Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

284 Appendix 4
(b) Formation of radical at C-2, followed by fragmentation of C-3–C-4
bond to give alkenyl radical, which re-attacks acrylic acid at C-2 to give
product radical.
(2) (a) Abstraction of H* to give radical intermediate; cyclisation onto ester
carbonyl to give cyclopropyl intermediate; fragmentation to give product
radical.
(b) Possibly SAM-dependent radical reaction, or generation by protein
radical or haem Fe5O.
(3) Abstraction of hydrogen atom from methyl group of toluene; attack
of radical on C5C of succinate to give product radical; abstraction of
hydrogen atom from glycine a-CH 2 to give product, and regenerate glycyl
radical.
(4) Generation of tyrosyl radical and cysteine radical (one-electron oxidations
by copper centre); radical coupling to form C2S bond, followed by rearomatisation.
(5) Attack of methanol at quinone C5O to form hemiketal intermediate,
followed by abstraction of adjacent hydrogen by hemiketal O and elimination
of formaldehyde (using the other C5O as electron sink), to give
reduced PQQ.
Chapter 12
(1) In principle the reaction is reversible, which would allow the ribozyme to
integrate itself into a piece of RNA (reminiscent of the life cycle of some
viruses).
(2) A cyclic phosphonate ester would be a good transition state analogue. Such
an analogue was used to elicit antibodies capable of catalysing this lactonisation
reaction with an enantiomeric excess of 94%.
(3) At neutral pH the tertiary amine will be protonated. When exposed to the
immune response, this will elicit a complementary antibody containing a
negatively-charged carboxylate group in close proximity. This group functions
as a base for the elimination reaction. This ‘bait and switch’ trick has
been used in other cases also.
(4) (a) Might expect to mimic cysteine proteases, hydrolysing aromatic ester
(and possibly amide) substrates.
(b) Might expect to mimic FAD-containing oxidases, oxidising aromatic
amines and aromatic thioethers (to sulphoxides).