Pathway of Cytosolic Starch Synthesis in the Model Glaucophyte Cyanophora paradoxa (original) (raw)
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The unique features of starch metabolism in red algae
Proceedings of the Royal Society B: Biological Sciences, 2001
Red algae (Rhodophyceae) are photosynthetic eukaryotes that accumulate starch granules outside of their plastids. The starch granules from red algae (£oridean starch) show structural similarities with higher plant starch granules but lack amylose. Recent studies have indicated that the extra-plastidic starch synthesis in red algae proceeds via a UDP glucose-selective a-glucan synthase, in analogy with the cytosolic pathway of glycogen synthesis in other eukaryotes. On the other hand, plastidic starch synthesis in green cells occurs selectively via ADP glucose in analogy with the pathway of glycogen synthesis in prokaryotes from which plastids have evolved. Given the emerging consensus of a monophyletic origin of plastids, it would appear that the capacity for starch synthesis selectively evolved from the a-glucan synthesizing machinery of the host ancestor and its endosymbiont in red algae and green algae, respectively. This implies the evolution of fundamentally di¡erent functional relationships between the di¡erent subcellular compartments with regard to photosynthetic carbon metabolism in these organisms. It is suggested that the biochemical and molecular elucidation of £oridean starch synthesis may o¡er new insights into the metabolic strategies of photosynthetic eukaryotes.
Starch metabolism in green algae
Starch - Stärke, 2013
Starch plays a central role in the life cycle as one of the principal sources of chemical energy. This polysaccharide accumulates in plastids in green algae and land plants, and both organisms have acquired various enzyme isoforms for each step of the metabolic pathway. Eukaryotic green microalgae present the critical photosynthetic functions as higher plants. However, due to the small size of their genome, gene redundancy is decreased, a feature that makes them an excellent model for investigating the properties of photosynthetic physiology. In the last decade, there has been an increasing demand for starch in many industrial processes, such as food, pharmaceutical, and bioethanol production. Thus, a better understanding of starch biosynthesis, in particular the structure-function relationship and regulatory properties of the enzymes involved in its production may provide a powerful tool for the planning of new strategies to increase plant biomass, as well as to improve the quality and quantity of this polymer.
The relocation of starch metabolism to chloroplasts: when, why and how
Trends in Plant Science, 2008
Plastid endosymbiosis was accompanied by the appearance of a novel type of semi-cristalline storage polysaccharide (starch). Interestingly, starch is found in the cytoplasm of Rhodophyceae and Glaucophyta but is localized to the chloroplast stroma of Chloroplastida. The pathway is presumed to have been cytosolic in the common ancestor of the three Archaeplastida lineages. The means by which in green plants and algae an entire suite of nuclear-encoded starch-metabolism genes could have had their protein products rewired simultaneously to plastids are unclear. This opinion article reviews the timing and the possible reasons underlying this rewiring and proposes a hypothesis that explains its mechanism. The consequences of this mechanism on the complexity of starch metabolism in Chloroplastida are discussed.
Nature of the Periplastidial Pathway of Starch Synthesis in the Cryptophyte Guillardia theta
Eukaryotic Cell, 2006
The nature of the periplastidial pathway of starch biosynthesis was investigated with the model cryptophyte Guillardia theta. The storage polysaccharide granules were shown to be composed of both amylose and amylopectin fractions with a chain length distribution and crystalline organization very similar to those of starch from green algae and land plants. Most starch granules displayed a shape consistent with biosynthesis occurring around the pyrenoid through the rhodoplast membranes. A protein with significant similarity to the amylose-synthesizing granule-bound starch synthase 1 from green plants was found as the major polypeptide bound to the polysaccharide matrix. N-terminal sequencing of the mature protein proved that the precursor protein carries a nonfunctional transit peptide in its bipartite topogenic signal sequence which is cleaved without yielding transport of the enzyme across the two inner plastid membranes. The enzyme was shown to display similar affinities for ADP and UDP-glucose, while the V max measured with UDP-glucose was twofold higher. The granule-bound starch synthase from Guillardia theta was demonstrated to be responsible for the synthesis of long glucan chains and therefore to be the functional equivalent of the amylose-synthesizing enzyme of green plants. Preliminary characterization of the starch pathway suggests that Guillardia theta utilizes a UDP-glucose-based pathway to synthesize starch.
Biocatalysis and Biotransformation, 2006
Since the initial discovery showing that ADPglucose (ADPG) serves as the universal glucosyl donor in the reaction catalyzed by starch synthase, the mechanism of starch biosynthesis in both leaves and heterotrophic organs has generally been considered to be an unidirectional process wherein ADPG pyrophosphorylase (AGPase) exclusively catalyzes the synthesis of ADPG and acts as the major limiting step of the gluconeogenic process. There is however mounting evidence that ADPG linked to starch biosynthesis is produced de novo in the cytosol by means of sucrose synthase (SuSy). In this review we show and discuss the numerous pitfalls of the 'classic' view of starch biosynthesis. In addition, we describe many overlooked aspects of both ADPG and starch metabolism. With the overall data we propose an 'alternative' model of starch biosynthesis, applicable to both photosynthetic and heterotrophic tissues, according to which both sucrose and starch biosynthetic processes are tightly interconnected by means of an ADPG synthesizing SuSy activity. According to this new view, starch metabolism embodies catabolic and anabolic reactions taking place simultaneously in which AGPase plays a vital role in the scavenging of starch breakdown products. Sucrose UDPG G1P UDP Fru SuSy UGPase PPi UTP G6P PGM Starch ATP PPi G1P ADPG AGPase G6P PGM ADP Amyloplast Cytosol ADP ATP Sucrose UDPG G1P UDP Fru UGPase PPi UTP G6P PGM ATP PPi G1P G6P PGM Amyloplast Cytosol Sucrose UDPG G1P UDP Fru UGP PPi UTP G6P PGM ATP G1P G6P PGM t l ADP ATP ADP ATP 2Pi APPase Figure 2. Schematic representation of the classic model of Suc-starch conversion in heterotrophic tissues of di-cotyledonous plants.
2000
In Chlamydomonas reinhardtii, the presence of a defective STA11 locus results in significantly reduced granular starch deposition displaying major modifications in shape and structure. This defect simultaneously leads to the accumulation of linear malto-oligosaccharides (MOS). The mutants of STA11 were showed to lack d-enzyme, a plant ␣-1,4 glucanotransferase analogous to the Escherichia coli amylomaltase. We have cloned and characterized both the cDNA and gDNA corresponding to the C. reinhardtii d-enzyme. We now report allele-specific modifications of the d-enzyme gene in the mutants of STA11. These allele-specific modifications cosegregate with the corresponding sta11 mutations, thereby demonstrating that STA11 encodes d-enzyme. MOS production and starch accumulation were investigated during day and night cycles in wild-type and mutant C. reinhardtii cells. We demonstrate that in the algae MOS are produced during starch biosynthesis and degraded during the phases of net polysaccharide catabolism. . Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/
PLANT PHYSIOLOGY, 2003
In Chlamydomonas reinhardtii, the presence of a defective STA11 locus results in significantly reduced granular starch deposition displaying major modifications in shape and structure. This defect simultaneously leads to the accumulation of linear malto-oligosaccharides (MOS). The mutants of STA11 were showed to lack d-enzyme, a plant ␣-1,4 glucanotransferase analogous to the Escherichia coli amylomaltase. We have cloned and characterized both the cDNA and gDNA corresponding to the C. reinhardtii d-enzyme. We now report allele-specific modifications of the d-enzyme gene in the mutants of STA11. These allele-specific modifications cosegregate with the corresponding sta11 mutations, thereby demonstrating that STA11 encodes d-enzyme. MOS production and starch accumulation were investigated during day and night cycles in wild-type and mutant C. reinhardtii cells. We demonstrate that in the algae MOS are produced during starch biosynthesis and degraded during the phases of net polysaccharide catabolism.
Starch Overproduction by Means of Algae
Algal Biorefineries, 2013
This chapter provides an overview of the state of knowledge of starch production as an ultimate energy reserve in algae. It includes a survey of recent discoveries on controls that direct the metabolism of algal cells towards starch hyper-accumulation with the aim of providing starch-enriched biomass for the production of bioethanol as a biofuel of the future. We also outline basic research from the 1960s, from which the recent starch research stems, although the use of algal starch for biofuel production was not considered at that time. The principles of, and basic approaches to a directed synthesis of starch are described in both laboratory experiments and large scale-up outdoor photobioreactors.