1. Enzymes
Enzymes are highly efficient, selective biological catalysts which
accelerate the approach of metabolic reactions to equilibrium
without changing that equilibrium. Enzymes provide alternative
pathways with lower activation energies. Without enzymes
metabolic reactions would not take place quickly enough for cells
to live. Most enzymes are globular proteins.
The tertiary structure is important for the enzyme to carry out its
functions. A specific area called the active site of the enzyme
molecule binds other molecules called substrates to produce
required products.
Enzymes are relatively large molecules and form many contact
points with the substrate. In enzyme catalyzed reactions an
intermediate step forms when the enzyme binds to the substrate
called the enzyme substrate complex, the enzyme holds the
substrate in the required orientation for a reaction to occur, by
placing the substrate next to specific amino acids and co factors.
These binding points results from hydrogen bonds, Van der Waals
interactions and ionic interactions, during these chemical reactions
the enzymes themselves are not consumed in the end, one enzyme
can convert many substrate molecules into product.
Enzymes are specific in the types of reactions they catalyze and the
types of substrate they act on; they have particular shapes which
allow them to be unambiguous with which substrate they bind to.

2. Fig.I.1 Enzyme Activity
Table: 1.1 Enzyme Specificity
Type of Specificity Characteristics
Absolute Enzymes that act on a particular
substrate only
Group Enzymes that act either
relatively or absolutely on
substrates in close relations with
a particular functional group
Steriochemical Enzymes that act on particular
sterioisomers
Linkage Enzymes that act on a particular
bond regardless of the rest of the
molecules structure

3. In metabolism the enzyme usually does not react alone, sometimes
the enzyme cannot convert the substrate to product by itself; this is
facilitated by molecules called co-factors. These enzymes usually
have a binding site for the substrate and another for the co-factor,
the enzyme only catalyses for these to molecules and no other, the
enzyme does this by binding to the substrate first then the cofactor
preventing catalysis of the co factor with anything else.
Fig.I.2: Enzyme activity with co factor.
Inhibitors are compounds that bind enzymes; they disrupt their
activity by preventing either the formation of the enzyme substrate
complex or its breakdown to enzyme and product. There are two
types of inhibition reversible and irreversible. There are different
types of reversible inhibition:

4. Fig.I.3: Reversible Inhibition.
a) Competitive Inhibition
• Binds to the active site of the free enzyme only, has same
shape as substrate.
• If the enzyme is allosteric the binding of the inhibitor to
one site prevents the binding of substrate at another in,
the inhibitor may not have the same shape as the
substrate.
• Km increases.
• Vmax is unchanged.
• Both cases prevent the formation of the enzyme substrate
complex.
b) Noncompetitive Inhibition
• Binds to either the active site of the free enzyme or the
active site of the enzyme substrate complex, preventing
either the formation of the ES complex or the of the ES
complex to E+P.
• the substrate still binds to the enzyme

5. • The inhibitor alters the enzymes shape in such a way
that the enzyme becomes inactive. The inhibitor does
not have the same shape as the substrate.
• Vmax decreases.
• Km is unchanged.
•
c) Uncompetitive inhibition
• Binds to the active site of the enzyme substrate complex
only.
• does not compete with the substrate
• Prevents breakdown to enzyme and product. The inhibitor
does not have the same shape as the substrate.
• Km and Vmax both decrease.
• Ratio of Vmax/Km is unchanged.
d) Mixed Inhibition
• Binds to free enzyme
• Binding of either the inhibitor or substrate to the
enzyme decreases the enzymes affinity to the other.
• Km may increase or decrease.
• Vmax always decreases.
Irreversible inhibitors bind to enzymes by covalent bonds.

6. Fig.I.3: Graphs of factors affecting rate of enzyme catalyzed
reactions.
a) Effect of increasing substrate concentration
As concentration of substrate increases collision frequency
between substrates and active sites of enzymes increases, in turn
increasing rate. Because the number of enzyme molecules is fixed,
eventually all the enzyme active sites become filled with substrate
and the rate reaches a maximum without any further increase.
b) Effect of increasing temperature.
Increasing temperature increases the kinetic energy in the system
causing more molecules to have sufficient energy to get over the
energy barrier, increasing collisions between substrate and active
sites, further increase in temperature causes the bonds keeping the
protein together to vibrate, when the enzyme reaches optimum

7. temperature these bonds vibrate so much that they break. The
enzyme’s tertiary structure denatures and changes the shape of the
active site making the enzyme inactive. This denaturation is
cooperative; as soon as the optimum temperature is surpassed the
entire enzyme breaks down, it does not break down bond by bond.
Table :1.2 Enzyme Classification.
Group number Class Function Examples
1. Oxidoreductases catalyze oxidation Cytochrome
reduction reactions. oxidase, lactate
dehydrogenase
2. Transferases Transferases Acetate kinase,
catalyze group alanine deaminase
transfer reactions-
the transfer of a
functional group
from one molecule
to another.
3. Hydrolases In hydrolysis Lipase, sucrase
reactions, C-O, C-N,
and C-S bonds are
cleaved by addition
of H2O in the form
of OH- and H+ to the
atoms forming the
bond.
4. Lyases Lyases cleave C-C, Oxalate
C-O, C-N, and C-S decarboxylase,
bonds by means isocitrate lyase
other than
hydrolysis or
oxidation.
5. Isomerases Isomerases just Glucose-phosphate
rearrange the isomerase, alanine
existing atoms of a racemase
molecule, that is,
create isomers of the
starting material.
6. Ligases Acetyl-CoA
synthetase, DNA
ligase

8. Ligases synthesize
C-C, C-S, C-O, and
C-N bonds in
reactions coupled to
the cleavage of high
energy phosphate
bonds in ATP