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Bifunctional glycosyltransferases catalyze both extension and termination of pectic galactan oligosaccharides
Author(s) -
Laursen Tomas,
Stonebloom Solomon H.,
Pidatala Venkataramana R.,
Birdseye Devon S.,
Clausen Mads H.,
Mortimer Jenny C.,
Scheller Henrik Vibe
Publication year - 2018
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1111/tpj.13860
Subject(s) - galactan , glycosyltransferase , biochemistry , cell wall , glycosidic bond , chemistry , polysaccharide , glycoside hydrolase , biosynthesis , galactose , phosphofructokinase 2 , stereochemistry , enzyme
Summary Pectins are the most complex polysaccharides of the plant cell wall. Based on the number of methylations, acetylations and glycosidic linkages present in their structures, it is estimated that up to 67 transferase activities are involved in pectin biosynthesis. Pectic galactans constitute a major part of pectin in the form of side‐chains of rhamnogalacturonan‐I. In Arabidopsis, galactan synthase 1 (GALS1) catalyzes the addition of galactose units from UDP‐Gal to growing β‐1,4‐galactan chains. However, the mechanisms for obtaining varying degrees of polymerization remain poorly understood. In this study, we show that At GALS1 is bifunctional, catalyzing both the transfer of galactose from UDP‐α‐ d ‐Gal and the transfer of an arabinopyranose from UDP‐β‐ l ‐Ara p to galactan chains. The two substrates share a similar structure, but UDP‐α‐ d ‐Gal is the preferred substrate, with a 10‐fold higher affinity. Transfer of Ara p to galactan prevents further addition of galactose residues, resulting in a lower degree of polymerization. We show that this dual activity occurs both in vitro and in vivo . The herein described bifunctionality of At GALS1 may suggest that plants can produce the incredible structural diversity of polysaccharides without a dedicated glycosyltransferase for each glycosidic linkage.