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ENZYMES- INTRO
Ms C
...
g
...
If an enzyme is
broken down into its component amino acids, its catalytic
activity is always destroyed
...

• Others require an additional chemical component called
a cofactor—either one or more inorganic ions, such as
Fe2, Mg2, Mn2, or Zn2 , or a complex
organic or metalloorganic molecule called a coenzyme

3

• A coenzyme or metal ion that is very tightly

or even covalently bound to the enzyme
protein is called a prosthetic group
• A complete, catalytically active enzyme together
with its bound coenzyme and/or metal ions is
called a holoenzyme
...


5

Active site
• Active catalytic site
...
Within the
catalytic site, functional groups provided by
coenzymes, tightly bound metals,
and, of course, amino acid residues of the
enzyme, participate in catalysis

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Catalysis occurs at the active site
-Cleft or pocket
- The catalytic sites of multimeric enzymes are often located at the
interface between subunits and recruit residues from more than one
monomer
-The three-dimensional active site – shields substrate from solvent
- facilitates catalysis
- Substrate binds to the active site
-aligns portions of the substrate that will undergo change with the
chemical functional groups of aminoacyl residues
- Active site also binds and orients cofactors or prosthetic groups
- Many amino acyl residues drawn from portions of the polypeptide
chain contribute to the extensive size and three-dimensional
character of the active site

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Some common features of active sites of enzymes
1
...
Three dimensional entity formed by groups from different parts of
the linear amino acid sequence
3
...
Active sites are clefts or pockets
...
The specificity of the binding depends on the precisely defined
arrangement of the atoms in an active site

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9

Enzyme specificity/ properties
Highly efficient
• Enhance the rates of the corresponding noncatalyzed reactions by
at least 106
• Neither consumed nor permanently altered as a consequence of the
reaction
Selective of reaction type
• Specific for the type of reaction they catalyze
• The ability of an enzyme to catalyze one specific reaction and
essentially no others is perhaps its most significant property
• Specific for a single substrate or a small set of closely related
substrates
Optical specificity• show absolute optical specificity for at least a portion of the
substrate molecule
– glycolytic pathway enzymes catalyze interconversions of D- but
not L- phosphosugars
– most mammalian enzymes act on L- but not D- amino acids

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Group specificity• Lytic enzymes act on specific chemical groupings e
...
pepsin and
trypsin on peptide bonds
– a large number of substrates may be attacked, thus lessening
the number of digestive enzymes that might be required

• Certain lytic enzymes exhibit a higher order of group specificity
– Chymotrypsin preferentially hydrolyzes peptide bonds in which
the carboxyl group is contributed by the aromatic amino acids
phenyalanine, tyrosine and tryptophan
– Carboxypeptidase and aminopeptidase split off amino acids one
at a time from the carboxyl- and amino-terminal end of
polypeptide chains respectively
– Although some oxidoreductases utilize either NAD+ or NADP+ as
electron acceptor most use exclusively one or the other
• Specificity enables cells to simultaneously conduct and
independently control a
broad spectrum of biochemical processes
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International classification of Enzymes
No
1

2

3
4

5
6

Class
Oxidoreductases

Type of reaction catalyzed
Enzymes catalyzing oxidoreductions between
two substrates, S and S’
S reduced + S’ oxidized = S oxidized + S’ reduced
Transferases
Enzymes catalyzing a transfer of a group, G
(other than hydrogen), between a pair of substrates S and S’
S-G + S’ = S’–G + S
Hydrolases
Enzymes catalyzing hydrolysis of ester, ether,
peptide, glycosyl, acid anhydride, C-C bonds
Lyases
Enzymes catalyzing addition of groups to
double bonds, or formation of double bonds by removal of groups
X Y
C ─ C = X-Y + C C
Isomerases
Enzymes catalyzing transfer of groups within
molecules to yield isomeric forms
Ligases
Enzymes catalyzing formation of C-C, C-S, CO, and C-N bonds by condensation reactions coupled to ATP
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hydrolysis

Summary of classification

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Substrates induce conformational change
-‘Lock and key model’ by Emil Fisher
He accounted for the specificity of enzyme-substrate interactions but
failed to account for the dynamic changes that accompany catalysis
...

-The induced fit model by Daniel Koshland (confirmed by
biophysical studies during substrate binding)
When a substrate approaches and binds to an enzymes it induces a
conformational change in the enzyme that is complimentary to that
of the substrate
...
The enzyme then induces reciprocal changes in the substrate to
facilitate transformation of substrate into products
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• This model recognizes that the substrate binding site is
not a rigid ―lock‖ but rather a dynamic surface created by
the flexible overall three-dimensional structure of the
enzyme
• The function of the conformational change induced by
substrate binding, the induced fit, is usually to reposition
functional groups in the active site in a way that
promotes the reaction
• It also improves the binding site of a co-substrate, or
activates an adjacent subunit on the enzyme
...
g

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