BIBC102 Metabolic Biochemistry

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BIBC102Metabolic Biochemistry

• Lecture 3
• January 15, 2008

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CHAPTER 6 Enzymes and Enzyme Kinetics
Michaelis-Menten (1912-1916)
k1 k2 E S
ES Products
k-1 E
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Derivation of Michaelis-Mentenequation

• 1. equilibrium assumption k-1 gtgt k2
• 2. steady state assumption dES/dt 0
• Note E Etotal ES

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Hmmm..??
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Garrett Grisham Biochemistry, 3rd Ed.
Fig. 13-08, p.413
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k2 x ET x S v
------------------------- Km
S
when S gtgt Km then v ? vmax
and vmax k 2 E T
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We use vo , the initial velocity/rate, because we
know S precisely only at the beginning of the
reaction, before much of the substrate has been
used up
k -1 k 2 Km
k 1
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d
Km Kd when k -1 gtgt k 2
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Fig. 6-11
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Fig. 6-12
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1 Km 1 1
x vo v
max S v max
y ax b
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Lineweaver-Burk Plot
Double Reciprocal Plot
box. 6-1
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- another definition

v max k2 k cat
------------ E T
TURNOVER NUMBER

• The max. number of molecules (moles) of product
  produced
• per molecule (mole) of enzyme per
  second

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molecules of substrate / molecule of enzyme .
second moles of substrate / mole of enzyme .
second
(moles/liter) of substrate / (moles/liter) of
enzyme . second
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H2O2 ? H2O 1/2O2
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Reactions with more than one substrate (more
typical)
Overall reaction S1 S2
P1 P2
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Fig. 6-13
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Lines intersect ternary complex is formed in the
reaction
LNC 6-14 a
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Lines intersect ternary complex is formed in the
reaction
1/S1
LNC 6-14 a
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This case is also referred to as the
Ping-Pong or double displacement mechanism
LNC 6-13 b
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Lines do not intersect no ternary complex is
formed
LNC 6-14 b
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Enzyme Inhibitors
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Fig. 6-15a
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Competitive Inhibition
a 1 I / KI
box. 6-2.1
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Fig. 6-15b
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Uncompetitive Inhibition
a 1 I / KI
box. 6-2.2
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Fig. 6-15c
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Mixed Inhibition
box 6-2.3
When a a, then we also speak of
noncompetitive inhibition
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Purpose/rationale the precursor is saved to be
used for other reactions
FEEDBACK INHIBITION
Typically, the product at the end of a
pathwayinhibits an enzyme early in the pathway
LNC 6-28
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Allosteric Regulationof Enzymes
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C catalytic subunit
R regulatory subunit
LNC 6-26
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ASPARTATE TRANSCARBAMYLASE two stacked catalytic
clusters each with 3 catalytic subunits and 3
regulatory clusters (two polypeptides)
LNC 6-27
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homotropic enzyme substrate is positive
modulator (e.g. hemoglobin)
LNC 6-29a
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No change in vmax, but change in Km
LNC 6-29b
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Both vmax and Km change
LNC 6-29c
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END of Lecture 3

• January 15, 2008