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PubMed1995

Starch and dietary fiber components are excreted and degraded to variable extents in ileostomy subjects consuming mixed diets with wheat- or oat-bran bread.

The study was conducted to determine if the excretion of starch and dietary fiber components varies in ileostomy subjects consuming diets high or low in dietary fiber. Excretion of starch, enzyme-resistant starch and dietary fiber components was studied in nine human subjects with ileostomies, who consumed (in a crossover design) a wheat bread-based diet (daily intake 274 g starch, 2.4 g enzyme-resistant starch and 14.4 g total dietary fiber) and a high fiber diet based on oat-bran bread (daily intake 243 g starch, 2.7 g enzyme-resistant starch and 40.2 g total dietary fiber). Food and excreta were collected on d 3 and 17. No significant differences in excretion of starch, enzyme-resistant starch or dietary fiber components were found on these 2 d in each dietary period. When subjects consumed the wheat bread-based diet they excreted (mean +/- SD) 3.3 +/- 1.7 g starch and 2.4 +/- 0.4 g enzyme-resistant starch daily, whereas when consuming the oat bran-based diet they excreted 4.5 +/- 3.1 g starch and 2.5 +/- 0.4 g enzyme-resistant starch. During both dietary periods subjects excreted significantly greater amounts of certain dietary fiber polysaccharide residues (fucose, galactose and uronic acid) than they ingested. This indicates a contribution of endogenous and/or microbial material to the dietary fiber value in ileostomy effluents. However, significantly less excretion of some dietary fiber polysaccharide residues, especially glucose residues, during the oat-bran bread-based dietary period was also noted. This was presumably caused by a degradation of mixed-linked (1,3),(1,4)-beta-D-glucans.

Adult↗
PubMed1985

Different behavior towards raw starch of three forms of glucoamylase from a Rhizopus sp.

Three forms of glucoamylase [EC 3.2.1.3] of a Rhizopus sp., Gluc1 (M.W. 74,000), Gluc2 (M.W. 58,600), and Gluc3 (M.W. 61,400), which have similar pH optima and specific activities towards soluble starch were studied as to their behavior towards raw starch. The pH optima for raw starch digestion were different, that is, 4.5 for Gluc1 and 5.0 for both Gluc2 and Gluc3. All the enzymes digested raw starch almost completely but at quite different rates; Gluc2 and Gluc3, which lack the N-terminal portions of Gluc1, were 22 and 25 times less effective, respectively, for raw starch digestion than Gluc1. Of the three enzymes, only Gluc1 tightly bound to raw starch. Binding of Gluc1 to raw starch occurred pH-dependently with a broad pH optimum of 4.5-5.5, but temperature and ionic strength affected it only slightly and little, respectively. The binding constant of Gluc1 for raw starch at pH 5.0 and 4 degrees C was estimated to be 1.2 X 10(5) M-1. Fragment H (M.W. 16,700), presumably released from the N-terminal part of Gluc1, not only bound to raw starch itself but also inhibited the binding of Gluc1 to raw starch. pap-Gluc (M.W. 57,000) and chymo-Gluc (M.W. 64,000), which are papain- and chymotrypsin-modified Gluc1, respectively, and lack the N-terminal portions of Gluc1, resembled Gluc2 and Gluc3 in raw starch binding as well as digestion. All these results indicate that Gluc1 has a raw starch-binding site, different from the active center, in the N-terminal region. Various substrates and analogs inhibited the binding of Gluc1 to raw starch, presumably due to steric hindrance.

Glucan 1,4-alpha-Glucosidase↗
PubMed2004

Light induces phosphorylation of glucan water dikinase, which precedes starch degradation in turions of the duckweed Spirodela polyrhiza.

Degradation of storage starch in turions, survival organs of Spirodela polyrhiza, is induced by light. Starch granules isolated from irradiated (24 h red light) or dark-stored turions were used as an in vitro test system to study initial events of starch degradation. The starch-associated pool of glucan water dikinase (GWD) was investigated by two-dimensional gel electrophoresis and by western blotting using antibodies raised against GWD. Application of this technique allowed us to detect spots of GWD, which are light induced and absent on immunoblots prepared from dark-adapted plants. These spots, showing increased signal intensity following incubation of the starch granules with ATP, became labeled by randomized [betagamma-33P]ATP but not by [gamma-33P]ATP and were removed by acid phosphatase treatment. This strongly suggests that they represent a phosphorylated form(s) of GWD. The same light signal that induces starch degradation was thus demonstrated for the first time to induce autophosphorylation of starch-associated GWD. The in vitro assay system has been used to study further effects of the light signal that induces autophosphorylation of GWD and starch degradation. In comparison with starch granules from dark-adapted plants, those from irradiated plants showed increase in (1) binding capacity of GWD by ATP treatment decreased after phosphatase treatment; (2) incorporation of the beta-phosphate group of ATP into starch granules; and (3) rate of degradation of isolated granules by starch-associated proteins, further enhanced by phosphorylation of starch. The presented results provide evidence that autophosphorylation of GWD precedes the initiation of starch degradation under physiological conditions.

Adenosine Triphosphate↗
PubMed1999

Degradation of starch-poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) bioplastic in tropical coastal waters.

Extruded bioplastic was prepared from cornstarch or poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) (PHBV) or blends of cornstarch and PHBV. The blended formulations contained 30 or 50% starch in the presence or absence of polyethylene oxide (PEO), which enhances adherence of starch granules to PHBV. Degradation of these formulations was monitored for 1 year at four stations in coastal water southwest of Puerto Rico. Two stations were within a mangrove stand. The other two were offshore; one of these stations was on a shallow shoulder of a reef, and the other was at a location in deeper water. Microbial enumeration at the four stations revealed considerable flux in the populations over the course of the year. However, in general, the overall population densities were 1 order of magnitude less at the deeper-water station than at the other stations. Starch degraders were 10- to 50-fold more prevalent than PHBV degraders at all of the stations. Accordingly, degradation of the bioplastic, as determined by weight loss and deterioration of tensile properties, correlated with the amount of starch present (100% starch >50% starch > 30% starch > 100% PHBV). Incorporation of PEO into blends slightly retarded the rate of degradation. The rate of loss of starch from the 100% starch samples was about 2%/day, while the rate of loss of PHBV from the 100% PHBV samples was about 0.1%/day. Biphasic weight loss was observed for the starch-PHBV blends at all of the stations. A predictive mathematical model for loss of individual polymers from a 30% starch-70% PHBV formulation was developed and experimentally validated. The model showed that PHBV degradation was delayed 50 days until more than 80% of the starch was consumed and predicted that starch and PHBV in the blend had half-lives of 19 and 158 days, respectively. Consistent with the relatively low microbial populations, bioplastic degradation at the deeper-water station exhibited an initial lag period, after which degradation rates comparable to the degradation rates at the other stations were observed. Presumably, significant biodegradation occurred only after colonization of the plastic, a parameter that was dependent on the resident microbial populations. Therefore, it can be reasonably inferred that extended degradation lags would occur in open ocean water where microbes are sparse.

Biodegradation, Environmental↗
PubMed1991

Measurement of starch absorption in humans.

Although starch provides a large fraction of human caloric intake, there is limited information concerning the efficiency of intestinal absorption of this nutrient. Owing to the fermentation of starch by colonic bacteria, there is no quantitative test for starch absorption comparable to the fecal fat determination. The most accurate estimation of starch absorption has been obtained by intubating the terminal ileum and aspirating ileal contents following ingestion of a meal containing starch plus a nonabsorbable marker. Starch absorption is calculated from the ratio of starch:marker in the ileal aspirate relative to the ratio in the meal. Disadvantages of the technique are the requirement for ileal intubation and the possible adverse effect of intubation on the absorptive process. A more widely used technique to assess starch absorption involves measurement of breath hydrogen (H2) excretion after ingestion of starch. Malabsorbed starch is fermented by colonic bacteria with liberation of H2 that is absorbed and excreted in expired air. This test is simple and noninvasive and can provide quantitative measurements of starch malabsorption. Application of this technique has demonstrated that 5-10% of starch in wheat, potatoes, and corn is not absorbed by healthy subjects, while rice starch is nearly completely absorbed.

Diet↗
PubMed1986

Limits to starch digestion in the ruminant small intestine.

Site and extent of starch digestion by ruminant animals varies with species, grain type and processing method. Based on a review of 40 different experiments with cattle, between 18 and 42% of the dietary starch from corn and sorghum grains fed to cattle reaches the small intestine for digestion. With more extensive grain processing, a smaller quantity of starch reaches the small intestine. In the small intestine, from 47 to 88% of the presented starch is digested, while in the large intestine, 33 to 62% of the presented starch is digested. Though limits to digestion in and absorption from the small intestine can be demonstrated by infusing starch and glucose into the duodenum, enzymatic capacity does not appear to limit intestinal starch digestion since no plateau in the amount of starch disappearing from the small intestine is detected with typical diets. Yet, extent of digestion is incomplete. Other factors, such as time and surface exposure may limit small intestinal digestion of starch. Processing methods to reduce particle size or alter the protein matrix, which cements starch granules together, will increase the extent of digestion both in the rumen and in the small intestine. Performance data from growing cattle fed processed corn and sorghum grains indicate that starch was used 42% more efficiently if it was digested in the small intestine rather than in the rumen. Though total tract starch digestibility is of primary concern, results support the concept that energetic efficiency of growing ruminants is greater if starch is digested in the small intestine rather than in the rumen.

Animal Feed↗
PubMed2001

Effects of postrumen starch infusion on milk production and energy metabolism in dairy cows.

Two experiments were conducted to determine effects of postrumen starch infusion on milk production and energy and nitrogen utilization in lactating dairy cows. In experiment 1, four cows in early lactation fed grass silage and concentrates were continuously infused into the duodenum with water or 700, 1400, or 2100 g of purified maize starch daily for 10 to 12 d in a 4 x 4 Latin square design with 2-wk periods. Starch infusion increased milk yield linearly and decreased milk fat concentration in a quadratic manner such that increases in fat-corrected milk and calculated milk energy yield were minimal except at the highest rate of infusion. Changes in milk energy output suggest that even at the highest infusion rate metabolizable energy supplied by infused starch was used for tissue energy or oxidized. In experiment 2 energy and nitrogen balance were measured in four cows in late lactation fed a mixture of dehydrated lucerne, grass silage, and concentrates during the last 6 d of 2-wk abomasal infusions of 1200 g of purified wheat starch daily or water in a balanced switchback design with 5-wk periods. Measurements of fecal starch concentration indicated nearly all the starch infused was digested, but decreased fecal pH and apparent nitrogen digestion suggested an increase in hindgut starch fermentation. Starch infusion decreased urine nitrogen output in part because of increased tissue nitrogen retention but had no effect on milk nitrogen output. In absolute terms, numerical decreases in feed energy intake and energy digestion reduced the recovery of starch energy infused as digestible and metabolizable energy, but in terms of changes in total energy supply with starch infusion, 79% was recovered as metabolizable energy. Starch infusion had no effects on heat or milk energy but increased net energy for lactation due to a numerical increase in tissue energy, implying that in late-lactation cows, starch digested postruminally was used with high efficiency for tissue energy retention as protein and fat.

Abomasum↗
PubMed1999

The influence of alterations in ADP-glucose pyrophosphorylase activities on starch structure and composition in potato tubers.

In order to examine whether alterations in the supply of precursor molecules into the starch biosynthetic pathway affected various characteristics of the starch, starch was isolated from potato (Solanum tuberosum L.) tubers containing reduced amounts of the enzyme ADP-glucose pyrophosphorylase (AGPase). It was found that although the type of crystalline polymorph in the starch was not altered, the amylose content was severely reduced. In addition, amylopectin from the transgenic plants accumulated more relatively short chains than that from control plants and the sizes of starch granules were reduced. The starch granules from the transgenic plants contained a greater amount of granule-bound starch synthase enzyme, which led to an increase in the maximum activity of the enzyme per unit starch tested. The K(m) for ADP-glucose was, at most, only slightly altered in the transgenic lines. Potato plants containing reduced AGPase activity were also transformed with a bacterial gene coding for AGPase to test whether this enzyme can incorporate phosphate monoesters into amylopectin. A slight increase in phosphate contents in the starch in comparison with the untransformed control was found, but not in comparison with starch from the line with reduced AGPase activity into which the bacterial gene was transformed.Key words: ADP-glucose pyrophosphorylase. Amylopectin structure. Amylose. Solanum (starch. tuber). Starch granule size. Starch phosphorylation

Journal Article↗
PubMed1982

Biosynthesis of Starch in Proplastids of Germinating Ricinus communis Endosperm Tissue.

Electron photomicrographs of endosperm tissue from germinating seed of Ricinus communis L. cv. Hale show proplastids which contain prominent starch grains. The content of starch in endosperm tissue increased from 500 micrograms per seed, in imbibed seed, to 1,100 micrograms per seed in 5-day-old seedlings. The maximum net rate of starch deposition was 1.1 nanomoles glucose incorporated per minute per seed. About 200 micrograms of starch remained in the endosperm 9 days after imbibition. Starch content followed the same developmental pattern as the content of sucrose, free reducing sugars, and other metabolic processes found in this tissue. Two key enzymes of starch synthesis, adenosine diphosphoglucose (ADPG) pyrophosphorylase and ADPG-starch glucosyl transferase (starch synthetase) exhibited maximum activities at 4 and 5 days after germination, respectively. The maximum activity of ADPG pyrophosphorylase was 8.17 nanomoles ADPG formed per minute per seed, whereas starch synthetase exhibited an activity of 125 nanomoles glucose incorporated per minute per seed. These levels of enzyme activity are sufficient to account for the starch synthesis observed. Other enzymes which may be involved in starch synthesis include 3-phosphoglycerate kinase which showed an activity of 8.76 units per seed, triose-P isomerase (2.56 units per seed), fructose-1,6-bisphosphate aldolase (0.99 units per seed), fructose-1,6-bisphosphatase (0.23 units per seed), phosphoglucose isomerase (12.6 units per seed), and phosphoglucomutase (9.72 units per seed). The activities of these enzymes were similar to previously reported values.Starch synthetase was found in association with the fraction containing proplastids isolated from endosperm tissue. Of the total starch synthetase activity in the endosperm, 38% was particulate. Forty-four% of the total particulate activity of starch synthetase placed on sucrose gradients was associated with the band containing proplastids. The proplastids contained 98% of the ribulose 1,5-bisphosphate carboxylase carboxylase activity placed on the gradient.

Journal Article↗
PubMed2026

Integrated transcriptomic and metabolomic analysis reveals candidate regulatory networks associated with starch accumulation in tetraploid potato.

Potato (Solanum tuberosum L.) tuber starch is a major determinant of crop quality and industrial value, yet the regulatory mechanisms underlying starch accumulation in autotetraploid cultivars remain poorly resolved. Here, we performed integrated transcriptomic and metabolomic analyses using a segregating tetraploid population derived from parents with contrasting starch content. Extreme phenotypes were selected to systematically dissect the molecular basis of starch accumulation. Transcriptome profiling revealed extensive transcriptional reprogramming between high- and low-starch genotypes, with differentially expressed genes significantly enriched in carbohydrate metabolism, particularly the starch and sucrose metabolism pathway. Notably, multiple transcription factor families, including AP2/ERF, MYB, and bHLH, were prominently represented, suggesting coordinated regulatory control. Metabolomic analysis identified substantial metabolic divergence, with differentially accumulated metabolites predominantly enriched in starch and sucrose metabolism as well as secondary metabolic pathways. Most metabolites exhibited negative associations with starch content, indicating competitive carbon allocation between primary and secondary metabolism. Integrative multi-omics analysis further resolved a core regulatory module comprising key structural genes and transcription factors tightly associated with starch-related metabolites. In particular, genes involved in sucrose cleavage and ADP-glucose metabolism, together with trehalose-6-phosphate synthase (TPS) and UDP-glucose-associated pathways, emerged as critical nodes linking carbon flux to starch biosynthesis. Correlation network analysis suggested that AP2/ERF-, MYB-, and bHLH-type transcription factors modulate these pathways by coordinating structural gene expression and metabolic flux distribution. Collectively, our study establishes a transcriptional-metabolic framework for starch accumulation in tetraploid potato, highlighting the central role of carbon allocation and signaling intermediates in shaping starch content, and providing candidate targets for molecular breeding and genome editing.

Solanum tuberosum↗
PubMed1995

Stimulation of fluid intake by maltodextrins and starch.

Maltodextrin solutions are more potent stimulators of fluid intake than are starch suspensions, even though these two substances are both glucose polymers. When rats were offered a choice of two fluids, one containing maltodextrin and the other containing the same concentration of starch, rats preferred maltodextrin; this preference increased over successive days for rats given 7.4% or 10% carbohydrate but not for rats given 1% carbohydrate. Rats offered fluids containing artificial flavors mixed with maltodextrin and starch acquired preferences for flavors paired with 10% or 7.4% maltodextrin but not for flavors paired with 1% maltodextrin. In an attempt to determine whether this greater preference for maltodextrin is due to an innate flavor preference, rats were given a choice of maltodextrin containing sucrose octaacetate vs. plain starch; these rats initially preferred starch but subsequently ingested more maltodextrin flavored with sucrose octaacetate than plain starch. Direct evidence for a postingestive factor was obtained by giving rats intragastric infusions of either maltodextrin, starch, or vehicle whenever they drank a dilute saccharin solution. Both carbohydrates stimulated fluid intake, but maltodextrin did so more rapidly (within 1 day rather than the 2 days of training required with starch infusions). These results indicate that some difference in the postingestive effects of maltodextrin and starch contributes to the greater intake-stimulating effect of maltodextrin. The postingestive factor responsible for the difference between maltodextrin and starch has not been identified, but it is not the amount of utilizable energy because rats prefer maltodextrin over starch even when the starch suspension contains more calories than does the maltodextrin solution. It is proposed that the postingestive factor differentiating these substances is rate of digestion.

Animals↗
PubMed2005

Damaged starch characterisation by ultracentrifugation.

The relative molecular size distributions of a selection of starches (waxy maize, pea and maize) that had received differing amounts of damage from ball milling (as quantified by susceptibility to alpha-amylase) were compared using analytical ultracentrifugation. Starch samples were solubilised in 90% dimethyl sulfoxide, and relative size distributions were determined in terms of the apparent distribution of sedimentation coefficients g*(s) versus s(20,w). For comparison purposes, the sedimentation coefficients were normalised to standard conditions of density and viscosity of water at 20 degrees C, and measurements were made with a standard starch loading concentration of 8 mg/mL. The modal molecular size of the native unmilled alpha-glucans were found to be approximately 50S, 51S and 79S for the waxy maize, pea and maize amylopectin molecules, respectively, whilst the pea and maize amylose modal molecular sizes were approximately 14S and approximately 12S, respectively. As the amount of damaged starch increased, the amylopectin molecules were eventually fragmented, and several components appeared, with the smallest fractions approaching the sedimentation coefficient values of amylose. For the waxy maize starch, the 50S material (amylopectin) was gradually converted to 14S, and the degradation process included the appearance of 24S material. For the pea starch, the situation was more complicated than the waxy maize due to the presence of amylose. As the amylopectin molecules (51S) were depolymerised by damage within this starch, low-molecular-weight fragments added to the proportion of the amylose fraction (14S)--although tending towards the high-molecular-weight region of this fraction. As normal maize starch was progressively damaged, a greater number of fragments appeared to be generated compared to the other two starches. Here, the 79S amylopectin peak (native starch) was gradually converted into 61 and 46S material and eventually to 11S material with a molecular size comparable to amylose. Amylose did not appear to be degraded, implying that all the damage was focused on the amylopectin fraction in all three cases. Specific differences in the damage profiles for the pea and maize starches may reflect the effect of lipid-complexed amylose in the maize starch.

Amylopectin↗
PubMed2004

Starch acetate microparticles for drug delivery into retinal pigment epithelium-in vitro study.

Starch acetates are novel biodegradable polymers which undergo slower degradation and swelling than native starch. Retinal pigment epithelium (RPE) is an important target tissue in ocular treatment. The cellular uptake of starch acetate microparticles and degradation of starch acetate by cultured human RPE-cell line (D407) was examined. Calcein-containing starch acetate microparticles were prepared by a modified water-in-oil-in-water double-emulsion technique. The cellular uptake of the starch acetate microparticles was analysed using flow cytometry and confocal microscopy. Degradation of starch acetate films by the homogenate of lysed RPE cells was determined by gel permeation chromatography. The effect of the microparticles on RPE cell viability was determined by the MTT colorimetric assay. The mean diameter (D50%) of microparticles was 11 microm. During 3-h incubation in RPE-cell culture, 8.1 +/- 0.8% of D407 cells took up starch acetate microparticles. Confocal microscopy confirmed the internalisation of microparticles. Incubation of the starch acetate film in the RPE-cell homogenate considerably decreased the molecular weight of starch acetate in the film during 24 h. The viability of cultured RPE cells was at least 82% after 24-h incubation with the microparticles. The present results show that the starch acetate microparticles are taken up by the RPE cells and the polymer can be degraded by the enzymes in these cells. Starch acetate microparticles may be suitable for drug delivery to the RPE.

Cell Line↗
PubMed1998

What is the threshold amount of starch necessary for full gravitropic sensitivity?

In preparation for microgravity experiments, we studied the kinetics of gravitropism in seedlings of wild-type (WT) Arabidopsis and three starch-deficient mutants. One of these mutants is starchless (ACG 21) while the other two are intermediate starch mutants (ACG 20 and ACG 27). In root cap cells, ACG 20 and 27 have 51% and 60% of the WT amount of starch, respectively. However, in endodermal cells of the hypocotyl, ACG 20 has a greater amount of starch than ACG 27. WT roots and hypocotyls were much more responsive to gravity than were the respective organs of the starchless mutant, and the intermediate starch mutants exhibited reduced gravitropism but had responses that were close to that of the WT. In roots, ACG 27 (more starch) was more responsive than ACG 20 (less starch), while in hypocotyls, ACG 20 (more starch) had a greater response than ACG 27 (less starch). Taken together, our data are consistent with the starch-statolith hypothesis for gravity perception in that the degree of graviresponsiveness is proportional to the total mass of plastids per cell. These results also suggest that (in roots) 51-60% starch is close to the threshold amount of starch needed for full gravitropism and that the gravity sensing system is "overbuilt."

Arabidopsis↗
PubMed2001

An investigation of the action of porcine pancreatic alpha-amylase on native and gelatinised starches.

The action of pancreatic alpha-amylase (EC 3.2.1.1) on various starches has been studied in order to achieve better understanding of how starch structural properties influence enzyme kinetic parameters. Such studies are important in seeking explanations for the wide differences reported in postprandial glycaemic and insulinaemic indices associated with different starchy foodstuffs. Using starches from a number of different sources, in both native and gelatinised forms, as substrates for porcine alpha-amylase, we showed by enzyme kinetic studies that adsorption of amylase to starch is of kinetic importance in the reaction mechanism, so that the relationship between reaction velocity and enzyme concentration [E0] is logarithmic and described by the Freundlich equation. Estimations of catalytic efficiencies were derived from measurements of kcat/Km performed with constant enzyme concentration so that comparisons between different starches were not complicated by the logarithmic relationship between E0 and reaction velocity. Such studies reveal that native starches from normal and waxy rice are slightly better substrates than those from wheat and potato. After gelatinisation at 100 degrees C, kcat/Km values increased by 13-fold (waxy rice) to 239-fold (potato). Phosphate present in potato starch may aid the swelling process during heating of suspensions; this seems to produce a very favourable substrate for the enzyme. Investigation of pre-heat treatment effects on wheat starch shows that the relationship between treatment and kcat/Km is not a simple one. The value of kcat/Km rises to reach a maximum at a pre-treatment temperature of 75 degrees C and then falls sharply if the treatment is conducted at higher temperatures. It is known that amylose is leached from starch granules during heating and dissolves. On cooling, the dissolved starch is likely to retrograde and become resistant to amylolysis. Thus the catalytic efficiency tends to fall. In addition, we find that the catalytic efficiency on the different starches varies inversely with their solubility and we interpret this finding on the assumption that the greater the solubility, the greater is the likelihood of retrogradation. We conclude that although alpha-amylase is present in high activity in digestive fluid, the enzymic hydrolysis of starch may be a limiting factor in carbohydrate digestion because of factors related to the physico-chemical properties of starchy foods.

Animals↗
PubMed1997

Solution structure of the granular starch binding domain of Aspergillus niger glucoamylase bound to beta-cyclodextrin.

BACKGROUND: Carbohydrate-binding domains are usually small and physically separate from the catalytic domains of hydrolytic enzymes. Glucoamylase 1 (G1) from Aspergillus niger, an enzyme used widely in the food and brewing industries, contains a granular starch binding domain (SBD) which is separated from the catalytic domain by a semi-rigid linker. The aim of this study was to determine how the SBD binds to starch, and thereby more generally to throw light on the role of carbohydrate-binding domains in the hydrolysis of insoluble polysaccharides. RESULTS: The solution structure of the SBD of A. niger G1 bound to beta-cyclodextrin (betaCD), a cyclic starch analogue, shows that the well-defined beta-sheet structure seen in the free SBD is maintained in the SBD-betaCD complex. The main differences between the free and bound states of the SBD are observed in loop regions, in or near the two starch-binding sites. The two binding sites, each of which binds one molecule of betaCD, are structurally different. Binding site 1 is small and accessible, and its structure changes very little upon ligand binding. Site 2 is longer and undergoes a significant structural change on binding. Part of this site comprises a flexible loop, which appears to allow the SBD to bind to starch strands in a range of orientations. CONCLUSIONS: The two starch-binding sites of the SBD probably differ functionally as well as structurally; site 1 probably acts as the initial starch recognition site, whereas site 2 is involved in specific recognition of appropriate regions of starch. The two starch strands are bound at approximately 90 degrees to each other. This may be functionally important, as it may force starch strands apart thus increasing the hydrolyzable surface, or alternatively it may localize the enzyme to noncrystalline (more hydrolyzable) areas of starch. The region of the SBD where the linker to the catalytic domain is attached is flexible, allowing the catalytic site to access a large surface area of the starch granules.

Aspergillus niger↗
PubMed1999

Physicochemical properties of maize starches expressing dull and sugary-2 mutants in different genetic backgrounds.

Isogenic lines carrying the du or su(2) genes in five different maize inbred lines were developed, and the effect of the starch-modifying genes on general, thermal, pasting, and gel textural properties of starch was studied. Swelling power of du and su(2) starches was significantly lower than that of normal starch. The peak viscosity of du starches was reduced, peaks were sharper, and the setback was lower than in normal starches. The hardness of du starch gels increased and the adhesiveness decreased severalfold during 7 days of storage, indicating unstable gel texture. The su(2)/su(2) starches had flatter viscosity peaks than normal starches, with extremely low viscosity and almost zero breakdown and setback. In the su(2) starch gels hardness and adhesiveness did not change significantly during 7 days of storage. For most measured properties, there was considerable variation among different inbred backgrounds carrying the same mutant genotypes. Genetic background and starch-modifying genes can both be manipulated to select specific desired starch properties.

Calorimetry, Differential Scanning↗
PubMed2007

The diurnal metabolism of leaf starch.

Starch is a primary product of photosynthesis in leaves. In most plants, a large fraction of the carbon assimilated during the day is stored transiently in the chloroplast as starch for use during the subsequent night. Photosynthetic partitioning into starch is finely regulated, and the amount of carbohydrate stored is dependent on the environmental conditions, particularly day length. This regulation is applied at several levels to control the flux of carbon from the Calvin cycle into starch biosynthesis. Starch is composed primarily of branched glucans with an architecture that allows the formation of a semi-crystalline insoluble granule. Biosynthesis has been most intensively studied in non-photosynthetic starch-storing organs, such as developing seeds and tubers. Biosynthesis in leaves has received less attention, but recent reverse-genetic studies of Arabidopsis (thale cress) have produced data generally consistent with what is known for storage tissues. The pathway involves starch synthases, which elongate the glucan chains, and branching enzymes. Remarkably, enzymes that partially debranch glucans are also required for normal amylopectin synthesis. In the last decade, our understanding of starch breakdown in leaves has advanced considerably. Starch is hydrolysed to maltose and glucose at night via a pathway that requires recently discovered proteins in addition to well-known enzymes. These sugars are exported from the plastid to support sucrose synthesis, respiration and growth. In the present review we provide an overview of starch biosynthesis, starch structure and starch degradation in the leaves of plants. We focus on recent advances in each area and highlight outstanding questions.

Amylopectin↗