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

Particulate membrane preparations isolated from cambial cells and differentiating and differentiated xylem cells of pine (Pinus sylvestris L.) trees synthesised [(14)C]glucans using either guanosine 5'-diphosphate (GDP)-D-[U-(14)C]glucose or uridine 5'-diphosphate (UDP)-D-[U-(14)C]glucose 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-(14)C]glucose 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-(14)C]glucose 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 apparent K m and V max of 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 bound to the membrane.