Csd3 (also known as HdpA) is a bi-functional enzyme with delta,delta-endopeptidase activity and delta,delta-carboxypeptidase activity. The N-terminal domain is also known as domain 1 and is composed of an alpha/beta fold consisting of a five-stranded antiparallel beta-sheet and three short alpha-helices. Domain 1 blocks the active-site cleft of the LytM domain, with the protruding helix alpha-3 contributing to the Zn2+ coordination sphere. The fold of domain 1 is very remotely related to monellin/cystatin superfamily proteins, some of which act as inhibitors of cysteine peptidases [1].

1: Structure of Csd3 from Helicobacter pylori, a cell shape-determining metallopeptidase. An DR, Kim HS, Kim J, Im HN, Yoon HJ, Yoon JY, Jang JY, Hesek D, Lee M, Mobashery S, Kim SJ, Lee BI, Suh SW; Acta Crystallogr D Biol Crystallogr. 2015;71:675-686. PMID:25760614