Arginine Decarboxylase from Escherichia coli

Abstract

Abstract The inducible arginine decarboxylase of Escherichia coli B has a sedimentation coefficient, s°20, ω of 23.3 S and a molecular weight of 850,000. It dissociates to five subunits of identical size which have a sedimentation coefficient of 8.0 S and a molecular weight of 165,000. Dissociation occurs when the buffer cations are monovalent, the pH is greater than 6.5, and the concentration of monovalent cations is less than 0.05 m. It proceeds more rapidly at elevated temperatures and low protein concentrations. The characteristics of this dissociation suggest that it is caused by the creation of a new electrostatic repulsion, possibly by ionization of the carboxyl groups of glutamic or aspartic acid. The dissociated species can be reassociated fully with an activity loss of less than 10% by dialysis against a buffer containing divalent cations or a high concentration of monovalent cations at a pH less than 6. If the dissociated species is exposed to an inhibitory substrate analogue such as α-chloroδ-guanidovaleric acid, as many as five species can be observed in the ultracentrifuge. Two of these correspond to the 8 S and 23 S species; the other three have sedimentation coefficients of approximately 12.1 S, 16.4 S, and 19.5 S. The five sedimentation coefficients are consistent with a molecular weight ratio of 5:4:3:2:1, providing excellent support for the conclusion that the 23 S species is composed of five 8 S subunits. Preliminary experiments indicate that the 8 S subunit is itself a dimer; this is consistent with the observation of 10 pyridoxal-P binding sites per (associated) molecule. Measurements of decarboxylase activity indicate that the 23 S species is almost certainly active, whereas the 8 S species is either less active or inactive.