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

Red algae (Rhodophyceae) are photosynthetic eukaryotes that accumulate starch granules in the cytosol. Starch synthase activity in crude extracts of Gracilaria tenuistipitata Chang et Xia was almost 9-fold higher with UDP[U-14C]glucose than with ADP[U-14C]glucose. The activity with UDP[U-14C]glucose was sensitive to proteolytic and oxidative inhibition during extraction whilst the activity with ADP[U-14C]glucose appeared unaffected. This indicates the presence of separate starch synthases with different substrate specificities in G. tenuistipitata. The UDPglucose: starch synthase was purified and characterised. The enzyme appears to be a homotetramer with a native M(r) of 580 kDa and displays kinetic properties similar to other alpha-glucan synthases such as stimulation by citrate, product (UDP) inhibition and broad primer specificity. We propose that this enzyme is involved in cytosolic starch synthesis in red algae and thus is the first starch synthase described that utilises UDPglucose in vivo. The biochemical implications of the different compartmentalisation of starch synthesis in red algae and green algae/plants are also discussed.