Enzymic Hydrolysis of Methyl 2,3-O-Acetyl-5-Deoxy-α and β-D-Xylofuranosides - An Active-Site Model of Pig Liver Esterase

Abstract

The regioselective enzymic hydrolysis of methyl 2,3-di-O-acetyl-5-deoxy-α-D-xylofuranoside (1) and methyl 2,3-di-O-acetyl-5-deoxy-β-D-xylofuranoside (2) in the presence of pig liver esterase (PLE) was studied by GLC. Diacetate 2 gave exclusively methyl 3-O-acetyl-5-deoxy-β-D-xylofuranoside (6) while diacetate 1 produced both methyl 2-O-acetyl-5-deoxy-α-D-xylofuranoside (3) and methyl 3-O-acetyl-5-deoxy-α-D- xylofuranoside (4) in low yield. At high conversion, methyl 5-deoxy-α-D-xylofuranoside (7) was the only product. The first-order rate constants, Michaelis constants, and maximal velocities were determined for 1, 2, and the monoacetates 3 - 6. The results were interpreted on the basis of a recent active-site model for PLE.

Notes

The rate constant k₃ can be neglected against k₂ because only traces of diol 8 were present in a reaction mixture. Thus, the sum (k₂ + k₃) is actually k₂.

According to the initial rates of the formation of both 3 and 4 in the early stage of 1 deacetylation, the ratio k₃/k₂ can be estimated as 1.8. It means that k₃ = 0.55 × 10⁻⁴ s⁻¹ and k₂ = 0.30×10⁻⁴ s⁻¹.

Although no rate constants are given in ref.,¹¹ the half-time of the hydrolysis can be roughly estimated.

The binding of hydrophobic groups must occur in the H_s area rather than in the H_L site, but the cyclopentyl ring is marginally too large for optimum fit into H_s. It may extend partially into H_s pocket.⁵