Structural basis for catalysis and inhibition of N-glycan processing classI alpha 1,2-mannosidases

Endoplasmic reticulum (ER) class I alpha1,2-mannosidase (also known as ER a lpha -mannosidase I) is a critical enzyme in the maturation of N-linked oli gosaccharides and ER-associated degradation. Trimming of a single mannose r esidue acts as a signal to target misfolded glycoproteins for degradation b y the proteasome. Crystal structures of the catalytic domain of human ER cl ass I alpha1,2-mannosidase have been determined both in the presence and ab sence of the potent inhibitors kifunensine and 1-deoxymannojirimycin. Both inhibitors bind to the protein at the bottom of the active-site cavity, wit h the essential calcium ion coordinating the O-2' and O-3' hydroxyls and st abilizing the six-membered rings of both inhibitors in a C-1(4) conformatio n. This is the first direct evidence of the role of the calcium ion. The la ck of major conformational changes upon inhibitor binding and structural co mparisons with the yeast alpha1,2-mannosidase enzyme product complex sugges t that this class of inverting enzymes has a novel catalytic mechanism. The structures also provide insight into the specificity of this class of enzy mes and provide a blueprint for the future design of novel inhibitors that prevent degradation of misfolded proteins in genetic diseases.