Structural basis for the methylation mechanism in methyl-transferase BchU involved in bacteriochlorophyllcbiosynthesis

Microbial (endo)xylanases (endo--1,4-xylanases, E.C.3.2.1.8)are frequently used to modify the functionality of (arabino)xylan in feed and food applications.In cereals two types of endogeneous proteinaceous xylanase inhibitors were discovered, i.e.TAXI-type (T.aestivum xylanase inhibitor) [1] and XIP-type (xylanase inhibiting protein) [2] inhibitors.Probing the inhibition activities for different isolated TAXIisoforms against both A. niger and B. subtilis xylanases, led to the definition of TAXI-I type, with inhibiting activity against both GH family 11 endox, and TAXI-II type, with inhibition specificity against the B. subtilis endoxylanase only [3].Studies on the molecular identification and characterization of the TAXI-encoding genes showed that TAXIs are a new class of plant proteins to which a function as plant protective microbial glycoside hydrolase inhibitor can be ascribed [4].Both structures of TAXI-I in complex with A. niger xylanase and in complex with B. subtilis xylanase were determined.Detailed comparison of both complexes reveals the xylanase and inhibitor amino acid residues that are critical for complex-binding and stabilization.Extrapolation towards TAXI-II type inhibitors shows that TAXI-I inhibition activity is independent of the pH optima of xylanases, while TAXI-II inhibition of xylanases with low pH optima will be weak or absent [5].

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