• Introduction of enzymes
• Active site
• Thermodynamic changes
• Covalent catalysis
• Acid base catalysis
• Catalysis by bond strain
• Catalysis by proximity and orientation
• Biological catalysts
• Neither consumed nor permanently altered
• All enzymes are proteins in nature except ribozymes
which are RNA in nature
• Highly efficient
• Act as selective catalysts
Site where actual
Substrate –bound by
Specificity of enzyme
depend on arrangement
of atoms in active site
The catalytic efficiency of enzyme is explained by
•Processes at the active
• acquire a transitional state.
• The difference in energy level of
transitional state and substrate is called
Only a few substrate can cross this barrier to
be converted to products.
That is why rate of uncatalysed reaction is
When enzyme is present it provides an
alternative pathway for conversion of substrate
Enzymes accelerate reaction rate by
providing transition states with low
activational energy for formation of
Hence reaction rate is enhanced by many
folds in the presence of enzymes
The total energy of the system remains the
same and equilibrium state is not disturbed
Processes at active site
Acid base catalysis
Catalysis by bond
• Enzyme form covalent linkages with substrate
forming transient enz-subs complex with very
low activational barrier.
• Enzyme is then released unchanged and
unconsumed and substrate is converted into
• This process is mostly undertaken by transferases and
• The hydrolytic enzyme especially proteolytic enzyme
works in this manner for cleavage of peptide bonds in
• Proteolytic enzymes mostly have serine at their active
site so OH goup of serine makes a transient complex
with COOH group of peptide bond with subsequent
cleavage by water.
• Mostly undertaken by oxido-reductases.
• Mostly at the active site, either histidine is present
which act both as a proton donor and a proton
• Sometimes aspartic acid, glutamic acid and
cysteine residues are also present which
participate in Hydrogen transfer reaction.
• Mostly undertaken by lyases.
• The enz-subs binding causes reorientation of the structure of
substrate in the site due to being in a strain condition.
• Thus transitional state is readily acquired and enzyme
maintains that transitional state where the bond is in the
unfavourable state and is eventually broken.
• So enzyme induces a strain in the bond which is required to be
• This mechanism is mostly undertaken by ligases.
• The rate of reaction is ↑ by bringing substrate closer to each
other at the a.site.
• A region of high substrate conc is produced at the a.site.
• The substrate molecule is placed at bond forming
• Since substrate is placed at optimal
• The probability of collision ↑ and
substrate is eventually converted into
• This mechanism involve the condensation of
• Class notes
• Biochemistry for medics