Starch degradation

Abstract

Degradation of starch (and of glycogen as well) converts carbohydrates accumulated as metabolically inert storage products back into forms that are usable for various biosynthetic and catabolic routes. Starch and glycogen share their basic biochemistry, but distinct physicochemical and enzymatic differences exist. Structural (dis)similarities of hydroinsoluble starch and hydrosoluble glycogen are briefly discussed. Various types of starch-degrading enzymes and their mode of action are presented. Features frequently observed in starch-degrading enzymes, such as carbohydrate-binding modules and secondary binding sites, are discussed including kinetic implications. Approaches to identify proteins functional in vivo starch degradation and their limitations are discussed. Three types of in vivo starch degradation are distinguished: degradation of transitory starch, mobilization of reserve starch in dead tissue, and that in living cells. Most of the current biochemical knowledge of starch degradation relates to mobilisation of transitory starch. We discuss this process in the context of cellular organisation and location/distribution of starch granules within the cell. Transitory starch degradation is initiated at the granule surface and includes local transitions from a hydroinsoluble ordered to a soluble state. Transition is facilitated by iterating cycles of phosphorylating and dephosphorylating reactions that act on starch-related glucosyl residues. In the stroma, four main products are synthesised by highly interconnected paths and are then exported into the cytosol. Plastidial transporters and cytosolic downstream processes are presented which link starch degradation to biosynthetic or degradative routes all originating from the cytosol. Finally, current views on plastidial transitory starch degradation as controlled by the cytosol are discussed.