Transcript Enzyme

Enzyme
• Halo enzyme= apoenzyme+ cofactor
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coenzyme(non covalane)
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organic
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prostatic(covalan)
• Cofactor
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inorganic
activitor
• Artificial enzymes, termed abzymes, have
been synthesized by making antibodies
against chemicals that are transition-state
analogs.
• Abzymes have been designed to catalyze
over 100 different chemical reactions
• Metalloenzymes contain a tightly bound
transition metal, such as Zn2+ or Fez+,
Carbonic anhydrase
• In mid-2004, information was available for
over 83,000 different enzymes from 9800
different organisms;
• International Union of Biochemistry and
Molecular Biology (IUBMB) to characterize
each enzyme
Enzyme nomenclature
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1- Ordinary
Substrate name+ enzyme action+ase
Urea + H2O
2NH3+H2CO3
Urea hydrolase = Urease
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2- Classic IUB
EC=1.2.3.4
Butyrylcholinesterase BChE (EC.3.1.1.8
acetylcholinesterase (ACHE, EC.3.1.1.7)
1- Oxido Reductase
Alcohol dehydrogenase
• Alcohol dehydrogenase an enzyme that oxidizes an
alcohol to an aldehyde has the IUBMB number
1.1.1.1.T his indicates
• ( 1) that the enzyme is involved in an oxidationreduction reaction (first number),
• (2) it removes hydrogen as a hydride ion with NAD+
as the electron accepror (second n umber),
• and (3) substrates for the enzyme can be most
primary alcohols (third number).
• (4 ) The last number is reserved to different substrat
each enzyme that catalyzes the same overall
reaction but with different substrates. Lactate
dehydrogenas e (l.l .1.27
Alcohol DH
Glucose oxidase
2- Transferase
catalyze transfer of groups such as methyl or glycosyl
groups from a donor molecule to an acceptor
molecule
Alanin Amino Transferase (ALT)= Glutamic pyruvat Transferase(GPT)
Ping Pong
Hexo Kinase or Glucokinase
• For example, the enzyme commonly
called “hexokinase” is designated “ATP:Dhexose-6-phosphotransferase
• E.C. 2.7.1.1.” This identifies hexokinase as
a member of class 2 (transferases),
subclass 7 (transfer of a
• phosphoryl group),
• sub-subclass 1 (alcohol is the phosphoryl
acceptor).
• Finally, the term “hexose-6” indicates that
the alcohol phosphorylated is that of
carbon six of a hexose.
3- Hydrolase
• catalyze the hydrolytic cleavage of C-C, C-O,
C-N, P-O, and certain other bonds, including
acid anhydride bonds.
• ButyrylCholine +H2O
Choline+Butyric Acid
• Butyrylcholinesterase BChE
(EC.3.1.1.8)
• acetylcholinesterase (ACHE, EC.3.1.1.7)
• RNases and DNases are enzymes that
hydrolyze the phospho—ester bond
4-Lyase
catalyze cleavage of C-C, C-O, C-N, and other bonds
by elimination, leaving double bonds, and also add
groups to double bonds
Hydrolysis of Asparaginase
and Leukemia
• Some forms of juvenile leukemia require
the nonessential amino acid asparagine
• hydrolase enzyme that converts
asparagine into aspartic acid
• This form of (the enzyme was encoded in
polyethlenglycol)
• the enzyme is commercially available and
is approved for human treatment.
5- Isomerase
catalyze geometric or structural changes within a single molecule
(moving a group or a double bond within the same molecule)
• Glucose6Phosphate
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Fructose6Phosphate
Hexose6Phosphate isomerase
6- ligase or synthetase
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Urea+NH3+ATP
Carbomyl phoshate +
ADP
Cabomylphosphate synthetase
Oxaloacetate+AcetylCoA Citrate+CoA
Citrate syntase
Induced-fit model
Lack-and-key model
• Hydrophobic portions of the substrate bind
such that they are in a hydrophobic portion
of the protein, referred to as a
hydrophobic pocket
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ISOZYMES ARE DISTINCT ENZYME
FORMS THAT CATALYZE THE
SAME REACTION
Higher organisms often elaborate several
physically distinct versions of a given enzyme,
each of which catalyzes the same reaction.
• Like the members of other protein families, these
protein catalysts or isozymes arise through
gene duplication.
• Isozymes may exhibit subtle differences in
properties such as sensitivity to
Enzyme kinetic
•E+S
ES
E+P
Active site
pH
Acid phosphatase
Alkaline Phosphatase
Histidine is a general base or
general acid pki=7
Enzyme is a protein
Temperature
Stability enzyme
Vant hofe law
[E]
[S]
Time
Zero order in substrate
V=Vmax.[S]
Km+[S]
Michele's Minton Equ
Line Weaver Burk Equ
• An example of the importance of Km is the
physiological
utilization of glucose. Glucose can
be phosphorylated by two different kinases to form
glucose6 –phosphate Liver contains
• both hexokinase and glucokinase that catalyze the
identical reaction of
• glucose+ ATP glucose6 -phosphate+ ADP.
• For hexokinase the Km-for glucose is 0. mM whereas
for glucokinase it is 5 mM. lf then the concentration of
blood sugar is low, as occurs in the fasted state,
hexokinase is used to phosphorylated glucose, but
when blood glucose increases
• after feeding, the high K-enzyme also function
The Hill Equation Describes the Behavior
of Enzymes That Exhibit Cooperative
Binding of Substrate
• While most enzymes display the simple
saturation kinetics depicted in Figure 8–
3 and are adequately described by the
Michaelis-Menten expression,
• some enzymes bind their substrates in a
cooperative fashion analogous to the
binding of oxygen by hemoglobin
• Enzymologists therefore employ a graphic
representation of the Hill equation
• originally derived to describe the cooperative
binding of O2 by hemoglobin. Equation (43
log vi/(Vmax − vi)
log[S]
• A graph of log vi/(Vmax − vi) versus log[S] gives
a straight line (Figure 8–7), where the slope of
the line n is the Hill coefficient,
• an empirical parameter whose value is a
function of the number, kind, and strength of the
interactions of the multiple substrate-binding
sites on the enzyme.
• When n = 1, all binding sites behave
independently, and simple Michaelis-Menten
kinetic behavior is observed.
• If n is greater than 1, the enzyme is said to
exhibit positive cooperatively
• Binding of the first substrate molecule then
enhances the affinity of the enzyme for binding
additional substrate.
• The greater the value for n, the higher the
degree of cooperativity and the more sigmoidal
will be the plot of vi versus [S].
• A perpendicular dropped from the point where
the y term log vi/(Vmax − vi) is zero intersects
the x axis at a substrate concentration termed
S50,
• the substrate concentration that results in halfmaximal velocity.
• S50 thus is analogous to the P50 for oxygen
binding to hemoglobin
•Inhibitors and
activators
Inhibitors
•Reversible
•Irreversible
Competitive Inhibitors Typically
Resemble Substrates
• The effects of competitive inhibitors can be overcome by
raising the concentration of the substrate.
• Most frequently, in competitive inhibition the inhibitor, I,
binds to the substrate-binding portion of the active site
and blocks access by the substrate.
• The structures of most classic competitive inhibitors
therefore tend to resemble the structures of a substrate
and thus are termed substrate analogs.
• Inhibition of the enzyme succinate dehydrogenase by
malonate illustrates competitive inhibition by a substrate
analog
Competitive
Simple Noncompetitive Inhibitors Lower
Vmax but Do Not Affect Km
• In noncompetitive inhibition, binding of the inhibitor does
not affect binding of substrate. Formation of both
• EI and EIS complexes is therefore possible. However,
• while the enzyme-inhibitor complex can still bind
substrate,
• its efficiency at transforming substrate to product,
• reflected by Vmax, is decreased. Noncompetitive
• inhibitors bind enzymes at sites distinct from the
substrate• binding site and generally bear little or no structural
• resemblance to the substrate
Uncompetitive
ES+ I ESI
Succinate dehedrogenase
Irreversible
Choline esterase
Di iso propylphosphate Fluoride
(DIPF
GAL3PDH
• A Case of Poisoning Emergency room personnel encounter
many instances of pesticide poisoning and must be equipped
to recognize and treat these cases.
• Many of the common insecticides are organophosphate
compounds that irreversibly inhibit the action of acetylcholine
esterase(AChE) in the postsynaptic fibers of the cholinergic
neurons (p. 948) by
• Forming stable phosphate esters with a specific serine in the
active site of the esterase Inhibition of AChE prevents the
hydrolysis of acetylcholine in the synapse resulting in
constant stimulation of the end organs of these neurons.
• The most prominent effects of pesticide poisoning in humans
are paralysis of the respiratory muscles and pulmonary
• edema.
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If given early enough, a drug like pralidoxime can
displace the alkyl phosphate from the p€sticide bound to the
active site serine and regenerate an active AchE
Alostric enzyme
Glycogenphosphorylase phosphorylated form active
Glycogensyntase phosphorylated form inactive
Glycogephosphorylase dephosphorylated form inactive
Glycogensyntase dephosphorylated form active