Rice endosperm-specific plastidial α-glucan phosphorylase is important for synthesis of short-chain malto-oligosaccharides

Previous genetic studies have indicated that the type L a-glucan phosphorylase (Phol) has an essential role during the initiation process of starch biosynthesis during rice seed development. To gain insight into its role in starch metabolism, we characterized the enzymatic properties of the Phol recombinant form. Phol has significantly higher catalytic efficiency toward both linear and branched alpha-glucans in the synthesis direction than in the degradation direction with equilibrium constants for the various substrates ranging from 13 to 45. Phol activity is strongly inhibited by its own reaction product (Pi) in the synthesis reaction (K-i = 0.69 mM) when amylopectin is the primer substrate, but this inhibition is less pronounced (K-i = 14.2 mM) when short alpha-glucan chains are used as primers. Interestingly, even in the presence of Pi alone, Phol not only degrades maltohexaose but also extends them to synthesize longer MOSs. Production of a broad spectrum of MOSs (G4-G19) was stimulated by both Pi and Glc1P in an additive fashion. Thus, even under physiological conditions of high Pi/Glc1P, Phol extends the chain length of short Moss which can then be used as subsequent primer by starch synthase activities. As ADP-glucose strongly inhibits Phol activity, Pho1 likely operates only during the initial stage and not during maturation phase of starch synthesis. (C) 2009 Elsevier Inc. All rights reserved.