This domain corresponds to an amidase function. Many of these proteins are involved in cell wall metabolism of bacteria. This domain is found at the N-terminus of Swiss:P43675, where it functions as a glutathionylspermidine amidase EC:3.5.1.78 [1]. This domain is found to be the catalytic domain of PlyCA [4]. CHAP is the amidase domain of bifunctional Escherichia coli glutathionylspermidine synthetase/amidase, and it catalyses the hydrolysis of Gsp (glutathionylspermidine) into glutathione and spermidine [5].

1: Glutathionylspermidine metabolism in Escherichia coli. Purification, cloning, overproduction, and characterization of a bifunctional glutathionylspermidine synthetase/amidase. Bollinger JM Jr, Kwon DS, Huisman GW, Kolter R, Walsh CT; J Biol Chem 1995;270:14031-14041. PMID:7775463

2: The CHAP domain: a large family of amidases including GSP amidase and peptidoglycan hydrolases. Bateman A, Rawlings ND; Trends Biochem Sci 2003;28:234-237. PMID:12765834

3: Amidase domains from bacterial and phage autolysins define a family of gamma-D,L-glutamate-specific amidohydrolases. Rigden DJ, Jedrzejas MJ, Galperin MY; Trends Biochem Sci 2003;28:230-234. PMID:12765833

4: PlyC: a multimeric bacteriophage lysin. Nelson D, Schuch R, Chahales P, Zhu S, Fischetti VA; Proc Natl Acad Sci U S A. 2006;103:10765-10770. PMID:16818874

5: Structure and mechanism of Escherichia coli glutathionylspermidine amidase belonging to the family of cysteine; histidine-dependent amidohydrolases/peptidases. Pai CH, Wu HJ, Lin CH, Wang AH; Protein Sci. 2011;20:557-566. PMID:21226054