Lesson 3: Review: 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.
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.
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 form binding.
The inhibitors do not effect 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.
