engleski

AUTOCATALYTIC PROCESSING OF ASPARTATE DECARBOXYLASE

Escherichia coli L-aspartate-a-decarboxylase (ADC), encoded by the gene panD, catalyses the formation of b-alanine from L-aspartate, a step in the biosynthetic pathway of pantothenate (vitamin B5). ADC is a (homo)tetramer with its active sites located between adjacent subunits1. The protein is initially translated as an inactive proenzyme, which is autocatalytically activated: the maturing process involves a cleavage of Gly24-Ser25 peptide bond and transformation of S25 to pyruvoyl. We crystallised the inactive proenzyme, the completely activated form, and the non-processing S25A mutant of ADC. The proenzyme and activated ADC are isostructural (hexagonal crystals) to the previously solved, partially activated ADC,1 with two polypeptide chains (half of the tetramer) in the asymmetric unit. The S25A mutant crystallise in the tetragonal space group I422 with cell parameters a = 73.1, c = 111.1 A. The structure was solved by molecular replacement using AMoRe with native ADC as search model. The asymmetric unit contains only one chain which is related to the other three subunits of the tetramer by the crystallographic four-fold axis. The proenzyme differs from the S25A mutant only in the OH group of the Ser25 sidechain; however, they adopt different conformations in the region Ala18 to Ser25, indicating a possible conformational change during autocatalytic activation of ADC. _____________ [1] Albert et al., Nature Struct. Biol. 5 (1998) 289-293.

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