Characterization of two isoalleles and three mutations in both isoforms of purified recombinant human porphobilinogen deaminase

Abstract

Defects in the enzyme porphobilinogen deaminase (PBG‐D) are associated with acute intermittent porphyria (AIP). Human PBG‐D is transcribed into a housekeeping or an erythroid form as a result of differential promoter usage and splicing. In addition, three pairs of isoallelic forms have been described. However, whether the enzymatic properties of housekeeping and erythroid forms differ is unknown. In this study the two isoallelic forms, K210 and E210, were cloned and expressed in Escherichia coli together with three mutations associated with a clinical AIP phenotype. The mutations were introduced in the K210 isoallelic background and expressed as both the housekeeping and the erythroid form. The proteins were expressed as GST fusions and purified to homogeneity. Initial experiments revealed that the GST‐PBG‐D fusions and the purified PBG‐D obtained by proteolytic removal of the GST moiety had enzymatic properties that were indistinguishable. Consequently, all analyses with mutant PBG‐D were performed on the GST‐fusion proteins. Comparison of the wild‐type proteins revealed a significant difference in Km between isoalleles with a Km of 9 µM for K210 and 7 µM for E210, whereas no significant difference in activity or kinetics between the housekeeping and the erythroid isoforms was observed. The mutant proteins showed 0.3–1.0% wild‐type activity, depending on mutation. There was a clear correlation between yield of recombinant protein and CRIM status of patients. Furthermore, co‐expression of the mutant proteins with the bacterial chaperone GroESL did not affect protein yield or function to any significant extent, supporting the view that the investigated mutations primarily influence structure and function and not folding of the proteins.