Download Enzymes

Enzymes
• Large molecules made of various amino
acids
• Act as catalysts to speed up reactions w/out
being destroyed
– Highly specific
– Lowers energy of activation level
Enzymes lower
the energy of
activation for a
reaction
Enzyme Kinetics
E + S  ES complex  E + P
• Effect of substrate concentration
– 10 test tubes of fixed [E]
– Add gradations of [S]
– Measure rate of reactions
• Vmax occurs when
enzyme active sites
are saturated with
substrate
• Km (MichaelisMenten constant)
reflects affinity of
enzyme for its
substrate
• smaller the Km, the
greater the affinity an
enzyme has for its
substrate
Enzyme Kinetics
• [S] generally < than its Km
– Only uses fraction of enzyme catalytic ability
– Enzyme is able to respond to changes in [S]
• Isozymes (isoenzymes) are variations of
same enzyme
– Four isozymes of hexokinase
• Three have low Km and fourth has a high Km
Hexokinase can
phosphorylate
glucose even
with a low
blood [glucose];
a high Km
prevents liver
from taking up
blood glucose
when [glucose]
is low
Regulation of Enzymes
• Enzymes concentration
– Will increase Vmax but not Km
– Vmax proportional to [E]
• Competitive inhibition
Regulation of Enzyme Kinetics
• Competitive inhibition
–
–
–
–
have similar geometric shape
Compete with enzyme for substrate
Can be overcome by  [S]
Will not affect Vmax but will  Km
Regulation of Enzyme Kinetics
• Allosteric regulation (noncompetive
inhibitor/stimulator)
– Allosteric enzymes don’t follow MichaelisMenton kinetics; rather, most follow a
sigmoidal model
– Does not bind to active site on E
• Changes shape of E which either  of  ability to
bind with S
– Will change Vmax but not Km
Regulation of Enzyme Kinetics
• Phosphorylation
– cAMP activates protein kinase
– activates catabolic enzymes
– inactivates anabolic (synthetic) enzymes
• Effect of temperature, pH
Metabolic Pathway
E1
E2
E3
A ------> B ------> C ------> D
initial substrate
final substrate
First step is usually irreversible
and controlled by an allosteric enzyme