Starch Synthesizing Reactions and Paths: in vitro and in vivo Studies

Abstract

Essentially all living systems are capable of synthesizing large glycans, such as arabinogalactan, xylan, cellulose, fructan, laminarin, glycogen or the two starch-related polyglucans, amylose and amylopectin. Each of these glycans exists as a large group of chemically closely related but polydisperse biopolymers rather than as a distinct macromolecule. The biological functions exerted by glycans are highly heterogeneous and range from shaping and/or protecting cells or tissues to intra-cellular storage of reduced carbon including the realm of molecular interactions between plant cells or between a plant and a non-plant organism. Glycans have an enormous impact on human diet.1,2) Furthermore, polysaccharides are gaining increasing importance for several biotechnological applications and energy-converting processes that all are based on photosynthesis-driven biosynthesis of carbohydrates. Naturally occurring polyglucans are usually composed of either of two cyclic hemiacetals, designated as α-or β-glucopyranosyl residues. Both six-membered rings are interconnected by only one or two type(s) of precisely defined glucosidic linkages. The anomeric form of the pyranosyl residues and the type(s) of interglucose bonds largely determine the structure of the entire polysaccharide.As an extracellular product of plant cells, cellulose is composed of β-D-glucopyranosyl residues that are interconnected by β-1,4-bonds. By contrast, starch and glycogen are typical intracellular storage carbohydrate of plants and animals, respective. Both starch and glycogen are exclusively built of α-D-glucopyranosyl residues that are linked by two types of interglucose bonds, α-1,4-and α-1,6-linkages. Despite the chemical similarity, important physicochemical differences exist: Native starch is deposited as water-insoluble particles designated as granules. Native starch particles are capable of an essentially unlimited growth. Glycogen is a hydro-soluble polydispers molecule which appears to possess a strict upper size limit (for review see Ref. 9 11)).In photosynthesis-competent eukaryots and their heterotrophic derivatives starch granules act as the almost ubiquitously occurring storage carbohydrate that allows central carbon metabolism and growth to continue when photosynthesis is not functional and, at the same time, to prevent carbon starvation. Starch may, however, share these functions with other reduced carbon compounds, such as sucrose, fructans or even soluble non-carbohydrates and both accumulation and utilization of the various compounds are remarkably flexible. In green algae, mosses, ferns and higher plants, starch granules are synthesized inside the plastidial compartment but in many other eukaryotic algae the cytosol is the princi- We then give a schematic presentation of the structure of starch synthases from Arabidopsis thaliana and discuss the action of the different isoforms. The two types of phosphorylases that are common in plants are described and their in vivo actions are presented. We describe functional properties of...

Homologs of PROTEIN TARGETING TO STARCH Control Starch Granule Initiation in Arabidopsis Leaves

“…PHS1 is present in the chloroplast in Arabidopsis leaves, although its role in starch synthesis or degradation remains unclear (Zeeman et al, 2004). However, its role in generating MOS may be redundant, as all Arabidopsis SS enzymes could elongate maltose into longer MOS (including maltodecaose) in vitro after an extended incubation period (Brust et al, 2013). It is difficult to estimate the frequency of such occurrence in the stroma, as longer MOS are often below the level of detection.…”

Intraspecific sequence variation and differential expression in starch synthase genes of Arabidopsis thaliana

“…Furthermore, in zymograms performed with leaf extracts some AtSS isozymes are recovered as multiple bands but products of other AtSS genes are not detectable at all (although the respective recombinant proteins exhibit enzyme activity). Thus, zymograms do not reflect the genetic complexity of the starch synthases [52]. …”

Initiation Process of Starch Biosynthesis

“…9.4). As stated above, Arabidopsis SSIV was also able to synthesize , and maltoheptaose (DP7) at 30 ı C for about 22 h in the presence of 3.5 mM ADPglucose and 1.2 mM MOS as primer (Brust et al 2013) glucan by using maltotriose and maltose as primer, and this synthesis capacity was much higher than that of SSIII (Szydlowsky et al 2009). …”

“…However, recently, Brust et al (2013) showed that SSI, SSII, and SSIII from Arabidopsis were capable of acting on maltose as glucan acceptor when they were incubated for a prolonged period (22 h), although glucose could not be replaced by maltose (Fig. 9.4).…”

“…5.3.1.1 in Chap. 5), the synthesis of MOS from maltose and maltotriose (Brust et al 2013), and the synthesis of branched glucan by forming functional interaction with BE .…”