Download Enzymes

1
Metabolism: The Regulation of Enzyme
Activity and Synthesis
I.
II.
Regulation
A.
Without adequate control of metabolic processes:
Cells become disorganized and die
B.
Metabolic pathways are regulated so that:
Cell components are present in correct amounts
C.
Chemical composition of cell's surroundings changes constantly:
Microbial cell responds to environmental change by:
Switching catabolic pathways when new nutrients become available
D.
Constitutive enzymes:
Enzymes in metabolic pathways essential to life:
Always present in cells
E.
Many enzymes not needed all the time:
Cells regulate:
Location
Activity
Synthesis
Types of Regulation
A.
Three types
1.
Metabolic Channeling:
Important in eukaryotic cells:
Compartmentalization:
Metabolites and enzymes located in different parts of the
cell:
Differential distribution of enzymes among separate organelles
Concentration gradients within the cytoplasm
2.
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Control of Enzyme Activity
Enzymes often stimulated or inhibited to rapidly alter pathway
activity:
Allosteric Regulation:
Allosteric Enzymes:
Regulatory enzymes
Effector (modulator):
Small molecule alters the activity of allosteric
enzyme:
Binds reversibly to allosteric site on enzyme:
Changes conformation of the active site:
Alters activity of the enzyme
2
Two types of effectors:
Positive effector:
Increases enzyme activity
Negative effector:
Decreases enzyme activity
Allosteric inhibition:
Feed back inhibition:
End-product of pathway inhibits activity of first
enzyme (pacemaker enzyme) in pathway:
Enzyme's synthesis not affected:
Activity of enzyme controlled:
Enzyme combines with end-product of metabolic pathway:
Active site blocked or its shape changed:
Can no longer combine with its substrate:
End product is not made
Enzyme-end-product combination reversible:
When more end product is needed enzyme is released:
Synthesis can resume
Catalytic activity of Allosteric enzymes:
Regulated through reversible binding of endproduct to a site other than the active site
Feedback relations may be very complex if branched
pathways are involved
In branched pathway first enzyme after branch is usually
the allosteric enzyme
Regulation of enzyme activity allows:
Fine tune metabolic activity
Rapidly adjust metabolic activity
3.
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Regulation of enzyme synthesis:
Number of enzyme molecules may be controlled:
Involves controling expression of genetic material:
3
Covers longer time periods:
Complements regulation of enzyme activity
Saves energy and raw material
Maintains balance between the amounts of various cell proteins
Allows cells to adapt to long term environmental change
III.
Control of gene expression
A.
Involves
Regulation of mRNA synthesis:
Induction and repression
Result from changes in rate of transcription:
B.
Induction:
Enzyme synthesized only in presence of its specific substrate:
Catabolic enzymes often induced
In absence of lactose E. coli lacks:
-galactosidase:
Catalyzes the hydrolysis of lactose  galactose and glucose
mRNA that codes for the -galactosidase is not transcribed
When carbon source is lactose:
3000 -galactosidase molecules present in cell:
mRNA that codes for the -galactosidase is transcribed
-galactosidase:
Inducible enzyme
Level rises in the presence of small molecule, the inducer
Inducer:
Substance which causes induction (allolactose derivative of lactose)
C.
Operon:
Genes (and their controlling elements) located next to each other in the
DNA molecule
D.
Operator gene:
Area of DNA that controls RNA transcription
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R
P 0 A B C
────┴/\/\/\/\┴───┴─────┴─────┴──────┴───────┘
OPERON
R = REPRESSOR GENE
P = PROMOTER
0 = OPERATOR GENE
I = INDUCER (allolactose)
RP = REPRESSOR PROTEIN
CR = CO-REPRESSOR (END-PRODUCT)
E.
Repressor protein:
Synthesized all the time:
Sits on operator gene:
Blocks formation of mRNA
No enzymes can be made
F.
Inducer (specific substrate): combines with repressor protein:
Repressor protein no longer fits on the operator gene:
 mRNA is synthesized
Protein (enzymes) is transcribed
G.
H.
Enzymes allow cell to metabolize inducer
Repression:
Synthesis of enzyme occurs only when end-product of biochemical
pathway is not present
Repressor protein:
Synthesized all the time:
Can't combine with operator gene:
Structural genes are transcribed:
Enzymes necessary for biosynthetic pathway produced:
Specific end-product formed
When concentration of end-product increases:
End-product combines with repressor protein
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Combination of repressor protein and co-repressor (end-product) sit on
operator gene and turn it off:
No mRNA made:
 no enzyme is produced
Enzyme production is repressed
I.
IV.
Induction and repression:
Common in bacteria
Not common in eukaryotic cells
CATABOLITE REPRESSION
A.
Occurs in bacteria and yeast:
Presence of glucose inhibits synthesis of enzymes involved in the utilization of
other sugars:
Lactose
Galactose
Maltose
Arabinose
When glucose present with other sugars:
Glucose always utilized first
After glucose is used:
Enzymes necessary for utilization of other sugars made.
B.
In operons subject to catabolite repression:
RNA polymerase binds firmly to promoter: Only if catabolic activator
protein (CAP) is already bound to the promoter
C.
Promoter:
Region in DNA where RNA polymerase must bind before it can make the
linkage between ribonucleotides and phosphate
CAP only binds to promoter if first combined with cyclic adenosine
monophosphate (cAMP)
Presence of glucose causes:
Large decrease in intracellular concentration of cyclic AMP
Glucose inhibits synthesis or breaks down cAMP:
In presence of glucose intracellular level of cAMP low:
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CAP can't attach to promoter:
RNA polymerase can't attach to promoter
No RNA can be transcribed
No protein synthesis occurs
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