Chemistry 340 Exam 2 Lecture 8 |
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Enzyme Regulation
Major concepts:
1. What are isozymes, and what is their function in enzyme regulation?
2. How does enzyme regulation occur using proteolysis? What are the advantages and disadvantages of using this method of regulation?
3. How does enzyme regulation occur covalently? What are the advantages and disadvantages of using this method of regulation?
4. How does enzyme regulation occur allosterically? What are the advantages and disadvantages of using this method of regulation?
5. What two types of enzymes are needed for the most common type of covalent regulation? How do these enzymes recognize their protein substrates, and how does modification affect the substrates?
6. What are the characteristics of allosterically regulated enzymes?
Core knowledge:
1. What is a proenzyme, and how is it converted to an enzyme?
2. What is amplification, and what is required for amplification to occur?
3. What are the reactions catalyzed by protein kinase and protein phosphatase? How do these reactions affect the activity of glycogen phosphorylase?
4. What are modulators, and how do they affect enzyme activity?
Reasons for regulation (changing enzyme activity without changing [enzyme])
1. Cell conditions change, and cells are different from each other.
2. Enzyme synthesis is energetically expensive and slow.
NOTE: Only a few enzymes are regulated;
regulation of those enzymes effectively controls the activity of other enzymes.
Types of enzyme regulation (self-regulation by an organism)
A. Isozymes = variations in proteins from one cell type to another
B. Proteolysis: an inactive proenzyme or zymogen is converted to an active enzyme
by a protease
C. Reversible covalent modification: a functional group is transferred to an enzyme,
which causes a change in enzyme conformation = change in activity.
Removing the functional group restores the original conformation.
D. Allosteric regulation: a modulator binds noncovalently to an enzyme,
which causes a change in enzyme conformation = change in activity.
Dissociation restores the original conformation.
Isozymes = slightly different versions of an enzyme, typically found in different cells
Basically allows cells of different types to respond differently.
Regulation is determined by the protein synthesized in a cell.
Variation may be small or large, KM, V max , enzyme specificity, enzyme mechanism, etc.
Example: hexokinase and glucokinase
Hexokinase is found in all cells. KM< 0.1 mM; substrates = glucose and other sugars;
inhibited by its product, glucose-6-P
Glucokinase (hexokinase IV) is found in liver cells only. KM ∼ 5 mM; substrate = glucose;
has cooperative binding with Hill constant = 1.5
Allows liver to remove glucose from blood after a meal when blood [glucose] is high.
Questions to ask about each of the other types of regulation:
1. Is it reversible?
2. Is another enzyme required?
3. Does it result in activation only, inhibition only, or both (under different circumstances)?
4. How rapid is it?
5. Is it a response to conditions inside the cell or outside the cell?
Proteolysis: enzyme is synthesized in an inactive form that must be converted to the active form by a protease that hydrolyzes one or more peptide bonds
proenzyme or zymogen + 2O → active enzyme (+ peptide)
Irreversible process
Examples: digestive enzymes chymotrypsin and trypsin, coagulation cascade, complement
Advantages:
1. One protease can activate many proenzyme molecules (amplification).
2. A destructive enzyme isn't activated inside the cell.
Disadvantage: Can't be reversed.
Covalent modification: enzyme is modified when a functional group is transferred to it g
conformation change = change in activity.
The original conformation (activity) is restored when the functional group is removed by a
different enzyme.
Reversible process
Functional groups: phosphoryl , AMP, UMP, methyl
For phosphoryl, a protein kinase that recognizes a specific S, T, or Y transfers P from ATP
to the –OH → adding a large group with a negative charge g conformation change
The phosphoryl can be removed by a protein phosphatase that hydrolyzes the phosphate
ester bond.
Example: glycogen phosphorylase b (less active) + 2 ATP →
glycogen phosphorylase a (more active) – (Pi)2 catalyzed by phosphorylase kinase
glycogen phosphorylase a – (Pi)2 + 2 2O → glycogen phosphorylase b + 2 Pi
catalyzed by protein phosphatase
Both phosphorylase kinase and protein phosphatase recognize a specific sequence.
Advantages:
1. Each phosphorylase kinase can modify several glycogen phosphorylase molecules.
2. With multiple modifications, enzyme activity can be set to different levels.
3. Can involve activation or inhibition
Disadvantages:
1. Slower than allosteric regulation
2. Costs a small amount of energy (ATP)
Allosteric regulation: a modulator forms a noncovalent association with the enzyme at an allosteric binding site that is not the active site. The association causes a change in enzyme conformation and therefore a change in enzyme activity.
1. Allosteric enzymes usually have multiple subunits (multiple active sites)
cooperative binding of substrate = sigmoidal rate curves ( not Michaelis-Menton kinetics)
Instead of using KM, use [S]0.5 or K0.5 (although this is relatively recent).
2. Modulators can be
a. positive (= activators that stabilize R conformation and reduce K0.5 )
b. negative (= inhibitors that stabilize T conformation and increase K0.5)
3. Modulators can be
a. homotropic (substrate = homotropic activator; increases cooperative binding)
b. heterotropic
4. V max may be reduced by an inhibitor, but for most enzymes V max remains the same.
5. Advantages:
a. Quick because the modulator easily dissociates from the enzyme: M + E D EM
b. No other enzyme required, no energy required
c. Can involve activation or inhibition
6. Disadvantages:
a. Each modulator can affect the activity of only one enzyme molecule.
b. Primarily a response to conditions within the cell, not conditions within an organism