[30] Carboxymethyl Esterase of Bacterial Chemotaxis

This chapter illustrates assay, purification, and properties of carboxylmethyl esterase of bacterial chemotaxis. Methylation of a set of transmembrane proteins, originally termed methyl-accepting chemotaxis proteins (MCPs) and now known to be receptors, play a central role in bacterial chemotaxis. Bacterial membranes are prepared with protein methyl groups ³H-labeled either in vitro or in vivo. Esterase activity results in the production of [³H]methanol, which is distilled and counted by liquid scintillation spectrometry. One unit of activity hydrolyzes 1 fmol of methyl ester/min. Specific activity is expressed as the number of enzyme units per mg protein. All purification procedures are generally carried out at 4°. The enzyme activity is maximum at pH 5.5. Stability of the esterase is inversely related to its activity. Maximum stability and low activity is observed in phosphate buffer at pH 7 with 50-100 mM added NaCl, while [2-(N-morpholino)ethanesulfonic acid] MES and low salt afford minimum protection against inactivation but high enzyme activity. Activity is also inhibited by a variety of residue-specific reagents.