Lesson 3: Metabolic Pathways Flashcards
Metabolism
Metabolism is the integrated
network of all the biochemical
reactions of life.
Metabolic pathways are the chemical
processes that occur in all cells that
maintain life.
Cellular respiration and
photosynthesis are two key
metabolic pathways
Metabolic Pathways
Metabolic pathways consist of enzyme catalysed chains or
cycles of reactions.
Simple metabolic pathways involve a series of steps, each
controlled by an enzyme, which convert substrates into a
final product.
Enzymes
Enzymes are proteins that act to speed up
chemical reactions by lowering the
activation energy required for the reaction
to occur.
Enzymes are not used up in the reaction.
Metabolic reactions that occur in living
things must occur at body temperature,
which is never high enough for activation
energy.
With the use of enzymes, biological
reactions release more energy than they
require for activation and are said to be
exothermic.
In the structure of every enzyme is a specially shaped
region called the active site.
Substrates bind to the active site of an enzyme because
their shapes are complementary. This is called the enzyme
– substrate complex.
The Induced – Fit Model
Developed by Daniel Koshland in 1958.
The lock and key model of enzyme action cannot account for the binding
and simultaneous change that is seen in many reactions.
It also does not account for the fact that some enzymes can bind with
multiple similarly shaped substrates.
This model states that the substrate induces a slight change in the actives
site so it can fit perfectly.
Factors Affecting Enzyme Action
Temperature
In the human body the optimal temperature
for reactions is 37˚C. Deviations from this affect the reaction rate.
At low temperatures the molecules of
enzyme and substrate are moving slowly, therefore, fewer collisions occur per unit time.
At high temperature the atoms within the enzyme molecules are moving more energetically causing a strain on the bonds that hold the atoms together. Eventually they break and the enzyme is denatured and no longer functional.
Factors Affecting Enzyme Action
Temperature
pH
Not all enzymes have the same
optimal pH.
Enzymes are affected by pH because
the amino acids that make up the
molecule contain positive and
negative regions around the active
site.
Excess H+ or OH- can lead to
bonding in the charged areas which
could affect the matching process.
Factors Affecting Enzyme Action
Concentration of Substrate
Reactions are a product of collisions.
The greater the concentration of the
substrate the more collisions per unit
time that can occur.
This increases the rate of the reaction
to a limit determined by the amount
of enzyme present. Once this limit is
reached increasing substrate
concentration has no effect on
reaction rate.
Inhibitors
Enzyme inhibitors are substances that reduce or prevent an
enzymes activity.
There are two types of inhibitors:
Competitive
Non-competitive
Competitive Inhibitors
Have a structure similar to that of the substrate that would normally bind
with the enzyme.
They compete with the substrate to occupy the active site of the enzyme and prevent the substrate from binding.
The inhibitors do not affect the enzyme and do not form products so they tend to remain in the active site.
The rate of reaction is decreased because substrate cannot enter the active site of the enzyme.
At high concentrations of substrate the effects of inhibition are decreased as the substrate can outcompete the inhibitor.
Non-competitive Inhibition
Also combine with enzymes but not at the active site.
They bind to another part of the enzyme where they either
Partly block access to the active site
Cause a change in the shape of the enzyme so that the substrate cannot
enter the active site.
Increasing the concentration of the substrate has no effect on inhibition
Control of Metabolic Pathways
Metabolic pathways are often controlled by end-product inhibition.
This process uses the end product of the pathway to inhibit an enzyme in
that pathway.
May be competitive or non-competitive. However in most cases it is non-competitive.
This prevents over production.
Overproduction wastes energy or could be toxic.
Enzymes that are acted upon by a non – competitive
end-product inhibitor are known as allosteric enzymes.
The product is called an allosteric inhibitor and binds to the allosteric site.
An example of end-product inhibition is found in the pathway of threonine being converted to isoleucine.
Negative Feedback
As the end product begins to accumulate the inhibitory effects become
more prevalent and production decreases.
When the end products starts to be used up, the inhibitory effects are
reduced and more product can be created.
