Metabolism & Enzymes
What I should know
• There are 2 types of metabolic pathwaysAnabolic and catabolic
• the pathways - can have reversible and
irreversible steps and alternative routes.
Cell metabolism
• Is the collective term for all the biochemical
reactions that occur in a living cell
• Reactions are turned on/off or sped up/slowed
down according to the cell's immediate needs and
overall functions
• the numerous pathways involved in building
up and breaking down cellular components
must be monitored and balanced in a
coordinated fashion
• To do this, cells organise reactions into
various enzyme-powered pathways
Types of Metabolic Pathways
• Metabolic pathways are chains of enzyme
catalysed reactions in which the product of one
reaction is the substrate for the next
• In some cases these steps can be reversed,
whereas others are irreversible
• Anabolic & Catabolic Pathways
• Reversible & Irreversible Pathways
Anabolic Pathways
• Synthesis reactions (building up)
• Requires energy in the form of ATP to proceed
• E.g. in DNA replication: DNA polymerase adds
free nucleotides to the end of the newly-forming
DNA, causing elongation (building up) of the new
strand
• E.g. Amino acids -----> Protein
Catabolic Pathways
• Breakdown reactions
• Involves the release of energy to form ATP
• E.g. Hydrogen peroxide (harmful) is broken down,
by catalase in the liver, into Oxygen & water
(harmless)
• E.g. Proteins ---> amino acids
Two types of metabolic pathways
• Anabolic pathways
require energy and are
involved in the
biosynthesis of complex
molecules from simpler
molecules
• Catabolic pathways
release energy and
involve the breakdown
of molecules
Irreversible/Reversible Pathways
• Glucose diffusing into a cell from a
high conc outside to a low conc
inside is irreversibly converted to
intermediate 1 by enzyme A
Glucose
Enzyme A
Intermediate 1
Enzyme B
• Advantage: maintains a low conc of
glucose inside the cell, promoting
the entry of glucose
Intermediate 2
Enzyme C
Intermediate 3
Many
enzymes
Pyruvate
Irreversible/Reversible Pathways
• If more intermediate 2 is
produced than is required, some
can be converted back into
intermediate 1, (reversible)
• It can then be used in an
alternative pathway
• E.g. to build glycogen (in animals)
or starch (in plants)
• Alternative routes also allows
steps in a pathway to be bypassed
e.g. sorbitol
• The reversible and irreversible
steps allow this pathway to be kept
under strict and precise control
Glucose
Enzyme A
Intermediate 1
Enzyme B
Intermediate 2
Enzyme C
Intermediate 3
Many
enzymes
Pyruvate
Do you know ?
• There are 2 types of metabolic pathwaysAnabolic and catabolic
• the pathways - can have reversible and
irreversible steps and alternative routes.
Control of Metabolic Pathways
What you should know
• Metabolic pathways are controlled by the presence or
absence of particular enzymes in the metabolic pathway
and through the regulation of the rate of reaction of key
enzymes within the pathway.
• Regulation can be controlled by intra- and extracellular
signal molecules.
• Induced fit and the role of the active site of enzymes
including shape and substrate affinity.
• How enzymes affect activation energy.
• The effects of substrate and end product concentration
on the direction and rate of enzyme reactions.
• Enzymes often act in groups or as multi-enzyme
complexes.
• Control of metabolic pathways through competitive, noncompetitive and feedback inhibition
Enzymes
• Enzymes are biological catalysts
which are essential to the
maintenance of life
• They form an enzyme-substrate
complex that accelerates the
rate of reaction
• Molecules of a substrate have
an ‘affinity’ (chemical
attraction) for the enzyme’s
active site
Enzyme Properties
• Enzymes are globular proteins
• They possess a small region
called the active site where
the reaction occurs
• Enzymes are specific in the
reaction that they catalyse
• Enzymes are only required in
small amounts and remain
unchanged at the end of the
reaction
Activation energy
•The energy
required to break
chemical bonds in
the reacting
chemicals is
called the
activation energy
•Enzymes lower
the activation
energy
Induced Fit
• Substrate molecule induces a slight change in the
shape of the active site
• This allows the substrate molecule to fit perfectly
and change to its working conformation
• The change in shape of the active site facilitates the
reaction
Metabolic Pathways and Enzymes
• Some enzymes are associated with
other enzymes involved in a particular
pathway to form multi-enzyme
complexes
• In reality, DNA/RNA polymerase
aren’t just single enzymes.
• Rather, they are massive multienzyme complexes possessed of
multiple catalytic activities
Control by Enzymes
• Metabolic reactions are controlled by the
presence or absence of particular enzymes in
metabolic pathways
• If appropriate enzyme is present= reaction
continues
• If appropriate enzyme is absent= pathway stops
Example of control by genes
• Some organisms have the ability to turn on/off
genes which code for enzyme production (called
‘enzyme induction’)
• It is known to occur in humans, but the process is
very complicated
• A good example is E.Coli, a bacteria which have the
ability to turn on/off a gene to produce an enzyme
in order for them to undergo respiration
• Please look at pages 86-87 of the TB and we’ll make
notes on this example
• Please copy Fig. 6.18 into your notes.
• Underneath the diagram, make notes on
the ‘Absence of Lactose’ and the
‘Presence of Lactose’
• What benefit is there to the E.Coli?
The Effect of Substrate and End
Product Concentration
FACTORS AFFECTING ENZYME
ACTIVITY
•
•
•
•
•
Temperature
pH
substrate concentration
enzyme concentration
inhibitors
INCREASING SUBSTRATE
CONCENTRATION
• Increasing substrate conc increases rate of reaction
as more active sites become occupied
• This continues until all of the active sites become
occupied
• further increases in substrate conc make no
difference to the rate of the reaction thereafter
• If displayed on a graph, the
line would increase, to a point,
where it would then level off
* See also fig. 6.12 in TB
Intra/Extracellular Signals
- Intracellular
signal molecules
are those that
effect the cell’s
metabolism &
originate from
within the cell
- Extracellular
signal molecules
effect the cell’s
metabolism, but
come from the
cell’s environment
Regulating Enzyme activity
A cell has to control enzyme activity. It does this by:
• Controlling number of enzyme molecules in cell
• Keeping reactions (and enzymes) in compartments
eg. Mitochondria or lysosome.
• Changing enzyme shape (most effective)
Enzyme activity
• Enzymes are always present due to constant
expression. This means they need to be
controlled.
• Enzymes can be inhibited by other things binding
to them
Competitive and Non-competitive
Inhibition
Inhibition
•Competitive –
A molecule
with similar
molecular
shape to the
substrate
competes for
the active site
and reduces
the
concentration
of available
enzyme
•Noncompetitive –
A molecule
binds to the
enzyme at
another site;
it changes
the
conformation
of the
enzyme and
its active
site. Thus,
catalytic
efficiency is
reduced.
Control of a Pathway using its
enzymes
• Metabolic pathways can be controlled by the
switching on or off of the first enzyme in the
pathway
• If the first enzyme is switched off the rest of the
pathway stops due to the lack of intermediates
• The first enzyme can be inhibited by high levels of
the final product from the pathway
End-Product Inhibition
• Used in the control of metabolic pathways
• The end-product of the pathway inhibits the activity
of the first enzyme in the pathway
• This is energetically efficient as it avoids the
excessive production of the intermediates of a
pathway
Do you know ?
• Metabolic pathways are controlled by the presence or
absence of particular enzymes in the metabolic pathway
and through the regulation of the rate of reaction of key
enzymes within the pathway.
• Regulation can be controlled by intra- and extracellular
signal molecules.
• Induced fit and the role of the active site of enzymes
including shape and substrate affinity.
• How enzymes affect activation energy.
• The effects of substrate and end product concentration
on the direction and rate of enzyme reactions.
• Enzymes often act in groups or as multi-enzyme
complexes.
• Control of metabolic pathways through competitive, noncompetitive and feedback inhibition