A membrane-bound enzyme complex synthesising glucan and glucomannan in pine tissues

摘要

Particulate membrane preparations isolated from cambial cells and differentiating and differentiated xylem cells of pine (Pinus sylvestrisL.) trees synthesised 14Cglucans using either guanosine 5′-diphosphate (GDP)-D-U-14Cglucose or uridine 5′-diphosphate (UDP)-D-U-14Cglucose as glycosyl donors. Although these glucans had β-(1→3) and β-(1→4) linkages in an approximate ratio 1:1, the distribution of the linkages in the glucan synthesised from GDP-D-glucose was different from that synthesised from UDP-D-glucose. The synthesis of the mixed β-(1→3) and β-(1→4) glucan from GDP-D-U-14Cglucose was changed to that of β-(1→4) glucomannan in the presence of increasing concentrations of GDP-D-mannose. The glucan formed from UDP-D-U-14Cglucose was not affected by any concentration of GDP-D-mannose. The membrane preparations epimerized GDP-D-glucose to GDP-D-mannose; however, the low amount of GDP-D-mannose formed was not incorporated into the polymer becaus the affinity of the synthase for GDP-D-glucose was much greater than that for GDP-D-mannose. The glucan formed from GDP-D-glucose and the glucomannan formed from GDP-D-glucose together with GDP-D-mannose were characterized. The apparentKmandVmaxof the glucan synthase for GDP-D-glucose were 6.38 μM and 5.08 μM·min-1, respectively. No lipid intermediates were detected during the synthesis of either glucan or glucomannan. The results indicated that an enzyme complex for the formation of the glucomannan was