Download Enzymes - Kevan Kruger

Cell Metabolism - Enzymes
A)
Metabolism
1) Definition: Metabolism is the constantly occurring chemical reactions
that take place in a cell.
2) These chemical reactions occur in organized sequences from reactants
to end products.
3) This organized sequence of reactions is known as a metabolic pathway.
Example:
METABOLIC PATHWAY

A
Reactant
B


C
(Intermediate products)
D
Final product
B) Enzymes:
1) An enzyme is a catalyst (a substance that speeds up a reaction without
being consumed). Enzymes are proteins and are reusable. They work
in low concentrations and speed up the reaction rate.
Carbonic anhydrase (enzyme)
CO2
+
H2O
H2CO3
2) Each step of a metabolic pathway is controlled by one or more specific
enzyme(s). Enzymes allow these reactions to proceed at lower
temperatures than they would normally occur.
3) The reactant(s) that an enzyme acts upon is known as the substrate(s).
Enzymes work by forming a very temporary complex with the substrate.
The shape of the substrate changes, and it becomes more reactive.
Example (see p.103):
Enzyme + Substrate  Enzyme-Substrate Complex  Enzyme + Product
C)
Enzyme Structures
1) Enzymes are large globular proteins with very specific 3-D shapes
(tertiary structure).
2) Enzymes have grooves (or pockets), which may contain chemically
functional groups. These areas are called active sites and this is where
the substrate attaches.
LOCK AND KEY THEORY OF ENZYME ACTION
A. In order for a reaction to occur, the reactants (substrates) must be
brought close together.
B. The substrates bond to the active site on the enzyme, and are
brought close together. Sometimes the active site changes shape to bring the
substrates together.
C. The reaction occurs and the product(s) are released. The enzyme
goes back to its normal tertiary configuration. (shape)
COMPLEX
Review of LOCK AND KEY
According to this analogy, an enzyme acts like a key by combining with a
specific substrate and “unlocking” the substrate for further activity of the cell.
This is a useful analogy because the key (enzyme) must have the correct shape to fit
the lock (substrate). After the lock has been opened (reaction takes place) the key
(enzyme) is freed and unchanged so that it may be used repeatedly in the same
manner. The portion of the enzyme that is involved in the reaction is called the
active site.
3) When the substrate attaches to the enzyme, this causes stress in the
substrate, which will cause:
a) a substrate to break apart (in a degradative reaction). Another
word for this is CATABOLISM: breaking big molecules in to
smaller ones.
b) two substrates to form a bond (in a synthetic reaction). Another
word for this is ANABOLISM: making big molecules from
small.
Note: ANABOLISM and CATABOLISM make up METABOLISM.
4) Specific groove shapes and chemical groups in an active site means that
enzymes can only bond with one specific substrate or reactant.
D)
Enzyme Action
1) Enzymes operate by lowering the energy of activation needed for a
reaction to occur.
2) Usually, heat is used to speed up chemical reactions by increasing the
number of collisions that occur between reactants. Excessive heat,
however, destroys the tertiary structure of protein (denatures) and
therefore cannot be used to speed up reactions within living organisms.
3) Enzymes act as a CATALYST and are not consumed in a reaction.
This means that they can be used over and over again.
E)
Factors Affecting Reaction Rate With Enzymes
1) Concentration: The amount of enzyme and/or substrate available to
react can affect enzyme activity.
a) The reaction speeds up if there are high concentrations of
substrate, and it slows down if the [substrate] are depleted
Rate of Reaction
(product per unit of
time)
Time
b) The reaction speeds up if there are high concentrations of
enzyme, and it slows down if the [enzyme] are depleted.
Rate of Reaction
(product per unit of
time)
Time
2) Temperature: As temperature rises moderately, enzyme activity
generally increases (according to the kinetic molecular theory). At a
certain point, however, the enzyme becomes denatured (changes shape)
by the increase in temperature and no longer functions properly. All
enzymes have an optimal temperature (body temperature).
Rate of Reaction
(product per unit of
time)
37C
3) pH: The 3D shape of an enzyme can be affected by pH and all enzymes
have an optimal pH to work at (body pH, 6-8).
Rate of Reaction
(product per unit of
time)
pH 7
4) Inhibitors: Chemicals that interfere with the enzyme action. A
metabolic product can feedback on a metabolic pathway to control how
much product is made.
Example:
A

enzyme
#
1
B

enzyme
#
2
C

enzyme
#3
D
(ie: Thyroxin)
Product ‘D’ can temporarily attach to an active site on Enzyme #1. The
enzyme will be denatured and will not bind to reactant A. This is an
example of NEGATIVE FEEDBACK or FEEDBACK INHIBITION,
and it will stop the reaction.
When the concentration of the product F gets low again, the inhibitor (F)
will fall away from enzyme #1, and the the metabolic pathway is
reactivated. The enzyme #1 is free to react with substrate A again.
There are two types of feedback inhibition:
a) Competitive Inhibitors are chemicals that so closely resemble an
enzyme’s normal substrate that it can attach to the enzymes
active site.
If the inhibitor occupies the active site of the enzyme, the reaction
will be stopped.
If the Inhibitor is removed, the enzyme will become active again.
Example:
SUBSTRATE
or
SUBSTRATE + Inhibitor
Enzyme (E)
Enzyme-Inhibitor Complex
(EI)
PRODUCT
No Reaction!
No Product!
b) Non-competitive inhibitors are atoms or molecules that attach to
an enzyme at an allosteric site (not the active site) and this
denatures the enzyme.
Non-competitive inhibitors will destroy an enzyme by permanently
binding to the allosteric site. An example of this is heavy metals,
such as lead, in the nervous system.
Example: Feedback Inhibition and Control of Metabolic Rate
Thyroxin, the hormone that controls
the metabolic rate of all of the cells in
your body, is produced by the thyroid
gland in the neck. If the concentration
Pituitary Gland
of thyroxin in your body is high, your
metabolic rate will be raised, and if
thyroxin levels are low, your metabolic
TSH
rate will be low.
The thyroid gland is stimulated to
release thyroxin by a hormone produced
Thyroid Gland
in the pituitary gland called TSH
(thyroid stimulating hormone). But the
enzymes in cells of the pituitary that
Thyroxin
make TSH are inhibited by thyroxin.
Therefore, if thyroxin levels are high,
the pituitary stops producing TSH,
Body Cells
and if thyroxin levels are low, the
(Metabolic Rate)
pituitary makes the TSH.
Thus, the metabolic rate of cells in
your body are maintained by the
feedback inhibition of an enzyme
F)
Co-enzymes
Many enzymes are made up of 2 pieces.
1) The APOENZYME – the protein portion (inactive)
2) The CO-ENZYME – a non-protein portion
When these join together, the enzyme becomes active. The
Co-enzymes are often large molecules that the body cannot make on its
own. Most co-enzymes are VITAMINS which we get from food.
G)
Cell Energy - ATP
Most cell reactions (metabolism) require energy to occur. The energy
‘currency’ of cells is a molecule called ATP (Adenosine triphosphate)
1) Structure: made up of the nucleotide adenosine with 3 phosphates, the
last two of which are held together by a high energy bond.
2) It takes energy to make this phosphate bond (~P), and energy is
released when the bond is broken.
A – P ~ P ~ P
ATP (Adenosine Triphosphate)
A – P ~ P
+
P
+
Energy
ADP (Adenosine Diphosphate)
3) Where does the cell get ATP? It earns it from breaking food molecules
apart and using the energy that is released to make ATP. This is what
occurs when glucose is used during cellular respiration.
C6H12O6 + 6O2 + ADP  6CO2 + 6H2O + ATP (energy)
4) When the cell needs energy for metabolism, it “cashes in” ATP, a
metabolic reaction occurs, and the cell receives ADP as “change”.