The Intricate Pathway of Starch Biosynthesis and Degradation in the Monocellular Alga Chlamydomonas reinhardtii

Starch and glycogen define the two most widespread storage polysaccharides found in living cells. Both consist of a -1,4 linked and a -1,6 branched glucans but differ considerably in their structure and physicochemical properties. Because of its complex semi-crystalline organization, starch poses a particular challenge to understand the basis of its synthesis through hydrosoluble enzymes. This review summarizes the authors progress in understanding starch biosynthesis through the use of a unicellular model plant system. Comparative biochemistry of starch synthesis in the green alga Chlamydomonas reinhardtii and of glycogen synthesis in bacteria point to both duplication and specialization of the set of enzymes responsible for formation of the two types of glucosyl linkages. In addition, enzymes previously thought to be solely involved in polysaccharide catabolism turn out to be either also or exclusively involved in starch biosynthesis.