Download Ch.21Pt.4_000

Enzymes grouped in 6 major classes: (p. 643)
1. Oxidoreductases: Double-barreled name
catalyze the reduction or oxidation of a molecule.
2. Transferases:
catalyze the transfer of a group of atoms from one molecule to another.
e.g. transfer of phosphate between ATP and sugar
3. Hydrolases:
catalyze hydrolysis reactions & reverse rxns.
e.g. hydrolysis of an ester.
4. Isomerases:
Catalyze conversion of a molecule into isomer.
The cis-trans conversion of maleate & fumarate
5. Lyases:
Rxns which break a double bond & add a small molecule (water or NH3)
& the reverse rxn: elimination to form a double bond.
(Not involving hydrolysis or oxidation)
6. Ligases:
These enzymes catalyze reactions which make bonds to join together
(ligate) smaller molecules to make larger ones. (Uses ATP energy)
Each class has more specific subclasses:
1. Oxidoreductases:
oxidases and reductases
dehydrogenases (remove 2 H; make double bond)
2. Transferases:
transaminases (amino group),
kinases (phosphate group)
3. Hydrolases:
lipases, proteases, nucleases, carbohydrases, and phosphatases (break ester
and amide bonds)
4. Isomerases:
racemases (D to L or L to D forms)
mutases (structural isomer D)
5. Lyases:
dehydratases (remove H2O from substrate)
decarboxylases (remove CO2 from substrate)
deaminases (remove NH3 from substrate)
hydratases (add H2O to substrate)
6. Ligases:
synthetases (hook 2 substrates together)
carboxylases (add CO2 from substrate)
Catalysts lower activation energy by:
1. Aligning substrate molecules at correct angle
2. Holding substrate while rxn occurs (less E)
Substrate and active site on enzyme must fit.
Some substrate molecules will fit / others will not,
Some substrates will react and others will not = “specificity.”
The intermediate product when the substrate and
enzyme co-mingle
= enzyme-substrate complex
There are two models for enzyme activity:
1. Lock & Key
2. Induced Fit
The active site of
an enzyme is usually a
crevice formed
from a protein’s
2o and 3o structural
characteristics.
Usually is only a small
portion of protein
structure.
May be multiple active
sites
The lock-and-key model for enzyme activity: Only a substrate
whose shape and chemical nature complement those of the
active site can interact with the enzyme.
enzyme-substrate
complex
The induced-fit model for enzyme activity: The enzyme
active site is not exactly complementary in shape to
substrate, but is flexible enough to adapt to
the
shape of the substrate.
Close enough for
government work!
Enzyme-substrate complex is
held weakly by intermolecular forces:
•Hydrogen Bonding
•Salt bridges + electrostatic forces
•Hydrophobic & hydrophilic regions
•Van der Waals interactions
Enzyme action animation
Amino acid R group interactions
bind a substrate to enzyme’s active site
until reaction is complete
Enzyme specificity levels
Enzymes can vary in their specificity:
Some active sites are very shape specific and
bind to only one compound.
Others are more generalized and focus on a
similar structure found on various “related”
molecules.
1. Absolute specificity
2. Stereochemical specificity
3. Group specificity
4. Linkage specificity
Factors affecting Enzyme Activity
Four main factors affect the reaction rate:
• Temperature changes
• pH changes
• Changes in substrate concentration
• Changes in enzyme concentration
Effect of temperature
on the rate of an
enzymatic reaction.
Why does heat
first raise, then drop off
the reaction rate?
What bonds would
break due to increased
molecular vibration?
How do thermophile
bacteria survive?
Effect of pH changes
on an enzyme’s
turnover rate.
Why does low / high pH
affect an enzyme?
What bonds would
break and why?
What type of bonds
would be found in
enzymes found in
acidophile bacteria?
A graph showing
D in enzyme activity
with a D in
substrate concentration
(constant temp, pH, &
enzyme concentration).
Enzyme activity
remains constant
after a certain
substrate concentration
is reached.
Why?
You can only work so fast!
(Turnover #)
A graph showing the D in reaction rate
with a D in enzyme concentration for
an enzymatic reaction.
Temp, pH, and substrate concentration are constant.
Why does the graph
not level off?
Adding more workers to
the assembly line,
production speeds up!
Textbook Summary of Enzyme Activity
3 Types of enzyme inhibition:
1. Competitive inhibition
2. Noncompetitive inhibition
3. Irreversible inhibition
Substrate blocked
Competitive inhibition
•Molecule with similar structure to substrate
•Competes for active site bonding
•Blocks active site temporarily
•Enzyme activity reduced / stopped
•Reversible reaction
(weak interactions: H bonding, etc)
Increasing concentration of substrate can overcome
inhibition effect (out competes for active site)
e.g. antihistamines vs histadine decarboxylation
(which converts histadine to histamine)
Viagra: interferes with nitric oxide breakdown
(NO is a vasodilator)
Noncompetitive inhibitor
•Bonds to another site other than active site
•Tertiary structure of enzyme altered
•Affects shape of active site (conformation change)
•Lowers or blocks substrate affinity for active site
•Reversible
Increasing concentration of substrate does
not overcome inhibition effect
Heavy metal poisoning: stops enzyme action
Mercury 200 amu, Cadmium 112 amu, Lead 207 amu