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Higher Human Biology
Unit 1
Human Cells
KEY AREA 6: Metabolic Pathways
Human Cells Learning Intentions
KEY AREA 6 – Metabolic Pathways
1. Anabolic Pathways
2. Control of Metabolic Pathways
6a) Metabolic Pathways

Metabolism can be defined as all the chemical reactions that occur within the human
body

Metabolic pathways involve the building up and breaking down of molecules

Anabolic pathways require energy and involve biosynthetic (building up) processes e.g.
building up of amino acids to form proteins

Catabolic pathways release energy and involve the breakdown of molecules e.g.
breaking down of glucose in glycolysis

Anabolic and Catabolic pathways can have reversible and irreversible steps, and
alternative routes
6b) Activation Energy
 The rate of a chemical reaction is indicated by
the quantity of chemical change that occurs per
unit time
 Chemical change may involve joining together
simple molecules to form more complex
molecules (Anabolism) or may involve splitting of
complex molecules into simpler ones
(Catabolism)
 The energy needed to break chemical bonds in
the reactant chemicals is called the Activation
Energy
 The bonds break when the molecules of the
reactant have absorbed enough energy to make
them unstable (this is called the Transition
State) and the reaction can occur
 This energy input often takes the form of heat
energy and the reaction only proceeds at a high
rate if the chemicals are raised to a high
temperature
Non-catalysed Reaction
Catalysed Reaction
6c) Catalysts, Enzymes & Activation Energy
A Catalyst is a substance that:-
Lowers the activation energy required for a chemical reaction to proceed
-
Speeds up the rate of a chemical reaction
-
Takes part in the reaction but remains unchanged at the end of it
An Enzyme is a Biological Catalyst that:-
Lowers the activation energy required by the reactants to form the transition
state
-
Speeds up the rate of the reaction
-
Made of Protein
6d) Enzyme Specificity

An enzyme contains an active site which is a specific shape (due to the structure
and bonding of the amino acids in the polypeptide chain)

An enzyme only acts on one substrate because the enzyme is specific to its
substrate and the molecules of the substrate are complementary to the enzyme’s
active site for which they show an affinity (chemical attraction)

After the reaction, the products have a low affinity for the active site and are
released, leaving the enzyme free to repeat this process with new molecules of the
substrates
6e) Active Site Induced Fit

The active site of an enzyme is not a rigid
structure. It is flexible and dynamic

When a molecule of a substrate enters the
active site, the shape of the enzyme
molecule and the active site change
slightly, making the active site fit very
closely round the substrate molecule – this
is called induced fit

Induced fit ensures that the active site
comes into very close contact with the
molecules of substrate and increases the
chance of the reaction taking place
6f) Orientation of Reactants

When the reaction involves two (or
more) substrates, the shape of the
active site determines the
orientation of the reactants. This
ensures that they are held together
in such a way that the reaction can
take place

The active site holds the two
reactants closely together in an
induced fit, then weakens the
chemical bonds that must be broken
during the reaction. This reduces
the activation energy needed to
reach the transition state.
6g) Factors affecting Enzyme Action

To function efficiently, an enzyme requires a suitable: -
-
Temperature
-
pH
-
Substrate concentration
-
Enzyme concentration

Inhibitors may slow down the rate of an enzyme-controlled reaction or bring
it to a halt
6h) Enzymes & Substrate Concentration
As substrate concentration increases, rate of reaction increases and then the rate of
the reaction remains constant as the active sites are all occupied because the enzyme
concentration is the limiting factor
6i) Control of Metabolic Pathways by presence/absence
of Enzymes

Metabolic pathways can be controlled by
1. Presence of an Enzyme
2. Absence of an Enzyme
3. Key enzymes regulating the rate of a reaction within a metabolic pathway

Regulation of a pathway can be controlled by intra and extracellular signal
molecules

Genes for some enzymes are continuously expressed. These enzymes are always
present in the cell and their control involves regulation of their rate of reaction

Most metabolic reactions are reversible and the presence of a substrate or the
removal of a product will drive a sequence of reactions in a particular direction

Often an enzyme can catalyse a reaction in both a forward and a reverse
direction. The actual direction depends on the concentrations of the reactants &
the products.

Enzymes often act in groups or as multi-enzyme complexes e.g. DNA Polymerase
and RNA Polymerase
6j) Control of Enzymes by Gene Expression (switching
on/off genes)

Lactose Metabolism in Escherichia Coli
-
Lactose sugar is found in milk
-
Lactose is made of a molecule of glucose and a molecule of galactose
-
E.coli must break down lactose into glucose + galactose so it can make use of the
glucose
-
E.coli must break down lactose by using the enzyme β-galactosidase
-
In order to save energy and resources, E.coli only produce the enzyme when it’s
needed!

Along a section of DNA there is a regulator gene, and then the Lac Operon

The Lac Operon of E.coli consists of a operator gene and a structural gene
6k) Lac Operon
Absence of Lactose
-
Regulator gene codes for the production of a repressor molecule
-
Transcription & Translation occur to form the repressor protein molecule
-
The repressor combines with the operator gene
-
The operator gene “switches off” the structural gene
-
No B-galactosidase is produced
6l) Lac Operon
Presence of Lactose
-
Regulator gene codes for the production of a repressor molecule
-
Transcription & Translation occur to form the repressor protein molecule
-
The repressor combines with Lactose
-
The operator gene is free
-
The structural gene is “switched on”
-
B-galactosidase is produced and breaks down LactoseGlucose+Galactose
-
When all the lactose has been digested, the repressor becomes free again and combines with
the operator so the enzyme is no longer produced
6m) Control of Enzyme Action – Signal Molecules

Some metabolic pathways (e.g. Glycolysis) operate continuously

The genes that code for enzymes involved in continuous pathways are always
switched on, so control of these pathways is brought about by regulating enzyme
action

Signal molecules control (regulate) the action of some enzymes

In the Lac Operon, lactose is called a signal molecule as it causes a gene (the
structural gene) that is switched off to become switched on (expressed)

Signal molecules that originate from within the cell are called intracellular signal
molecules

Signal molecules that originate from out-with the cell (in the cell’s environment)
are called extra-cellular signal molecules
6n) Control of Enzyme Action – Inhibitors

An inhibitor is a substance that decreases the rate of an enzyme controlled reaction

A Competitive Inhibitor
-
has a similar molecular structure to the substrate
-
competes with the substrate for the active sites on the enzyme
-
blocks the active sites, so the rate of the reaction decreases

Competitive Inhibition can be reversed by increasing the substrate concentration
6o) Control of Enzyme Action – Inhibitors
A Non-Competitive Inhibitor
-
does not have a similar molecular structure to the substrate
-
does not combine directly with the active site
-
attaches to a non-active (allosteric) site and changes the shape of the enzyme molecule –
indirectly altering the shape of the active site so the rate of the reaction decreases
6p) Control of Enzyme Action - Regulators

Enzymes have active sites and nonactive (allosteric) sites

Enzymes can change their shape if
regulatory molecules bind to a nonactive (allosteric) site

If the regulatory molecule is an
activator, the enzyme changes to its
active state and enzyme activity is
stimulated

As activator molecules increase, the
enzyme reaction rate increases

If the regulatory molecule is a noncompetitive inhibitor, the enzyme
changes to its inactive state and
enzyme activity is inhibited

As inhibitor molecules increase, the
enzyme reaction rate decreases
6q) Control of Enzyme Action - Feedback Inhibition



Feedback inhibition is when the
end product binds to an enzyme
that catalyses a reaction early in
the pathway, which in turn
regulates the pathway.
This pathway is kept under finely
tuned control by negative
feedback
Feedback inhibition prevents
waste of resources
Human Cells Questions
KEY AREA 6 – Metabolism and Enzymes
1. Testing Your Knowledge 1
Page 85
Q’s 1-4
2. Testing Your Knowledge 2
Page 96
Q’s 1-4
3. Quick Quiz