Enzymes [2]: Lock and Key and Induced Fit (A Level Biology)


Uploaded by freeeschool on 17.10.2012

Transcript:
A Level Biology: Enzymes 2 – Induced Fit and Lock & Key Theories
Hi! Welcome to my second video on the series about Enzymes. Today, we are going to be looking
at Induced fit and the Lock-and-key hypothesis.
The last time, we looked at a few key terms and so one of the first ones was anabolic
reaction. In an anabolic reaction, enzymes are used to bring substrate molecules together
for form bonds between them.
In a catabolic one, they do the opposite of that. They break bonds between molecules and
therefore, they are used to break things down.
The Lock-and-key hypothesis assumes that the active site of the enzyme is rigid in shape.
This is our enzyme and this is the section where the substrate will bond is called the
active site. It is the bit where all enzymes bond to substrates.
It used to be the case where we thought that these active sites were rigid and therefore,
didn’t change their shape at all. So only a very, very specific-shaped substrate could
fit in. But current tests have shown and from looking at other top forms of data, we found
that doesn’t always necessary ring true.
The best current hypothesis or the best current idea that we have for this is called the Induced
fit hypothesis. That’s where the active site could be flexible and so what will happen
is our substrate will bond in and in this case, this an anabolic reaction. What will
happen is the active site will physically change shape to suit the shape of the substrate
and hence, the induced fit. What will happen when the enzyme leaves the active site, the
enzyme will revert back to its inactive state.
For a run of that again, you’ve got essentially an Induced Fit hypothesis whereby the active
site doesn’t necessarily fit the shape of the substrate. In an anabolic reaction here,
what happens is the active site will basically fit around it and therefore it is an induced
fit. Then once the substrate leaves, it reverts back to its original shape.
This diagram here shows us an example of an enzyme Lipase. Lipase breaks down fats. The
active site of this is specifically down to the tertiary structure of the protein. It
is the folding in from the secondary structure that gives this a very, very specific shape.
Initially, it is the order of amino acids and then the secondary structure and how that
folds in to form a tertiary structure that means only certain substrates could fit. This
is very, very specific when alone you break down certain types of fat. If you want an
enzyme to break down protein or proteate, then that will have a slightly different active
side and will have a very different tertiary structure.
Some of the important things to do with enzymes: They only change the rate of reaction. Rate
means how quickly it is happening. They don’t change the equilibrium or the end-products.
They have no effect on the type of product that is made. For instance, if you have the
breakdown of starch into glucose, the enzyme won’t change the actual end-product. It
will only speed up the reaction and they’re specific to one particular reaction. This
is down to their specific tertiary structure. They are present in very, very small amounts
due to high molecular activity and that means you need a very, very small amount of them
to get this huge marked effect.
This value here, the turn-over number is a number of substrate molecules transferred
per minute by one enzyme molecule. In this case, this is Catalase; amongst the quickest
enzymes we know. It is made by the liver and this should be 6x106. They are basically 6
followed by 6 so that’s 6,000,000 per minute, which is a staggering number.
If you join me on the next video, we will be looking at the rates of reaction and how
enzymes can be affected by different conditions.
[end of audio 4:15] A Level Biology: Enzymes 2 – Induced Fit
and Lock & Key Theory Page…2