Active Sites – What’s Happening?
• 1. Substrate is bound
• 2. A reaction is being catalyzed . . . .
• e. g.
Mechanism of Action: 2 major types
For HW- Skip #22.21 – too weird . . .
1. Lock-and-key model of enzyme mechanism.
• 2. Induced-fit model
Mechanism of Action: 2 major types
• 1. Lock-and-key model of enzyme mechanism.
• The enzyme is a rigid three-dimensional body.
• The enzyme surface contains the active site.
CH2 OPO 3 2 CH 2 OPO 3 2 Ph osph oh e xos e
CH2 OH
O
O
i some rase
H HO
OH
OH
H
HO
OH
H
OH
HO
 - D- Gl u cos e -6-ph os ph ate
 - D-Fru ctos e-6-ph osph ate
1.
(key)
NOTE: THE REACTION being Catalyzed
CH2 OPO 3 2 CH 2 OPO 3 2 Ph osph oh e xos e
CH2 OH
O
O
i some rase
H HO
OH
OH
H
HO
OH
H
OH
HO
 - D- Gl u cos e -6-ph os ph ate
 - D-Fru ctos e-6-ph osph ate
1.
2.
(lock)
CH 2 OPO 3 2 CH2 OPO 3 2 Ph osph oh e xos e
CH2 OH
O
O
i some rase
H HO
OH
OH
H
HO
OH
H
OH
HO
 - D- Gl u cos e -6-ph os ph ate
 - D-Fru ctos e-6-ph osph ate
2.
Mechanism of Action
2. Induced-fit model
The active site becomes modified
to accommodate the
substrate
CH2 OPO 3 2 CH 2 OPO 3 2 Ph osph oh e xos e
CH2 OH
O
O
i some rase
H HO
OH
OH
H
HO
OH
H
OH
HO
 - D- Gl u cos e -6-ph os ph ate
 - D-Fru ctos e-6-ph osph ate
1.
Induced fit – substrate changes
enzyme form/shape
once in active site
CH2 OPO 3 2 CH2 OPO 3 2 Phosphohe xose
CH2 OH
O
O
i some rase
H HO
OH
OH
H
HO
OH
H
OH
HO
 - D- Gl ucose -6-phosphate
 - D-Fructose-6-phosphate
2.
Mechanism of Action
• Figure 22.9 - Competitive inhibition.
• the inhibitor fits into the active site, thereby preventing the
substrate from entering.
Inhibitor got there 1st,
Ha ha
Mechanism of Action
• Figure 22.10
•
•
Noncompetitive inhibition.
the inhibitor binds to allosteric site, ( not active site)
This changes conformation of the active site.
Alosteric site
•
The substrate no longer fits
(properly).
See p. 562-3 Chem. Connections
Mechanism of Action
• Lock-and-key model & Induced-fit model
emphasize the shape of the active site.
• However, Chemistry of active site is most important.
• Just 5 amino acids participate in active sites of:
≤65% of the enzymes studies to date.
• Examples: His > Cys > Asp > Arg > Glu.
• Four have either acidic or basic side chains;
the fifth has a sulfhydryl group (-SH).
Confirming your knowledge
•
Which of the following is correct describing the
Induced-Fit Model of Enzyme action?
Substrates fit into the active site:
1.
b/c they are exactly the same size and shape
2.
By changing their size and shape to
match those of the active site
3. By changing the size and shape of the
active site Upon binding
Challenge Question
• Enzymes are long protein chains, > 100 Amino acids
• The Active SITE contains only a few amino acids
• Explain why other amino acids are present and what would
happen to the enzyme activity if significant changes were
made to the enzymes Structure.
Enzyme Regulation
1. Feedback control
2. Proenzymes
3. Isoenzymes
Enzyme Regulation
1. Feedback control: regulation process where the product
(of a series of enzyme-catalyzed reactions)
•
inhibits an earlier reaction in the sequence.
fee dback inhi bi ti on
(path 2)
A
E1
B
E2
C
(path 1)
E3
D
• The inhibition may be competitive or noncompetitive.
(path 1) e.g. cholersterol normally produced in liver, [<100mg/100mL ]
(path 2) if > [200mg/100mL] in plasma, liver stops producing . . .
Enzyme Regulation (cont.)
• 2. Proenzymes (zymogens): inactive form of enzyme
must have portion of polypeptide chain removed otherwise not active.
• e.g. trypsin, (a digestive enzyme)
• synthesized/stored as trypsinogen, no enzyme activity.
• Active only after a six-amino acid fragment is removed
• Removal of amino acid fragment changes primary + tertiary
structure,  active form.
• 3. Isoenzyme: different form of the same enzyme
( two different forms catalyze the same reaction(s))
Challenge Question
• Why not make fully active form of Enzyme(s) all
the time? (e.g. trypsinogen  trypsin)
The Allosteric Effect
• Figure 22.12 Binding of regulator to a site other than
the active site changes the shape of the active site.
Enzymes in Medicine
• Enzyme assays useful in medical diagnosis.
En zyme
Body Flu id
D iseas e Diagnosed
Alan ine amin otrans ferase (ALT)
Acid phosp hatase
Alk alin e phosp hatase (ALP)
Amylase
Serum
Hepatitis
Prostate cancer
Liver or bone dis ease
Pan creatic diseas e
Serum
Serum
Serum
As partate aminotransferase (AST) Serum,
Cereb rosp in al
fluid
Lactate dehydrogenas e (LD H)
Serum
Creatin e phosph ok inase (CK)
Serum
Ph os phohexose isomeras e (PHI)
Serum
Heart attack or
hep atitis
Heart attack
Heart attack
Heart attack
Monitor levels 24hr after M.I. or (heart attack)