Expression, Lipoylation and Structure Determination of Recombinant Pea H‐Protein in Escherichia coli

A synthetic gene encoding the entire mature H protein of the glycine decarboxylase complex from pea ( Pisum sativum L.) was constructed and expressed in Escherichia coli. The recombinant H protein, which after the induction period constituted more than half of the E. coli protein, was found in a soluble form. Activity measurements and mass‐spectrometry analysis of the purified protein showed that, in the absence or presence of 5[3‐(1,2)‐dithiolanyl]pentanoic acid (lipoic acid) in the culture medium, recombinant H protein could be produced as the unlipoylated apoform or as the lipoylated form, respectively. Addition of chloramphenicol to the culture medium after induction increased the proportion of lipoylated H protein. High rates of lipoylation of the H apoprotein were measured in vivo and in vitro , revealing that the recombinant pea H protein was an excellent substrate for the E. coli lipoyl‐ligase. The three‐dimensional structure of the recombinant H apoprotein was determined at a 0.25‐nm resolution. It was almost identical to the structure of the native pea leaf enzyme, which indicates that the recombinant protein folds properly in E. coli and that the lipoyl‐ligase recognizes a three‐dimensional structure in order to add lipoic acid to its specific lysine residue. It is postulated that the high level of expression and lipoylation of recombinant H protein may be due to the protein retaining the structure of the original enzyme.