Lipopolysaccharide biosynthesis without the lipids: recognition promiscuity of Escherichia coli heptosyltransferase I.

Heptosyltransferase I (HepI) is responsible for the transfer of l-glycero-d-manno-heptose to a 3-deoxy-α-D-oct-2-ulopyranosonic acid (Kdo) of the growing core region of lipopolysaccharide (LPS). The catalytic efficiency of HepI with the fully deacylated analogue of Escherichia coli HepI LipidA is 12-fold greater than with the fully acylated substrate, with a k(cat)/K(m) of 2.7 × 10(6) M(-1) s(-1), compared to a value of 2.2 × 10(5) M(-1) s(-1) for the Kdo(2)-LipidA substrate. Not only is this is the first demonstration that an LPS biosynthetic enzyme is catalytically enhanced by the absence of lipids, this result has significant implications for downstream enzymes that are now thought to utilize deacylated substrates.