Active site of the enzyme which demethylates receptors during bacterial chemotaxis

The CheB methylesterase catalyzes the hydrolysis of glutamyl methyl esters in bacterial chemoreceptor proteins. Studies with residue-specific inhibitors suggest that a cysteine residue is required. The nucleotide sequence of the CheB gene predicts a 349-amino acid protein with cysteine residues at positions 207 and 309. Oligonucleotide-directed mutagenesis was used to change each cysteine to an alanine. Whereas the Cys²⁰⁷-Ala mutation had essentially no effect on esterase activity, the Cys³⁰⁹-ala mutation caused a complete inactivation of the enzyme. Cys³⁰⁹ is located adjacent to a sequence of amino acids which is characteristic of the β-α-β motif found in a number of nucleotide binding proteins associated with receptor function in vertebrate tissues. A central feature of this sturcture is Gly-X-Gly-X-X-Gly. Mutation of the second glycine in this region (Gly²⁸⁴) to a valine also caused a complete loss of esterase activity.