Note: a dashed line (without arrowheads) between two compound names is meant to imply that the two names are just different instantiations of the same compound -- i.e. one may be a specific name and the other a general name, or they may both represent the same compound in different stages of a polymerization-type pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Carbohydrates Biosynthesis → Polysaccharides Biosynthesis → Glycogen and Starch Biosynthesis|
Some taxa known to possess this pathway include : Arabidopsis thaliana col [Delatte05], Chlamydomonas reinhardtii, Hordeum vulgare, Manihot esculenta, Nicotiana tabacum, Oryza sativa, Pisum sativum, Solanum tuberosum, Triticum aestivum, Zea mays
Starch and glycogen, megadalton-sized glucose polymers, are the major reservoir of readily available energy and carbon compounds in most living organisms, ranging from archaea, eubacteria and yeasts, up to higher eukaryotes including plants and animals [Zeeman10, Santelia11]. Only parasites seem to lack enzymes for the metabolism of these compounds [Henrissat02].
The structure of starch in higher plants differs from that of its counterpart a glycogen in animals and bacteria. Starch is a complex α-glucan that can be very difficult to adequately describe. Starch contains at least two different major sub-classes of α-glucans: amylose and amylopectin. Amylose contains up to several thousand α-glucosyl units linked almost exclusively in α(1->4) linkage with very few branches of α(1->6) linkage. Amylopectin, on the other hand is a much more branched molecule with many α(1->6) linkages and contains up to several million glucosyl residues. At least twelve different types of starch with different branching patterns and chain lengths have been reported [Robyt13]. To further complicate the situation, starch can appears in different crystalline and soluble forms which are difficult to define and depict using standard chemical structures.
It has been reported that cyanobacteria synthesize glycogen while red algae produce floridean-starch with structure that is intermediate between starch and glycogen, and that green algae accumulate amylopectin-like polysaccharides. However, some cyanobacteria (classified into group 5 by Honda et al [Honda99] have distinct α-polyglucans (which were designated as semi-amylopectin), making them a transition point between glycogen and starch biosynthesis [WingMing94, Nakamura05].
About This Pathway
Starch is synthesized in plastids, including chloroplasts in photosynthetic tissues and amyloplasts in non-photosynthetic tissues such as seeds, roots, and tubers. Starch synthesized in chloroplasts of photosynthetic tissues is degraded to maltose and glucose during the dark period (see starch degradation II). These sugars are exported to the cytosol and used in sucrose synthesis. Sucrose can be readily transported to non-photosynthetic tissues to support plant growth or for starch synthesis in amyloplasts.
The starch biosynthesis pathway depicted here includes both chloroplast and amyloplast pathways. The starting point for the chloroplast pathway is fructose-6-phosphate, a product of photosynthetic carbon fixation. The starting point for amyloplast pathway is glucose-1-phosphate, a product of sucrose degradation. Studies from potato, pea, and maize indicate that glucose-6-phosphate, in addition to glucose-1-phosphate, can be imported into the amyloplast and can serve as the starting point for starch biosynthesis [Tauberger00].
The role of plastidial α-phosphorylase enzymes ( 22.214.171.124) in starch biosynthesis remain controversial and may differ between species [Streb12, Ball09]. For example, mutations in the PHS1 gene of Arabidopsis have no effect on starch biosynthesis [Zeeman04].
Following the initial production of ADP-α-D-glucose, starch biosynthesis appears to involve reactions catalyzed by at least three classes of enzymes, i.e. starch synthases, starch branching enzymes and starch debranching enzymes [Ball09]. However, the exact steps involved and the order in which they are required for the formation of different types of starch may differ between species and even between different types of cells within the same species [Delatte05].
There is also evidence that Chlamydomonas reinhardtii might involve an additional enzyme in starch biosynthesis, namely, a disproportionating enzyme, DPE1. However the corresponding enzyme, DPE1 in Arabidopsis has been shown to play a part in starch degradation instead [Ball09, Streb12].
Developing a better understanding of starch biosynthesis and its regulation is an active area of research.
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Chae11: Chae, Lee (2011). "The functional annotation of protein sequences was performed by the in-house Ensemble Enzyme Prediction Pipeline (E2P2, version 1.0). E2P2 systematically integrates results from three molecular function annotation algorithms using an ensemble classification scheme. For a given genome, all protein sequences are submitted as individual queries against the base-level annotation methods. The individual methods rely on homology transfer to annotate protein sequences, using single sequence (BLAST, E-value cutoff <= 1e-30, subset of SwissProt 15.3) and multiple sequence (Priam, November 2010; CatFam, version 2.0, 1% FDR profile library) models of enzymatic functions. The base-level predictions are then integrated into a final set of annotations using an average weighted integration algorithm, where the weight of each prediction from each individual method was determined via a 0.632 bootstrap process over 1000 rounds of testing. The training and testing data for E2P2 and the BLAST reference database were drawn from protein sequences with experimental support of existence, compiled from SwissProt release 15.3."
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