SYNTHETIC SUBSTRATE-ANALOGS FOR UDP-GLCNAC-MAN-ALPHA-1-6R BETA(1-2)-N-ACETYLGLUCOSAMINYLTRANSFERASE-II - SUBSTRATE-SPECIFICITY AND INHIBITORS FOR THE ENZYME

UDP-GlcNAc:Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II (GlcNAc-T II; EC 2.4.1.143) is a key enzyme in the synthesis of comple x N-glycans. We have tested a series of synthetic analogues of the sub strate Man'''alpha 1-6(GlcNAc''beta 1-2Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T II. The enzyme att aches N-acetylglucosamine in beta 1-2 linkage to the 2'''-OH of the Ma n'''alpha 1-6 residue. The 2'''-deoxy analogue is a competitive inhibi tor (K-i = 0.13 mM). The 2'''-O-methyl compound does not bind to the e nzyme presumably due to steric hindrance. The 3'''-, 4'''- and 6'''-OH groups are not essential for binding or catalysis since the 3'''-, 4' ''- and 6'''-deoxy and -O-methyl derivatives are all good substrates. Increasing the size of the substituent at the 3'''-position to pentyl and substituted pentyl groups causes competitive inhibition (K-i = 1.0 -2.5 mM). We have taken advantage of this effect to synthesize two pot entially irreversible GlcNAc-T II inhibitors containing a photolabile 3'''-O-(4,4-azo) pentyl group and a 3'''-O-(5-iodoacetamido) pentyl gr oup respectively. The data indicate that none of the hydroxyls of the Man'''alpha 1-6 residue are essential for binding although the 2'''- a nd 3'''-OH face the catalytic site of the enzyme. The 4-OH group of th e Man beta-O-octyl residue is not essential for binding or catalysis s ince the 4-deoxy derivative is a good substrate; the 4-O-methyl deriva tive does not bind. This contrasts with GlcNAc-T I which cannot bind t o the 4-deoxy-Man beta- substrate analogue. The data are compatible wi th our previous observations that a 'bisecting' N-acetylglucosamine at the 4-OH position prevents both GlcNAc-T I and GlcNAc-T II catalysis. However, in the case of GlcNAc-T II, the bisecting N-acetylglucosamin e prevents binding due to steric hindrance rather than to removal of a n essential OH group. The 3'-OH of the Man'alpha 1-3 is an essential g roup for GlcNAc-T II since the 3'-deoxy derivative does not bind to th e enzyme. The trisaccharide GlcNAc beta 1-2Man alpha 1-3Man beta-O-oct yl is a good inhibitor (K-i = 0.9 mM). The above data together with pr evious studies indicate that binding of the GlcNAc beta 1-2Man alpha 1 -3Man beta- arm of the branched substrate to the enzyme is essential f or catalysis.