EXPLORING DONOR SUBSTRATE PROMISCUITY OF A THERMOSTABLE TRANSKETOLASE BY DIRECTED EVOLUTION

摘要

Enzymes catalyzing asymmetric carboligation reactions typically show very high specificity for their nucleophilic substrate. Transketolase (TK, EC 2.2.1.1) catalyses a reversible transfer of a hydroxylated C_2 fragment among phosphorylated ketoses and aldoses. Native TK converts a large variety of (2R)-hydroxyaldehydes as the electrophilic acceptor substrates, but apart from its natural phosphoketose donors TK accepts only hydroxy-pyruvate (hydroxylated donor) (Figure 1). In contrast, 1-deoxy-D-xylulose-5-phosphate synthase (DXS, EC 2.2.1.7) catalyzes the specific decarboxylative transfer of the acetyl moiety from pyruvate (non-hydroxylated donor) to glyceraldehyde-3-phosphate to yield 1-deoxy-D-xylulose 5-phosphate (DXP), which constitutes the first step into the non-mevalonate biosynthesis of terpenoids (Figure 1). Reactions of native TK and DXS are mutually exclusive in vivo.