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?
-
-
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
Page 1
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.
-
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
Page 2
SICM Tuition
Biology AS
Energy changes during a reaction
-
In a reaction, bonds are broken and formed
Breaking bonds requires energy (endergonic)
Forming bonds releases energy (exogonic)
-
ALL reactions begin with breaking bonds
The energy needed to do this is called the activation energy
-
All catalysts lower the activation energy for the reaction that they catalyse
The enzyme-substrate complex (3rd time!) allows the initial breaking of bonds
in the substrate to take place more easily
Draw a graph to show this information:
Intermediates
activation energy
with catalyst
activation energy
without catalyst
Activation energy is the
energy required to break
initial bonds and to start
the reaction
Reactants
(normally given as formula)
Energy
Products
(normally given as formula)
Reaction Pathway
Factors affecting enzyme action:
temperature
pH
concentration of the enzyme
concentration of substrate
presence of inhibitors
We will look into all of these in detail soon…
Page 3
SICM Tuition
Biology AS
Temperature
As temperature increases, the kinetic energy increases (Physics…which I
know almost none of you do because of how much you hate it…but it’s basic
Physics so it shouldn’t really be a problem)
More movement of substrate and enzyme due to the increased kinetic energy
More successful collisions (with activation energy and correct orientation)
More enzyme-substrate complexes are formed!!
So…as temperature increases, the rate of the reaction increases….
BUT
Beyond a point, higher temperatures change the shape of the enzyme molecule
-
Enzymes are globular proteins
Their tertiary structure is maintained by the bonds between the amino acid Rgroups of a polypeptide chain
o Do you remember what an amino acid looks like? Draw one…
R
R is a variable group – it
H
varies
with each amino acid
O
NH2 is the functional
N C C
group for an amine
COOH is the functional
group for a carboxylic acid
OH
H
H
High temperatures cause these bonds to break and so the tertiary structure is
damaged
If the 3D shape of the enzyme is damaged, then the substrate molecule cannot
bind to the active site and so the rate of the reaction will decrease
This process is irreversible and is called denaturation
-
Taking into account denaturation and the kinetic energy, there will be what is
known as the optimum temperature
o This is where the reaction will take place most rapidly
Draw a graph to show the affect of temperature on an enzyme. Explain what happens
at each stage.
Optimum temperature – enzymes and
substrates work at fastest rate.
Rapid increase in rate of reaction. As
temperature increases, the
enzyme/substrate molecules have
more kinetic energy so form complex
molecules more quickly.
C
B
D
Very slow rate of reaction.
Enzymes have little kinetic
energy and so they for enzyme
substrate complexes very
slowly.
Enzymes begin to be denatured. This happens
relatively quickly – over about 10ºC. Bonds in
the protein structure of enzyme are broken.
Active site loses its shape and is unable to fit the
substrate. Denaturation is permanent.
A
37
-273
Temperature (ºC)
Optimum temperature for enzymes vary, but many are around 40ºC.
Page 4
45