1. ENZYMES: KINETICS, INHIBITION, REGULATION

2. Kinetic properties of enzymes
Study of the effect of substrate concentration on the rate of reaction

3. Rate of Catalysis
At a fixed enzyme concentration [E], the initial velocity Vo is almost linearly proportional to substrate concentration [S] when [S] is small but is nearly independent of [S] when [S] is large
- Rate rises linearly as [S] increases and then levels off at high [S] (saturated)

4. Leonor Michaelis and Maud Menten – first researchers who explained the shape of the rate curve (1913)
During reaction enzyme molecules, E, and substrate molecules, S, combine in a reversible step to form an intermediate enzyme-substrate (ES) complex
E + S ES E + P k1 k2
k-1
k-2
k1, k-1, k2, k-2 - rate constant - indicate the speed or efficiency of a reaction

5. The Michaelis-Menten Equation
The basic equation derived by Michaelis and Menten to explain enzyme-catalyzed reactions is
Vmax[S]
vo =
Km + [S]
Km - Michaelis constant;
Vo – initial velocity caused by substrate concentration, [S];
Vmax – maximum velocity

6. Effect of enzyme concentration [E] on velocity (v)
In fixed, saturating [S], the higher the concentration of enzyme, the greater the initial reaction rate
This relationship will hold as long as there is enough substrate present

7. Enzyme inhibition
In a tissue and cell different chemical agents (metabolites, substrate analogs, toxins, drugs, metal complexes etc) can inhibit the enzyme activity
Inhibitor (I) binds to an enzyme and prevents the formation of ES complex or breakdown it to E + P

8. Reversible and irreversible inhibitors
Reversible inhibitors – after combining with enzyme (EI complex is formed) can rapidly dissociate
Enzyme is inactive only when bound to inhibitor
EI complex is held together by weak, noncovalent interaction
Three basic types of reversible inhibition: Competitive, Uncompetitive, Noncompetitive

9. Reversible inhibition Competitive inhibition
•Inhibitor has a structure similar to the substrate thus can bind to the same active site
•The enzyme cannot differentiate between the two compounds
•When inhibitor binds, prevents the substrate from binding
•Inhibitor can be released by increasing substrate concentration

10. Example of competitive inhibition
Benzamidine competes with arginine for binding to trypsin

11. Noncompetitive inhibition
• Binds to an enzyme site different from the active site
• Inhibitor and substrate can bind enzyme at the same time
•Cannot be overcome by increasing the substrate concentration

12. Uncompetitive inhibition
Uncompetitive inhibitors bind to ES not to free E
This type of inhibition usually only occurs in multisubstrate reactions

13. Irreversible Enzyme Inhibition Irreversible inhibitors
very slow dissociation of EI complex
Tightly bound through covalent or noncovalent interactions
Irreversible inhibitors
•group-specific reagents
•substrate analogs
•suicide inhibitors

14. Group-specific reagents
–react with specific R groups of amino acids

15. Substrate analogs
–structurally similar to the substrate for the enzyme
-covalently modify active site residues

16. Suicide inhibitors
•Inhibitor binds as a substrate and is initially processed by the normal catalytic mechanism
•It then generates a chemically reactive intermediate that inactivates the enzyme through covalent modification
•Suicide because enzyme participates in its own irreversible inhibition

19. Regulation of enzyme activity Methods of regulation of enzyme activity
• Allosteric control
• Reversible covalent modification
• Isozymes (isoenzymes)
• Proteolytic activation

20. Allosteric enzymes
Allosteric enzymes have a second regulatory site (allosteric site) distinct from the active site
Allosteric enzymes contain more than one polypeptide chain (have quaternary structure).
Allosteric modulators bind noncovalently to allosteric site and regulate enzyme activity via conformational changes

21. 2 types of modulators (inhibitors or activators)
• Negative modulator (inhibitor)
–binds to the allosteric site and inhibits the action of the enzyme
–usually it is the end product of a biosynthetic pathway - end-product (feedback) inhibition
• Positive modulator (activator)
–binds to the allosteric site and stimulates activity
–usually it is the substrate of the reaction

22. Example of allosteric enzyme - phosphofructokinase-1
(PFK-1)
PFK-1 catalyzes an early step in glycolysis
Phosphoenol pyruvate (PEP), an intermediate near the end of the pathway is an allosteric inhibitor of PFK-1
PEP

23. Regulation of enzyme activity by covalent modification
Covalent attachment of a molecule to an amino acid side chain of a protein can modify activity of enzyme

24. Phosphorylation reaction

25. Dephosphorylation reaction
Usually phosphorylated enzymes are active, but there are exceptions (glycogen synthase)
Enzymes taking part in phospho-rylation are called protein kinases
Enzymes taking part in dephosphorylation are called phosphatases

26. Isoenzymes (isozymes)
Some metabolic processes are regulated by enzymes that exist in different molecular forms - isoenzymes
Isoenzymes - multiple forms of an enzyme which differ in amino acid sequence but catalyze the same reaction
Isoenzymes can differ in:
kinetics,
regulatory properties,
the form of coenzyme they prefer and
distribution in cell and tissues
Isoenzymes are coded by different genes

27. Example: lactate dehydrogenase (LDG)
Lactate + NAD pyruvate + NADH + H+
Lactate dehydrogenase – tetramer (four subunits) composed of two types of polypeptide chains, M and H
There are 5 Isozymes of LDG:
H4 – heart
HM3
H2M2
H3M
M4 – liver, muscle
• H4: highest affinity; best in aerobic environment
•M4: lowest affinity; best in anaerobic environment
Isoenzymes are important for diagnosis of different diseases

28. Examples of specific proteolysis
Activation by proteolytic cleavage
• Many enzymes are synthesized as inactive precursors (zymogens) that are activated by proteolytic cleavage
• Proteolytic activation only occurs once in the life of an enzyme molecule
Examples of specific proteolysis
•Digestive enzymes
–Synthesized as zymogens in stomach and pancreas
•Blood clotting enzymes
–Cascade of proteolytic activations
•Protein hormones
–Proinsulin to insulin by removal of a peptide

30. Multienzyme Complexes and Multifunctional Enzymes
Multienzyme complexes - different enzymes that catalyze sequential reactions in the same pathway are bound together
Multifunctional enzymes - different activities may be found on a single, multifunctional polypeptide chain

31. Metabolite channeling
Metabolite channeling - “channeling” of reactants between active sites
Occurs when the product of one reaction is transferred directly to the next active site without entering the bulk solvent
Can greatly increase rate of a reactions
Channeling is possible in multienzyme complexes and multifunctional enzymes