C. Stehlin et al., DELETION MUTAGENESIS AS A TEST OF EVOLUTIONARY RELATEDNESS OF INDOLEGLYCEROL PHOSPHATE SYNTHASE WITH OTHER TIM BARREL ENZYMES, FEBS letters, 403(3), 1997, pp. 268-272
The role of the extra helix alpha(0) in the IV-terminal extension of t
he eight-fold beta alpha barrel of indoleglycerol phosphate synthase,v
as probed by point mutation and truncation, Replacing invariant leucin
e 5 by valine of the enzyme from Escherichia coli affected neither k(c
at) nor K-M, but deletion of 8 N-terminal residues decreased solubilit
y strongly, The similarly truncated variant from the hyperthermophile
Sulfolobus solfataricus was soluble, and had the same k(cat) value as
the wild-type protein but a 220-fold greater K-M value, These results
suggest that the N-terminal portion of helix alpha(0) provides for str
ong binding of the substrate, but is not essential for stabilizing the
bound transition state, Thus, three enzymes of tryptophan biosynthesi
s operate essentially as canonical eight-fold beta alpha barrels, as r
equired for their divergent evolution. (C) 1997 Federation of European
Biochemical Societies.