Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Transformations of Amino Acids 217
Figure 9.9 Active site of asparate aminotransferase (PDB Wle 1AJS). l-Aspartic acid bound to
pyridoxal 5 0 -phosphate coenzyme shown in black. Arg-292 (left), Arg-386 (top right) and catalytic
base Lys-258 (right) shown in red.
O
HN
H SH Br
NH 3
+
O
HN
H
S
NH 3
+
inactive
Figure 9.10 Cys-258 mutant of aspartate aminotransferase.
7% activity
by Arg-386 and the b-carboxylate by Arg-292. In principle, the substrate speciWcity
of this enzyme could be changed by replacing Arg-292 by other amino
acids. Mutation of Arg-292 to an aspartate residue gave an enzyme whose
catalytic eYciency for l-aspartate had dropped from 34 500 m 1 s 1 to 0.07
m 1 s 1 . However, the mutant enzyme was found to be capable of processing
l-amino acid substrates containing positively charged side chains which could
interact favourably with Asp-292, illustrated in Figure 9.11. So l-arginine,
l-lysine and l-ornithine (one carbon shorter side chain than lysine) were all
processed by the mutant enzyme, the best substrate being l-arginine with a
k cat =K M of 0:43 m 1 s 1 . This is an example of how the method of site-directed