This esterase enzyme is secreted by the bacterium Streptomyces scabies, a causal agent of the potato scab decease. Invasion into the plant host is brought about by the liplytic action of the secreted esterase, which is presumed to hydrolyse specific ester bonds within the suberin lipid which covers the plant tubers. Although the esterase possesses a catalytic triad similar to that of other serine hydrolases, the structure remains unique among known proteins. It is the first example of a naturally occurring enzyme in which a neutral hydrogen bond acceptor, a main chain carbonyl, replaces a carboxylic acid within the triad.