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Control of Metabolic
Pathways
Higher Human Biology
Unit 1 – Section 6
Metabolic Pathways
Learning Intentions
By the end of this section you should be able to:
• Explain the regulation of gene expression, enzyme
production and enzyme function.
• Describe the role of extra- and intracellular signal
molecules.
What is a Metabolic Pathway?
• A metabolic pathway consists of several
stages, each of which involves
conversion of one metabolite to another.
• Each step in the pathway is controlled
by a specific enzyme.
• Each step in a metabolic pathway is
driven by a specific enzyme.
•
•
•
Each enzyme is coded for by at least one gene.
If one enzyme is absent, the metabolic pathway stops.
Metabolic pathways can therefore be regulated through
gene expression.
Switching genes
On and Off.
• Some metabolic pathways (e.g. glycolysis
reactions in respiration) operate continuously.
• So the genes which code for these enzymes are
always expressed and ‘switched on’.
• However, other enzymes are only produced when
required by the cell, thereby saving resources
and energy.
Lactose Metabolism in Escherichia coli
• Lactose is a sugar that it found in milk.
• It is made up of a molecule of glucose and
galactose
• E.coli can only make use of the glucose for
respiration if it is released from the
galactose.
• The enzyme responsible for breaking up
lactose is B – galactosidase.
• E.coli has a gene that codes for B –
galactosidase but it only produces this
enzyme when lactose is present. Somehow
the gene that codes for B – galactosidase is
switched on in the presence of lactose. The
process of switching on a gene only when the
enzyme that it codes for is needed is called
enzyme induction.
Genetic Control
Control of growth is under genetic control with genes being switched on and
off at specific times.
One example of a genetic switch was discovered by Jacob and Monod who
demonstrated the method by which a gene making the enzyme ßgalactosidase could be regulated to ensure that :a) the enzyme was only switched on when its substrate lactose was
present.
b) valuable raw materials were not wasted.
c) valuable energy in the form of ATP was not wasted.
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1.
2.
3.
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Genes can work together to help regulate the production of an
enzyme
Structural gene is transcribed and translated into the enzyme ßgalactosidase which breaks down the sugar lactose
Operator gene switches on the structural gene
Regulator gene controls the functioning of the operator through the
production of a ‘repressor protein’
All three genes working together are known as an operon
The production of the enzyme B – galactosidase
by E. coli bacteria.
• In the absence of lactose (a sugar) the lactose digesting
enzyme ‘B – galactosidase’ is not produced by the
bacteria.
• But when lactose is present, the lactose digesting
enzyme ‘B – galactosidase’ is produced.
And here is an animation:http://www.youtube.com/watch?v=oBwtxdI1zvk
•
Since the lactose sugar starts the production of the enzyme it is
known as the inducer
Mutations of the operon genes
1. Regulator
The repressor protein would be faulty or not made so the operator would
be free to switch on the structural gene all the time. Therefore the
enzyme would be made continuously. Therefore wasting resources and
energy.
2. Operator
The operator would not be able to switch on the structural gene.
Therefore the enzyme would not be made. Therefore cell could not use
lactose as an energy source
3. Structural
The enzyme would be faulty or not be made. Therefore cell could not use
lactose as an energy source.
Regulation of gene expression by
‘signal molecules’.
• The activation of some
enzymes is controlled by signal
molecules.
• In the Lac operon system the
signal molecule is lactose
sugar.
• It combines with the
repressor molecule, allowing
the structural gene for B –
galactosidase to be expressed.
Intra- and Extracellular Signal
Molecules.
• The molecules that effect a cell’s metabolism and
originate from out with the cell (e.g. lactose) are
called extracellular signal molecules.
Hormones such as
Adrenaline are also
examples of extracellular
signal molecules.

Intra- and Extra-cellular Signal
Molecules.
• Molecules that
effect a cell’s
metabolism and
originate from
inside the cell
itself are called
intracellular
signal molecules.