Substrate stereoselectivity of poly(Asp) hydrolase-1 capable of cleaving β-amide bonds as revealed by investigation of enzymatic hydrolysis of stereoisomeric β-tri(Asp)s

We previously reported that poly(Asp) hydrolase-1 (PahZ1(KP-2)) from Pedobacter sp. KP-2 selectively, but not completely, cleaved the amide bonds between β-Asp units in thermally synthesized poly(Asp) (tPAA). In the present study, the enzymatic hydrolysis of stereoisomeric β-tri(Asp)s by PahZ1(KP-2) was investigated to clarify the substrate stereoselectivity of PahZ1(KP-2) in the hydrolysis of tPAA. The results suggest the following structural features of PahZ1(KP-2) at its substrate binding site: (1) the active site contains four subsites (2, 1, −1, and −2), three of which need to be occupied by Asp units for cleavage to occur; (2) for the hydrolysis to proceed, subsite 1 should be occupied by an l-Asp unit, whereas the other three subsites may accept both l- and d-Asp units; (3) for the two central subsites between which cleavage occurs, the (l-Asp)-(d-Asp) sequence is the most favorable for cleavage.