Production of D-phenylglycine from racemic (D,L)-phenylglycine via isoelectrically-trapped penicillin G acylase.

Abstract

Penicillin G acylase (PGA) is exploited for producing pure D-phenylglycine from a racemate mixture, via an acylation reaction onto a cosubstrate, the ester methyl-4-hydroxyphenyl acetate. The reaction, when carried in a batch, is severely hampered by the reverse process, by which the product, 4-hydroxyphenylacetyl-(L)phenyl glycine, upon consumption of L-phenylglycine, is converted by the enzyme back into free substrate and 4-hydroxyphenyl acetic acid via lysis of the amido bond. To prevent this noxious reaction, a multicompartment electrolyzer with isoelectric membranes (MIER) is used as enzyme reactor, operating in an electric field. PGA is trapped between pI 5.5 and pI 10.5 membranes, together with an amphoteric, isoelectric buffer (lysine). As the 4-hydroxyphenylacetyl-(L)phenyl glycine product is formed, it vacates the reaction chamber by electrophoretic transport and is collected close to the anode, in a chamber delimited by pI 2.5 and 4.0 membranes. The same fate occurs to the free acid 4-hydroxyphenyl acetic acid, formed upon spontaneous (and enzyme-driven) hydrolysis of the methyl ester in the reaction chamber. These combined processes leave behind, in the enzyme reaction chamber, the desired product, pure D-phenylglycine. The advantages of the MIER reactor over batch operations: the consumption of the L-form in the racemate is driven to completion and the enzyme is kept in a highly stable form, maintaining 100% activity after one day of operation, during which time the PGA enzyme, in the batch reactor, has already lost >75% catalytic activity.