Purification and characterisation of a novel starch synthase selective foruridine 5 '-diphosphate glucose from the red alga Gracilaria tenuistipitata

Red algae (Rhodophyceae) are photosynthetic eukaryotes that accumulate star ch granules in the cytosol. Starch synthase activity in crude extracts of G racilaria tenuistipitata Chang et Xia was almost 9-fold higher with UDP[U-C -14]glucose than with ADP[U-C-14]glucose. The activity with UDP[U-14C]gluco se was sensitive to proteolytic and oxidative inhibition during extraction whilst the activity with ADP[U-C-14]glucose appeared unaffected. This indic ates the presence of separate starch synthases with different substrate spe cificities in G. tenuistipitata. The UDP glucose: starch synthase was purif ied and characterised. The enzyme appears to be a homotetramer with a nativ e M-r of 580 kDa and displays kinetic properties similar to other alpha-glu can synthases such as stimulation by citrate, product (UDP) inhibition and broad primer specificity. We propose that this enzyme is involved in cytoso lic starch synthesis in red algae and thus is the first starch synthase des cribed that utilises UDP-glucose in vivo. The biochemical implications of t he different compartmentalisation of starch synthesis in red algae and gree n algae/plants are also discussed.