Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Appendix 4 283
(4) Use of PLP as a four-electron sink. Formation of amino acid-PLP adduct
followed by a-deprotonation. Removal of C-3 proS hydrogen using imine
as electron sink followed by elimination of phosphate gives bg-unsaturated
intermediate. Protonation at g-position, followed by attack of water at b-
position, and reprotonation at a-position.
(5) Attachment of PLP onto a-amino group followed by a-deprotonation gives
ketiminine intermediate. Hydration of g-ketone group is followed by cleavage
of bg-bond, using imine as an electron sink. C–C cleavage and reprotonation
proceeds with overall retention of stereochemistry.
Chapter 10
(1) One S centre is epimerised by the epimerase enzyme with introduction of an
a- 2 H, but if left to equilibrate both S centres would undergo enzymecatalysed
exchange with 2 H 2 O. R centre is decarboxylated, with replacement
by 2 H, so in principle three atoms of 2 H would be found in the
l-lysine product.
(2) Deprotonation at g-position to form dienol intermediate is followed by
reprotonation at a-position. 2-Chlorophenol is processed to give d-chloro
intermediate. Upon deprotonation at the g-position loss of Cl gives a gdunsaturated
lactone. This is processed by opening of the lactone, and
reduction of the ab-bouble bond by an NADH-dependent reductase.
(3) Protonation of the dienol at C-5 is followed by attack of water (or an active
site nucleophile) at the C-6 ketone. Cleavage of the C-5,C-6 bond is then
facilitated by the presence of an ab-unsaturated ketone group which can
act as an electron sink.
(4) Possibilities are: (i) reversible attack of an active site nucleophile at the
amide carbonyl, allowing free rotation of the tetrahedral intermediate; (ii) a
‘strain’ mechanism in which the enzyme binds the substrate in a strained
conformation close to the transition state for rotation of the amide bond.
Available evidence points to mechanism (ii), and it is thought that cyclosporin
A acts as a mimic of this strained intermediate. (Note: the immunosuppressant
activity is due to the complex formed between cyclosporin
A and this protein, which is called cyclophilin.)
(5) Concerted four-electron pericyclic reaction is a disfavoured process. Stepwise
reaction possible by transfer of phosphate group to active site group,
followed by reaction of enol intermediate with phosphoenzyme species.
Chapter 11
(1) (a) Abstraction of hydrogen atom by Ado2CH 2 radical at C-2, followed by
cyclisation onto C-4 to give a cyclopropyl radical; fragmentation of
other C2C bond in cyclopropane ring to give product radical.