Chemistry Reference
In-Depth Information
Figure 15.7
(Plate 18)
Coordination of the two metal ions in Group I intron. The two metal
ions have similar coordination geometry (hence symmetric) and can reverse their roles as 'A'
and 'B' metal ions in the successive steps of RNA splicing reaction. (Reproduced from Yang,
W.
et al
., Making and breaking nucleic acids: two-Mg
2+
-ion catalysis and substrate specifi city,
Mol. Cell
, 2006,
22
, 5-13) (See colour plate section)
don't require divalent cations, for example, hepatitis virus delta ribozyme,
113
RNase
A, type IB topoisomerase, phage l integrase and g
d resolvase.
114
Reactions resulting
in 5
-OH usually require one or two metal ions. Nucleases with
the b
b
a-metal motif, for example His-Cys and CIY-YIC homing and HNH endo-
nucleases
74,115 - 118
have no conserved catalytically essential carboxylates in the active
site and require one single divalent cation for catalysis. The single metal ion occupies
the same position relative to the scissile phosphate as metal ion B of the two-
metal-ion mechanism. These nucleases often depend on a conserved His to activate
a nucleophile. Without the requirement for specifi cally aligning two metal ions, a
variety of divalent cations are able to support one-metal-ion catalysis.
118
These types
of nucleases are either sequence nonspecifi c or specifi c. If sequence specifi c, the
cleavage and binding specifi city are equal and specifi city is achieved by noncatalytic
domains that recognize 20-40 bps, much longer than the 4-8 bps typically recognized
by type II REases.
119
′
- phosphate and 3
′
15.4.2 Prediction of Two - Metal - Ion Catalysis
Two - metal - ion catalysis, because of its unique specifi city and versatility, can be
readily predicted based on the composition of catalytic residues, the nature of
