Differences in catalytic properties between native isoenzymes of xyloglucan endotransglycosylase (XET)

Four isoenzymes of xyloglucan endotransglycosylase (XET; EC 2.4.1.207) were isolated from sprouting mung bean seedlings (M35, M45, M55a, M55b) and two from cauliflower florets (C30, C45). Purification in each case was by ammo nium sulphate precipitation, reversible formation of a covalent xyloglucan- enzyme complex, and cation-exchange chromatography. The isoenzymes differed in pH optimum (range 5.0-6.5), K-m for the nonasaccharide XLLGol (Gal(2).X yl(3).Glc(3).glucitol) as acceptor substrate, ability to utilise diverse ol igosaccharides as acceptor substrate, and ability to bind to carboxymethyl- cellulose (and thus possibly to other polyanions such as pectin in the cell wall). None of the isoenzymes was particularly cold-tolerant, unlike one X ET (TCH4) of Arabidopsis. The two cauliflower isoenzymes had higher K-m val ues for XLLGol (70-130 mu M) than the four mung bean isoenzymes (16-35 mu M ). We suggest that this difference is related to the major roles of the XET s in these two tissues: integration of new xyloglucan into the walls of the densely cytoplasmic cauliflower florets, and re-structuring of existing wa ll material in the rapidly vacuolating bean shoots. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.