Download Gene regulation

Activation or Switching on of Proteins (Enzymes)
Enzymes are responsible for the metabolism of cells. Some function continuously; some are present all
the time, but are ‘idle’, only working when given the appropriate signal; some are only needed if
particular substrates are present; some may be needed for particular aspects of development. Clearly
their action is regulated. Such regulation can be via chemical signals such as hormones or under genetic
control.
Role of Cyclic AMP in Protein Activation
You may recollect that cyclic AMP is a second messenger, allowing for transmission of messages across
the cell membrane. Hormones such as insulin and glucagon cannot pass through cell membranes.
Instead, there are receptors on the outer surface of the cell to which the hormone binds. This activates
a system that produces a second messenger – in the case of glucagon, adenylate cyclase brings about
the production of cyclic AMP from ATP. The cyclic AMP then activates intracellular enzymes like protein
kinase A (PKA) by altering their three dimensional structure. The activated enzyme then catalyses the
required reaction. In short, the required enzyme action is switched on.
cAMP
ATP
Peptide hormones cannot pass through the
membrane. Instead hormones like glucagon
and insulin bind to a receptor. Once bound it
produces a second messenger. Glucagon uses
the adenylate cyclase-cAMP pathway.
Adenylate cyclase is activated and cAMP is made
from ATP. There is a cascade effect on enzymes in
the cytoplasm which brings about the effect. For
example, cAMP activates protein kinase A (PKA) in
muscle cells which then switches on glycogen
phosphorylase and switches off glycogen synthase.
Glucagon activation of protein kinase A (PKA) by cyclic AMP in muscle cells brings about phosphorylation
of several enzymes in the cell. This switches on glycogen phosphorylase, but switches off glycogen
synthase.
Q. What do these enzymes normally do, and what effect will switching them on or off have upon
metabolism?
• Glycogen phosphorylase
• Glycogen synthase
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Genetic Control of Enzyme Production in Prokaryotes
As you already know, genes control protein synthesis. Some proteins are required all the time by cells
whilst others are only needed in specific circumstances and so are not made all the time. Such genes
must be capable of being switched on or off – it would be wasteful if they were made and not required!
Switching on is known as induction, whilst switching off is known as repression.
Jacob and Monod suggested how genes could be switched on or off. It involves a group of adjacent
genes that act together. They called this an operon. The operon consists of:

structural genes that code for the production of the enzymes

an operator gene that regulates the structural gene

a promoter region where the RNA polymerase attaches to, so allowing transcription
There is also regulator gene (not actually part of the operon) that codes for a repressor protein.
Behaviour of the repressor protein is determined by whether the gene is induced or repressed. Your
specification requires only that you know about the lac operon of Escherichia coli. It is however possible
that you may be provided with detail of another system and asked to compare it with the lac operon and
induction.
Gene/Enzyme Induction
Some genes are only switched on when the protein that they code for is required. Escherichia coli is a
gut bacterium that normally uses glucose for respiration. If it is grown in a medium that contains lactose
as the only respiratory substrate, it will use this. However, it needs to produce two enzymes before the
lactose can be used. These enzymes are lactose permease and β-galactosidase (lactase).
Q. What does each of these enzymes do?
• Lactose permease
• β-galactosidase (lactase)
The lac operon consists of the structural genes (lactose permease and β-galactosidase), the operator
region and promoter region. A regulator gene produces a repressor protein that binds to the operator
region when lactose is absent. The repressor protein therefore stops RNA polymerase attaching to the
promoter region. Consequently transcription and protein synthesis (of the enzymes) cannot take place.
In contrast, when lactose is present the lactose binds to the repressor protein. This changes its shape so
that the repressor protein can no longer bind to the operator gene. This means that RNA polymerase is
now able to bind to the promoter and so allow transcription and translation. Lactose permease and βgalactosidase are made so that lactose can be utilised.
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The following diagram summarises the key components of the lac operon system of E. coli. Add brief
notes to explain what is happening.
Lac operon – the basic components:
R
P
O
LP
β -Gase
Lactose not present or glucose as the respiratory substrate:
R
P
O
LP
β -Gase
O
LP
β -Gase
Lactose as the only respiratory substrate:
R
P
lactose
RNA polymerase
repressor
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Gene/Enzyme Repression
Some genes are normally switched on, but can be turned off when required. E. coli can normally make
its own amino acids. It makes tryptophan using the enzyme tryptophan synthetase. If however
tryptophan is supplied in the medium, E. coli stops making it – the gene is switched off.
The following diagram summarises the main components. Add brief notes to explain what is happening.
Tryp operon – the basic components:
R
P
O
Tryptophan synthetase
P
O
Tryptophan synthetase
P
O
Tryptophan synthetase
In the absence of tryptophan:
R
In the presence of tryptophan:
R
tryptophan
RNA polymerase
repressor
Q. With reference to the diagrams, in what key way is repression different from induction?
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