Two ways to
Regulate a
Metabolic Pathway
Control of Metabolism in Prokaryotes and Eukarotes
Two major ways to control metabolism
1. Allosteric control of Enzyme activity: negative feedback and
feedback inhibition
•
A quick short-term response
E1
A 
•
•
E2
B

C
E3
E4

D 
Product
Recall the role played by PFK (PhosphoFructoKinase) in the Allosteric
control of glycolysis
–
PFK Activated by _________________________
–
PFK Inhibited by _________________________
2. Regulation of Gene Expression (Prokaryotes)
•
Anabolic Pathways (e.g. tryptophan synthesis)
— Product accumulation inhibits the transcription (mRNA
synthesis) of genes coding for the enzymes needed to make
the product.
– Enzymes are not made unless they are needed
•
Catabolic Pathways (e.g. lactose breakdown)
— Presence of substrate activates the transcription (mRNA
synthesis) of genes coding for the enzymes needed to
breakdown the substrate.
— Enzymes are not made unless they are needed
2. Regulation of Gene Expression (Eukaryotes)
Much more complex in Eukaryotes than in Prokaryotes!
Some mechanisms include..
1.
Accessibility of genes  condensed (coiled) DNA prevents
transcription  RNA polymerase can’t access the promoter
— e.g. Barr bodies: One X chromosome is inactivated in females by
producing a tightly-wound structure called a Barr body
2.
Transcriptions factors  activate or inhibit transcription
— e.g. p53 protein is a transcription factor
3.
Chemical modification of DNA (e.g. DNA methylation)
— Permanently inactivates genes
The trp operon: a repressible operon
Anabolic Pathway for Tryptophan Synthesis from Precursor Molecule “A”
A
E1

B
E2
 C

E3
D 
E4
E5
E  Tryptophan
Control of Gene Expression in Prokaryotes
Vocabulary
•
Operon: Regulated cluster of structural genes that have a
common function (e.g. lac Operon, trp Operon)
— Structural genes code for mRNA
— Advantage of an Operon?
•
Promoter: Site on where RNA polymerase binds to DNA
•
Operator: Controls access of RNA polymerase to structural
genes
— Acts as an “on/off” switch for transcription
— Located between promoter and operon
The trp operon: regulated synthesis of repressible enzymes (Layer 1)
The trp operon: regulated synthesis of repressible enzymes (Layer 2)
Control of Gene Expression in Prokaryotes
Vocabulary (continued)
•
Repressor
— Protein that binds reversibly to operator
— Binding to operator blocks the transcription of the operon
•
Co-repressor (involved with repressible (anabolic) operons—e.g.
trp operon)
— Molecule that binds reversibly repressor protein
— Co-repressor binding activates the repressor
— Co-repressor-repressor complex binds to operator  operon not
transcribed
The lac operon: regulated synthesis of inducible enzymes
The lac operon: regulated synthesis of inducible enzymes
cAMP Receptor Protein (CRP)
Lac Operon Transcribed only if Lactose is present in the absence of Glucose
Lac Operon is not transcribed if glucose is present
cAMP (Cyclic AMP): Activates Lac Operon
When Lactose is present in the absence of Glucose
•
Cellular concentrations of cAMP increase as
cellular concentration of glucose decrease.
•
cAMP binds to an inactive CRP (Cyclic AMP
Receptor Protein)
•
cAMP-CRP complex  binds to promoter 
Lac Operon Transcribed
ALE 11. Question 7 on Page 11
It was through the
effects of mutations that
enabled Jacob and
Monod to decipher how
the lac operon works.
Predict how the following
mutations would affect
lac operon function in
the presence and
absence of allolactose.
Note: use this question
to test your knowledge
of the lac operon. Study
the how the lac operon
works, then attempt this
question, using only your
cerebral cortex as a
reference
Effect of mutation on lac operon when
Mutation
Mutation of regulatory
gene: Repressor will
not bind to allolactose
Mutation of operator:
Repressor will not
bind to operator
Mutation of regulatory
gene:Repressor will
not bind to operator
Mutation of promoter:
RNA polymerase will
not bind to promoter
Allolactose present
Allolactose absent
Figure 18.10 A hypothesis to explain how prions propagate
Figure 18.11 Replication of the bacterial chromosome
Figure 18.x7 E. coli
Figure 18.x8 E. coli dividing
Figure 18.x9 Bacterium releasing DNA with plasmids
Figure 18.x10 Plasmids
Figure 18.12 Detecting genetic recombination in bacteria
Figure 18.13 Transduction (Layer 1)
Figure 18.13 Transduction (Layer 2)
Figure 18.13 Transduction (Layer 3)
Figure 18.13 Transduction (Layer 4)
Figure 18.14 Bacterial mating
Figure 18.15 Conjugation and recombination in E. coli (Layer 1)
Figure 18.15 Conjugation and recombination in E. coli (Layer 2)
Figure 18.15 Conjugation and recombination in E. coli (Layer 3)
Figure 18.15 Conjugation and recombination in E. coli (Layer 4)
Figure 18.16 Insertion sequences, the simplest transposons
Figure 18.17 Insertion of a transposon and creation of direct repeats
Figure 18.18 Anatomy of a composite transposon
Unnumbered Figure (page 353) Bacterial and viral growth curves