Download Page 1 Enzymes OK….so now we`ve done all of that Chemistry stuff

SICM Tuition
Biology AS
Enzymes
OK….so now we’ve done all of that Chemistry stuff that you all love so much...let’s get
down to the real stuff…
Having just learnt about proteins, let’s now look at one of the fundamental substances
within the body. Enzymes are used for almost all chemical reactions in the body. But what
are enzymes and how do they work?
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Enzymes are catalysts. They “react” with substrates to form products
o What is a catalyst? (We will look at how some work later)
A catalyst is a substance that speeds up the rate of a reaction (by lowering the
activation energy) but it remains unchanged at the end of the reaction.
Without catalysts, 37˚C would be too slow to sustain life. We will look at how
different conditions affects enzymes later
An organism’s metabolism consists of thousands of different reactions and
each one has a different catalyst or enzyme
Metabolism consists of hundreds of reactions linked together where the
product from one reaction is the substrate of the next
So what is an enzyme?
Enzymes are proteins
Proteins are made up of amino acids
From this, we know that there is a wide range of possible structures that could
form (depending on the primary, secondary, tertiary structures)
There is, therefore always going to be an area that is only specific for certain
substances
This area is called the active site
It is where the enzyme comes into contact with the substrate
o The substrate is the substance that will react with the enzyme to for the
product
The way in which this works is still being debated, but there are two possible
hypotheses that you need to know about:
Lock and Key Hypothesis
during catalysis, the substrate molecule fits into the active site and interacts
with the amino acids by ionic and hydrogen bonding forming an ENZYMESUBSTRATE COMPLEX (yes…that’s in capitals and bold for a reason!
Learn it!)
A reaction occurs and the product leaves the active site
The precise shape of the active site must be complementary to the shape of the
substrate.
Active site
Enzyme
Enzyme-Substrate
complex
Substrate
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SICM Tuition
Biology AS
Induced Fit hypothesis
This is a more recent version of the mechanism
It states that the enzyme’s active site does not “fit” the substrate until the
substrate actually enters the site
The shape of the enzyme then moulds around the substrate
This then forms an ENZYME-SUBSTRATE COMPLEX (yes yes…that’s
the second time…)
Active site
Enzyme-Substrate
complex
The change in the active site brings the amino acids into their correct positions in the active
site so a reaction can occur. The active site then returns to its original shape
Enzyme
Substrate
Enzyme reactions are reversible
The enzyme will catalyse the reaction equally well in either direction
This refers to catabolic and anabolic reactions
o What do these words mean?
Catabolic reactions are the breaking down of molecules into smaller units.
Anabolic reactions are the building up of structural units from smaller units.
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The direction depends on the concentration of the substrate and product and
will go in the direction of the low concentration
An enzyme has no effect on the point of equilibrium: it just affects the time
taken to reach the equilibrium
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