here:
the pepsin, trypsin and cholinesterase. You can see here that in these cases, that pepsin
works best at low pH. Trypsin and cholinesterase, their optimum is much, much higher around
7. We can assume therefore that pepsin works best in an acidic environment and you would
expect it to work in something like the stomach. Trypsin and cholinesterase works best in a
kind of neutral environment, so you would expect to find that in other tissues where
the pH is quite neutral.
Obviously, if you would put pepsin in a similar condition with trypsin, then it wouldnÕt
work effectively and vice versa. If I would put trypsin and try to make it useful in the
stomach, it wouldnÕt work because it would denature.
What actually happens when an enzyme is denatured is some of the bonds that hold together the
active sites, some of the hydrogen bonds holding together the tertiary structure will break
down and therefore, the enzyme would be ineffective because it wouldnÕt have a shape that is
complementary to the substrate and therefore, no longer work.
If you join me in the next video, I will be looking at entrance and exam questions.
[end of audio Ð 06:42] A Level Biology: Enzymes 2 Ð Factor Affecting
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