1. REVIEW OF IMINE CHEMISTRY

2. Formation of Simple Imines
remove
primary
amine
aldehyde
or ketone
These reactions do not favor the formation of the
imine unless:
- the product (imine) is insoluble
(crystallizes or precipitates) or
- water is removed to drive the equilibrium
- the reaction is enzyme-mediated

3. Mechanism of Imine Formation
weak base addition - acid catalyzed
H-O H 2 1
H-O-H H +
H-O-H H +
acid-catalyzed
addition
proton exchanges
an imine .. +
H-O-H H + +
loss of water
(elimination)
H-O H
deprotonation

4. Hydrolysis of Simple Imines
REVERSAL
In an excess of aqueous acid, simple imines hydrolyze
back to the aldehyde or ketone and the amine from
which they were orginally formed …..
H3O+
Imines that are not soluble, however, are difficult to
hydrolyze.

5. COENZYME MECHANISMS

6. SIMPLIFIED COENZYMES PLP FAD Reduced to the chemically
significant portion - the part
that undergoes change.

7. Pyridoxal-5’-phosphate (PLP)
REACTS WITH AMINO ACIDS TO FORM IMINES ..
an amino acid +
- H2O
pyridoxal-5’-phosphate
PLP
( P5’P )
formation of
the imine +
imine

8. : .. THE IMINE CAN LOSE A HYDROGEN AND REPLACE IT ( or …. next page )
Enz-B: :
electron
flow
electron
flow
-H+ .. +
The aromatic ring is
restored by adding
a proton at another
site in the conjugated
system.
+H+
The positively-charged
nitrogen is an “electron
sink” attracting the
released pair of electrons.
It helps to acidify the
proton which is removed.
Electrons flow downward
through the conjugated
system to the nitrogen. + A

9. HYDROLYSIS + + + B C A
H2O
H2O
H2O

10. TRANSAMINATION

11. TRANSAMINATION
An amino group is transfered from one amino acid to an
a-ketoacid, thereby creating a new amino acid and converting
the original amino acid to an a- ketoacid.
PLP + +
transfer via pyridoxamine
+NH2
-NH2
pyridoxyl
pyridoxamine
pyridoxyl

12. Pyridoxyl-5’-phosphate (PLP)
Converts amino acids to a-ketoacids, and vice versa.
Biologically important in transamination reactions. .. +
an amino acid
pyridoxyl-5’-phosphate
- H2O
( PLP )
continued
formation of
the imine +
first imine

13. a-ketoacid :Enz Enz-H converts tautomerism H-Enz + Enz: first imine
H2O +
new imine
Removing the
amino group
hydrolysis of
the new imine +
pyridoxamine

14. TRANSFERRING THE AMINO GROUP
a different a-ketoacid
pyridoxamine
tautomerism
hydrolysis of the imine
These steps are the
reverse of those on
the previous slides.
a different
amino acid

15. SUMMARY TRANSAMINATION + PLP PLP
We will take a close look at the implications of this
process in the next lecture (uptake of nitrogen).